Thread: Attic venting

Does anyone out there have an opinion on the attached attic plan? No vents are indicated. The house is to be built in the Chicago area. There will be a downflow furnace/AC in the attic, but in its own insulated room. All ductwork will be insulated.
Should there be soffit vents to at least vent the soffit cavity?
Is open cell foam best for this application, or should it be closed cell?
Any help appreciated.

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The foam insulation should have a flame spead of less than 75 and smoke developed less than 450 per the 2009 IRC. The ceiling should have at least 1/2 gypsum applied. This system looks like a sealed attic system. The furnace needs combustian air. So far the application for open of closed cell, I would check with the manufactures installation instructions, if availible and see what they recommend. The think it would be ok to not vent the sofit only.

I'll tell whats missing is the rafter ties to negate the horizontal trust of the rafters. The ceiling joist / rafter ties are not connected to the rafters. Of couse the this would not apply if the ridge is an actual beam girder, like an LVL designed to support the roof.

Mr. Burkard brings up some good points to consider. Is the furnace getting it's combustion air from out side? And the structural issues.
In our area of California this design is considered an invitation for rot. In your area I would think that roof designs would be for vented space between the insulation and sheathing to avoid the chance of snow melt and ice dam formation, independent of the ice& water membrane. Most of our insulation installers prefer closed cell in the rafters, reducing the chance of absorption of any water vapor. The use of closed cell insulation would allow for less depth in the rafter bays and space for an vented air space above the insulation layer.

Originally Posted by William Cline

Mr. Burkard brings up some good points to consider. Is the furnace getting it's combustion air from out side? And the structural issues.
In our area of California this design is considered an invitation for rot. In your area I would think that roof designs would be for vented space between the insulation and sheathing to avoid the chance of snow melt and ice dam formation, independent of the ice& water membrane. Most of our insulation installers prefer closed cell in the rafters, reducing the chance of absorption of any water vapor. The use of closed cell insulation would allow for less depth in the rafter bays and space for an vented air space above the insulation layer.

Thank you both for replies.
I should clarify a couple of points not evident from plan:
1. Furnace will have own fresh air supply
2. Collar ties are specified (not shown on this section)
3. Ceiling below has 5/8" drywall

Just spoke to the insulation guy and he is OK with plan; i.e. no vents. Says that this is becomming a common system with attic furnace - expands the envelope to the rafters. Minimum R38 insulation is sufficient to keep the sheathing cold, so no ice damming issues. he also mentioned that people preferred open cell because in the event of a roof leak, it will be visible, whereas with closed cell, the leak cannot penetrate the insulation and so remains undetected. First I'd heard of that.

This design is getting more popular. The insulation on the ceiling doesn't make sense. If they are putting the insulation and air barrier on the underside of the roof that is the building envelope. This means means no vents are needed. The ceiling insulation should be omitted. I agree with the open cell as it will allow help prevent moisture from getting trapped behind it.

Originally Posted by Robert Ernst

This design is getting more popular. The insulation on the ceiling doesn't make sense. If they are putting the insulation and air barrier on the underside of the roof that is the building envelope. This means means no vents are needed. The ceiling insulation should be omitted. I agree with the open cell as it will allow help prevent moisture from getting trapped behind it.

The client wants insulation on ceiling as a sound barrier because of the attic furnace, although in my opinion most of the sound transmission will be through the ducts. Sound barrier is probably only necessary under the actual furnace.

Your right about the noise. They can have a section of flexible ducting and then mount the furnace on isolators and this will help. Fiberglass insulation is not very good for blocking sound. You can use Roxul which is much better. I agree that it should just be under the furnace. They also should have extra registers put in the ceiling to connect the house to the attic and allow a convective loop between the two.

I've never heard of making a decision on open or closed cell foam based on possibility of a roof leak. Normally, the choice is based on space and insulation needs. Closed cell foam is much denser and has a higher R-value per inch. In my opinion, R-35 is low for a roof in Chicago. Closed cell foam in the same installation will get you closer to R-60. However, with R-30 in the ceiling, they may be ok. My recommendation would be to not install the fiberglass batts from the ceiling, use rockwool as a sound barrier under the furnace, thermally connect the attic to the living space and use the money saved to put 9.5" of closed cell foam between the roof joists.

Agree with rock wool as better for sound, and also with R50 plus in rafter bays, but I'm a little confused as to why you guys think the living space should be thermally connected to the attic.

There should be one thermal barrier, either at the roof or the ceiling. Separate thermal barriers does not make sense. Sound deadening is a different animal but trying to get thermal performance from insulation in two different planes is going to be difficult.
I think installing registers to the attic through the ceiling will violate the fire blocking required at the ceiling, no?

Good point about fireblocking, given that the foam above is exposed. I'm think an isolated furnace room with rock wool insulation under and no insulation in the remainder of the ceiling.

If the attic is integrated into the thermal envelope of the building, then it should have have supply and return air as with any other room in the home. That being the case, the spray foam must be covered with a fire/ignition barrier. This can be achieved with a spray on retardant that your insulation contractor should be able to do, or with a traditional covering such as wallboard.
If you don't provide supply and return air to the attic and it is not vented to the outside, the air would get quite stale in a hurry. Additionally, the temperature in the attic would be not be controlled by the heater/AC, but rather by the thermal resistance of the roof insulation.
If it's not being used as living space, I don't think it will need to be heated and cooled to the same degree as the living space, but it should be tempered.

Originally Posted by Jim Luttrall

I think installing registers to the attic through the ceiling will violate the fire blocking required at the ceiling, no?

No.

That would only come into play if there was a separate dwelling unit above and the ceiling/floor system was a fire-rated system, but that is a roof above, so it would be highly unlikely that there would be any reason for a floor/ceiling fire-rated system.

The fireblocking you are thinking of is where the concealed wall stud cavities are fireblocked at the ceiling and at the floor, and at 10 foot intervals horizontally.

[quote=Jerry Peck;198552]No.

That would only come into play if there was a separate dwelling unit above and the ceiling/floor system was a fire-rated system, but that is a roof above, so it would be highly unlikely that there would be any reason for a floor/ceiling fire-rated system.

I should have mentioned this is a frame, two story house.
I get the fireblocking in the stud cavities to prevent a chimney effect in the event of fire, but I think penetrating a type x drywall ceiling with unducted register openings to an unfinished attic space would also be a fire hazard, even if the exposed foam was treated with a fire retardant.
Perhaps one way to keep the attic air "tempered" would be to install a small supply and return in the space to keep it circulating.

Originally Posted by Donal Hughes

Originally Posted by Jerry Peck

No.

That would only come into play if there was a separate dwelling unit above and the ceiling/floor system was a fire-rated system, but that is a roof above, so it would be highly unlikely that there would be any reason for a floor/ceiling fire-rated system.

I should have mentioned this is a frame, two story house.

Right ... a "two story house" ... which *does not have* a "separate dwelling unit above"

[quote\I get the fireblocking in the stud cavities to prevent a chimney effect in the event of fire, but I think penetrating a type x drywall ceiling ...

Why on earth are you using Type X drywall on the ceiling between the first floor and the second floor of *the same* dwelling? Certainly not required. If you are using Type X because you want to, the floor/ceiling system is still *not* a fire-rated system.

Are you not telling us something we need to know?

with unducted register openings to an unfinished attic space would also be a fire hazard, even if the exposed foam was treated with a fire retardant.

No it wouldn't, not in a single-family dwelling.

Perhaps one way to keep the attic air "tempered" would be to install a small supply and return in the space to keep it circulating.

Someone is not following what is going on ... either you are doing something other than what you are describing and I am not following you ... or ... you are not aware of what needs to be done and why.

Originally Posted by Jerry Peck

Why on earth are you using Type X drywall on the ceiling between the first floor and the second floor of *the same* dwelling? Certainly not required. If you are using Type X because you want to, the floor/ceiling system is still *not* a fire-rated system.

Are you not telling us something we need to know?








Someone is not following what is going on ... either you are doing something other than what you are describing and I am not following you ... or ... you are not aware of what needs to be done and why.

I've noticed that 5/8" drywall is generally spec'ed in this area for frame structures, (could be wrong, but I believe the City of Chicago require it), for all walls and ceilings. Also 5/8" is always Type X.

Sorry but I don't understand your last comment.

Originally Posted by Donal Hughes

I've noticed that 5/8" drywall is generally spec'ed in this area for frame structures, (could be wrong, but I believe the City of Chicago require it), for all walls and ceilings. Also 5/8" is always Type X.

Sorry but I don't understand your last comment.

The last comment was stating that either I was not following what you are doing and why, or ... the 'or' does not matter if 5/8" is required in you AHJ as that explains why I was not following you ... I have never heard of 5/8" type X being required in an installation such as you are describing.

Originally Posted by Jerry Peck

The last comment was stating that either I was not following what you are doing and why, or ... the 'or' does not matter if 5/8" is required in you AHJ as that explains why I was not following you ... I have never heard of 5/8" type X being required in an installation such as you are describing.

OK now I understand. Will check requirements in this jurisdiction, but the architect in this case specified 5/8" type X throughout. My thought on the second floor to attic penetrations was: why compromise a one hour ceiling with openings which could be avoided. In addition I'm thinking a future home inspector might write up these additional registers in the ceiling that are not connected to anything. I know I myself would likely question their function.

I should disclose that I've been hired by the owner as a consultant on this project, and I appreciate all ideas.

Consider running the wall sheathing all the way to the under side of the roof sheathing - this will provide a better air seal and help prevent issues with heat/moisture getting into the eve and causing paint peal/other issues. The detail does require that the eve be scabbed on after the fact (frame a ladder @ the pitch and toss it up) but it's worth the effort. Course they show a continuous 2x at the the rim in the area I'm discussing so they must have been planning on doing this anyway (no other way for two members to run though each other . This is indicated by the graphic convention of the member - blocking is a single line in the member (/), continuous is two (X). If that's the case then the wall sheathing should definately run up to the underside of the roof sheathing.

I'd also go closed cell in this case. While the installer is right as it'd pass the water though and let you "see the leak," but if you're making the choice based on that than you're instantly admitting you've got a sub-par roofing system/installer. No need to plan for a leak - design so you don't have one.

1. Open cell permits water (and thereby water vapor) to move though the insulation.
2. When water vapor comes in contact with something cold, it turns into dew.
3. It's already been mentioned that the rafters and sheathing are cold to prevent ice damns from forming.
4. Presence of water vapor (passes though open cell) + cold surface (ie your plywood and rafters) = dew.

I'd guess (note I'm not in Chicago though so it's a guess) that you'll have dew to form on the inside portion of the framing members, about 3 or 4 inches up. This will cause mold and possibly even rot if preventive measures aren't taken. A vapor barrier (6 mil sheets of plastic held in place with staples which pierce it and are never installed right around openings and punctures) should be applied over the rafters and insulation to keep the vapor out.

For this reason, my office prefers to use closed cell: you get better R value/$, and eliminate the worry of a pierced piece of plastic keeping the water out from your roof and wall assemblies.

Hope this helps,
-jacob

PS: Lastly, ceiling joists should run parallel to the rafters - this will make it easier for you to climb around up there to find that leak you're planning on having.

Thanks for the feedback all.
Update:
1. Roof already built, so in place of wall sheathing all the way up we have blocking (2x). Don't think there's any point adding pieces of sheathing over the blocking, rather the blocking will be sealed on all sides with foam.
2. Already argued Jacob's point about roof leak; I told insulation contractor we were NOT anticipating any leaks! So definately closed cell.
3. Ceiling joists are now running parallel to rafters.

Originally Posted by Donal Hughes

OK now I understand. Will check requirements in this jurisdiction, but the architect in this case specified 5/8" type X throughout. My thought on the second floor to attic penetrations was: why compromise a one hour ceiling with openings which could be avoided.

First, applying 5/8" Type X does not, in and of itself, make a 1 hour rated ceiling, nonetheless, though, I was ready to provide a suggestion for maintaining 'separation rating' of the 5/8" Type X for the ceiling, then I read the next sentence:

In addition I'm thinking a future home inspector might write up these additional registers in the ceiling that are not connected to anything.

The registers are not connected to anything? Then why are they there?

Regardless, let's go back to maintaining the 'separation' provided by the 5/8" Type X drywall on the ceiling - you could maintain that separation rating by installing fused fire-dampers in the registers, heat from a fire melts the fusible links and the dampers automatically close.

Originally Posted by Jerry Peck

Regardless, let's go back to maintaining the 'separation' provided by the 5/8" Type X drywall on the ceiling - you could maintain that separation rating by installing fused fire-dampers in the registers.

That's not necessarily true or correct. Depends on what has been specified in the assembly.

From:
UL | Dampers

Hope the imbedded links work:

Originally Posted by UL

Special notice regarding ceiling dampers: This is to advise that UL has not authorized the use of ceiling dampers in Design Number L528 (clickable link

BXUV.L528 - Fire Resistance Ratings - ANSI/UL 263
as published in UL's fire resistance directory.

This information is being provided in response to publications by other certification agencies that state that ceiling radiation dampers and insulated plenum boxes are acceptable for installation in partition types L-528.

For additional information regarding dampers, see the UL Marking Guide for Dampers for Fire Barrier and Smoke Applications and Ceiling Dampers
http://www.ul.com/global/documents/o...rs/dampers.pdf

(also attached).

The guide was developed for use by contractors, code and inspection authorities, installers, users and system designers to aid in the understanding of the scope of UL certification of dampers. The guide is also intended to assist in determining the suitability of these products for use in specific applications

However most authorities having jurisdiction who have adopted model codes and made ammendments to same, have reasons for having done so.

Density, little or no set backs, zoning, scores if not centuries of multiple generations of construction and construction types, myriad reasons why that is the case in the OPs area and many others (S.F. Calif, NYC, and many, Many others). An exception option for not otherwise having to retrofit sprinkler system for example, Insurance is yet another factor in planning.




.

An opinion? Don't put any appliance in the attic if you don't absolutely have to.

Originally Posted by Jerry Peck

First, applying 5/8" Type X does not, in and of itself, make a 1 hour rated ceiling, ...
.
Regardless, let's go back to maintaining the 'separation' provided by the 5/8" Type X drywall on the ceiling - you could maintain that separation rating by installing fused fire-dampers in the registers, heat from a fire melts the fusible links and the dampers automatically close.

Originally Posted by H.G. Watson, Sr.

That's not necessarily true or correct. Depends on what has been specified in the assembly.

Originally Posted by Donal Hughes

... but the architect in this case specified 5/8" type X throughout. My thought on the second floor to attic penetrations ...

To our knowledge, that is not a fire-rated system, which makes it not a UL Design, which makes it what I have been talking about.

*IF* the architect specified a UL Design #, then that UL Design # information would be in with the plans, or at the very least referenced on the plans - which would direct one to go seek out the specified UL Design #.

You are correct in that there is a UL Design # which does not allow for dampers, however, to this point, nothing in the information provided indicates that the ceiling is a UL Design # fire-resistance rated ceiling/roof system - nothing other than Donal thinking that because 5/8 Type X was used that it must therefore be a fire-resistance rated ceiling, which would actually be more correctly stated as a fire-resistance rated ceiling/roof assembly.

Add to that the simple fact that the UL Design you referred to is a ceiling/floor assembly, not a ceiling/roof assembly, and is therefore not even applicable to the discussion - but that was a good try Watson.

Originally Posted by Jerry Peck

First, applying 5/8" Type X does not, in and of itself, make a 1 hour rated ceiling, nonetheless, though, I was ready to provide a suggestion for maintaining 'separation rating' of the 5/8" Type X for the ceiling, then I read the next sentence:



The registers are not connected to anything? Then why are they there?

Regardless, let's go back to maintaining the 'separation' provided by the 5/8" Type X drywall on the ceiling - you could maintain that separation rating by installing fused fire-dampers in the registers, heat from a fire melts the fusible links and the dampers automatically close.

Sorry for not responding sooner but decided to clear out of Chicago for the weekend - we have some protesters in town!

I'd like to clarify that this is not a fire rated assembly, therefore no UL Design #. The registers "not connected to anything" I was referring to were suggested by a previous poster (Robert) to provide a convection loop between house and attic. So since this assembly has no fire rating then presumably the fusible ceiling dampers would be Ok (as per Jerry), but my inclination would be to avoid the extra ceiling openings/registers completely (unsightly from a cosmetic point of view apart from anything else) and "temper" the attic with supply/return from attic furnace.

Originally Posted by Donal Hughes

and "temper" the attic with supply/return from attic furnace.

There is no need to "temper" the attic.

With sealed attics, i.e., no ventilation and with the insulation covering across the soffit and up the underside of the roof sheathing, the temperature of the attic air above the ceiling is typically within 2-3 degrees of the air below the ceiling - this is because the drywall does really separate the two areas into distinct areas such that one area does not affect the other area - the two areas are almost like one area as both are within the thermal envelope.

[quote=Jerry Peck;198817]There is no need to "temper" the attic.


Interesting. So you don't think the attic air becomes "stale". I guess there would be a certain amount of leakage around the ceiling registers.

Originally Posted by Donal Hughes

Interesting. So you don't think the attic air becomes "stale". I guess there would be a certain amount of leakage around the ceiling registers.

There is no need for vents to the attic, except for possibly this:

Originally Posted by Donal Hughes

There will be a downflow furnace/AC in the attic, but in its own insulated room.

Is the furnace gas or electric?

If electric, no problem.

If gas, it will need combustion air and it will likely not get it from the attic, unless it is a direct vent furnace getting all air from the outdoors.

Originally Posted by Jerry Peck

There is no need for vents to the attic, except for possibly this:


Is the furnace gas or electric?

If electric, no problem.

If gas, it will need combustion air and it will likely not get it from the attic, unless it is a direct vent furnace getting all air from the outdoors.

Furnace will be natural gas, direct vent with all combustion air from exterior.
Thank you for your help.

Hi all,

We have used this exact system in Florida for a number of years. However, I'm not familiar with the uses in colder climates. It is super energy efficient and can even add some structurla strenght to the roof assembly. Here it is required to install a sprayed-on ignition resistant coating to meet the flame spread requirements. We don't see many gas-fired furnaces (especially in attics) so I can't comment on that. The furnace manufacturer should be able to give some guidance. I didn't see anyone mentioning the code, but the IRC and many state codes offer prescriptive methods or at least standards for unvented or semi-conditioned attic spaces. It is usually found in Section R806. Some methods require a transfer opening to communicate air between the ceiling and attic space.

Originally Posted by Donal Hughes

Agree with rock wool as better for sound, and also with R50 plus in rafter bays, but I'm a little confused as to why you guys think the living space should be thermally connected to the attic.

The attic would be considered a semi conditioned space. It also lends itself to a storage area too.

One of hte biggest challenges is to make a home air tight and the attic floor can be difficult to do. In this case there is the HVAC there. Plus you ,ight have can lights and other penetration of the ceiling.

From an energy efficiency standpoint bringing the HVAC inside the air barrier and thermal envelope far outways the added space inside the envelope.

I have heard the the argument for open cell based on water leaks and I really dont buy it. Many leaks are very small and will take time to be discovered and my actually be held in the upper layer. Plus people are not going to be in the attic often if ever. So it would take water dripping on to the ceiling and cause a wet spot in the living area to be noticable.

Building science says that a structure needs to be able to dry in, out or both. Depending on the roofing the roof may not be able to dry to the outside and it cant dry to the inside. Shingles and under layment will block drying up/outside. An option would be to provide an air channel under. What I dont like about either open or closed cell is the thermal bridging through the wood.

To me a better solution may be to put some if not all of the insulation to the exterior with rigid foam. A dense pack insualtion could be put on the bottom side of the roof deck. The foam would be an extera water layer and the deck could still dry to the interior. You would have multiple layers or air and water barriers. Couple that with a 50+ year or lifetime roof and the current owner would never have an issue and it would be a good roof when time to sell. Not to many shingle roofs last more than 20.

Checked the code (2009 IRC) and the requirements for unvented attic assemblies are :

Section R806.4 of the 2009 International Residential Code® (IRC), and Section R806.5 of the 2012 IRC have requirements for unvented (conditioned) attic assemblies. The overall insulation level in the roof assembly must meet the baseline requirements for energy efficiency as given in the IRC Section N1102 (or IECC, Section 402). Additionally, there are requirements related to the arrangement of the insulation so as to prevent moisture condensation in the roof assembly. In all but Climate Zones 2B and 3B, the IRC requires air-impermeable insulation in direct contact with the underside of the roof deck. A combination of an air-impermeable insulation and air-permeable insulation directly below it is permitted.
2009 IRC Table R806.4 and 2012 IRC Table R806.5 set minimum requirements for minimum rigid board insulation to be placed on top of the roof deck if air permeable insulation is installed in direct contact with the underside of the roof deck. The addition of the rigid board insulation will keep the monthly average condensing surface temperature at or above 45°F. Note that the additional rigid board insulation is not required if air impermeable insulation is installed in direct contact with the underside of the roof sheathing.

Apparently most open cell foams are air permeable with the exception of Icynene (which was specd by the architect in this case). So without rigid foam on top of roof deck it looks like closed cell or Icycene is required -climate zone 5.
Alternatively looks like spraying a couple of inches of closed cell against the underside of the sheathing to get the air impermeable barrier, and then making up the balance of the required R value with any other of insulation would also work.
This of course does not address thermal bridging accross the rafters. Perhaps spraying foam over all rafter edges would help.

Originally Posted by Robert Hronek

The attic would be considered a semi conditioned space.

Semi conditioned space is what it used to be called, however, and because it is within the thermal envelope of the structure it is now called conditioned space (even though there are no supplies to, or returns from, that space).

Originally Posted by Jerry Peck

Semi conditioned space is what it used to be called, however, and because it is within the thermal envelope of the structure it is now called conditioned space (even though there are no supplies to, or returns from, that space).

I agree it is conditioned space but you dont not manage it in the same way as you do an occupied room. It would get less heat/cold and is managed so as not to cause problems such as high moisture or mold. Most of the heat or cooling would be scavanged loss from the ducts and equipment. The cost to heat and cool would be less than occupied space.

Originally Posted by Robert Hronek

I agree it is conditioned space but you dont not manage it in the same way as you do an occupied room. It would get less heat/cold and is managed so as not to cause problems such as high moisture or mold. Most of the heat or cooling would be scavanged loss from the ducts and equipment. The cost to heat and cool would be less than occupied space.

OK so is the "scavanged loss from ducts and equiptment" sufficient? In this case furnace is in its own drywalled enclosure with combustion air from exterior and all duct joints sealed with duct sealant (assuming installer does a goog job). The only leakage might be around the ceiling registers but then they are usually a pretty tight fit.
I guess the bottom line question I'm asking: should we actively design for a minimal supply/return in the attic space?

Originally Posted by Donal Hughes

OK so is the "scavanged loss from ducts and equiptment" sufficient? In this case furnace is in its own drywalled enclosure with combustion air from exterior and all duct joints sealed with duct sealant (assuming installer does a goog job). The only leakage might be around the ceiling registers but then they are usually a pretty tight fit.
I guess the bottom line question I'm asking: should we actively design for a minimal supply/return in the attic space?

I said most. This a semi conditioned area....that is it is not kept at the same temp as the living area. There should be one or more supplies and returns depending on the size of the space. You are wanting to control moisture and temps to prevent mold or other moisture problems. It will be somewhat cooler or warmer than the living space. But with good insulation and no air leaks it will not require much heating/cooling.

Originally Posted by Robert Hronek

I agree it is conditioned space but you dont not manage it in the same way as you do an occupied room. It would get less heat/cold and is managed so as not to cause problems such as high moisture or mold. Most of the heat or cooling would be scavanged loss from the ducts and equipment. The cost to heat and cool would be less than occupied space.

Many tests have been done on those sealed attics over the past number of years in Florida and if you put a supply register in there you will be creating problems.

The migration of moisture, vapor, and temperature through the drywall is all that is necessary to allow the system to work properly.

Originally Posted by Robert Hronek

I said most. This a semi conditioned area....that is it is not kept at the same temp as the living area. There should be one or more supplies and returns depending on the size of the space. You are wanting to control moisture and temps to prevent mold or other moisture problems. It will be somewhat cooler or warmer than the living space. But with good insulation and no air leaks it will not require much heating/cooling.

Got it. Thanks for your help. I will have HVAC contractor include some attic s/r's.

Originally Posted by Donal Hughes

Got it. Thanks for your help. I will have HVAC contractor include some attic s/r's.

You will then become the designer of the system and when it fails *you* will be the one they go to for the solution and the monetary compensation for corrections.

Let a building science engineer or a mechanical engineer design the system, give them that liability.

You job is not to *design* the structure and the system, your job is to *verify* (through documentation) that the contractors did what was on the plans, drawings, and specifications - either this items is, or is not, as specified and here are the photos, code, manufacturer's installation instructions, etc., which supports your opinion that it does, or does not, meet the requirements which were specified by the engineers on the approved plans.

At most you might ask for an RFI (Request For Information) from the architect/engineer that there does, or does not, need to be any supplies or returns into the attic area - as in you are asking for verification that none are required if none are shown, or that they are required if they are shown.

That gives them a chance to re-evaluate their design, and it gives you a paper trail that you asked the question. But design it yourself? Not me.

Originally Posted by Jerry Peck

You will then become the designer of the system and when it fails *you* will be the one they go to for the solution and the monetary compensation for corrections.

Let a building science engineer or a mechanical engineer design the system, give them that liability.

You job is not to *design* the structure and the system, your job is to *verify* (through documentation) that the contractors did what was on the plans, drawings, and specifications - either this items is, or is not, as specified and here are the photos, code, manufacturer's installation instructions, etc., which supports your opinion that it does, or does not, meet the requirements which were specified by the engineers on the approved plans.

At most you might ask for an RFI (Request For Information) from the architect/engineer that there does, or does not, need to be any supplies or returns into the attic area - as in you are asking for verification that none are required if none are shown, or that they are required if they are shown.

That gives them a chance to re-evaluate their design, and it gives you a paper trail that you asked the question. But design it yourself? Not me.

Good points all; and what I should have stated in my last reply was that I would have the project architect include attic details in the permitted mechanical plan. We are very lucky in that we have a built in failsafe in this jurisdiction; an excellent building inspector who will not pass any alterations that are not on the architect's plan.

3 questions:
1) Roof leak - Holly crap, how would you find it?
2) why would you want to Condition unlivable space,
3) is this truly a energy efficient cost savings device or simply a cheaper easier way to install Insulation?

Conference teachers - this is a great topic for Vegas this Fall !!!!!!!

Nick J. Alati
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3 questions:
1) Roof leak - Holly crap, how would you find it?
2) why would you want to Condition unlivable space,
3) is this truly a energy efficient cost savings device or simply a cheaper easier way to install Insulation?

Conference teachers - this is a great topic for Vegas this Fall !!!!!!!

Nick J. Alati
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Originally Posted by Donal Hughes

This of course does not address thermal bridging accross the rafters. Perhaps spraying foam over all rafter edges would help.

Donal,
You can control the thermal bridging by wrapping the underside edges of the rafters with the closed cell foam, or by installing a continuous layer of rigid foam insulation to the underside of the rafters.

Originally Posted by nick alati

3 questions:
1) Roof leak - Holly crap, how would you find it?

That is the main question we all have for this type of roof insulation, and those of us from South Florida have been asking that question for, however-long-it-has-been years.

2) why would you want to Condition unlivable space,

Well, you are not really conditioning that space in the sense of providing supply and return, you are simply removing the heat/cold from the attic by stopping the heat/cold at the roof sheathing, the 'conditioning' effect happens automatically.

3) is this truly a energy efficient cost savings device or simply a cheaper easier way to install Insulation?

It really is energy efficient, and it is a simpler way to install the insulation, but it is not a cheaper way to install the insulation as it costs more to install that insulation.

In the long run, the only question which remains unanswered is the question about roof leaks and roof decking.

I'm wondering if everyone commenting in this thread realizes that the ceiling is not also insulated when using this system. By some of the remarks it does not seem so. Without ceiling insulation the attic will remain only a few degrees warmer or cooler (depending on the season) than the ambient temperature in the rooms below. Even though the HVAC system has a larger area (in cu. ft.) to condition, the system ductwork (which has less R-value that the attic/roof insulation) is operating within a conditioned space. So very little temperature is lost to the extremes in the attic. That's where the energy efficiency is made up. I agree with Jerry, the only real concern is concealed roof leaks will be harder to detect. But I think they can be detected with observation. On a sloped roof, the water will migrate to the roof edges. Since any moisture that makes it's way beneath the roof covering will not dry to a surface and evaporate, it accumulate and be forced to migrate downward. It wont take long before it appear on the soffits. It would also be forced into the capillaries of the rafter lumber and once saturation is reached would drip onto the ceiling below. Thats a good argument for not encapsulating the rafters or top chords of trusses. The worst scenario would be a very small leak that would not be perceptible for some time. It might do a fair amount of damage before it would be detected. Before this will occur, both roof covering and the underlayment would have to fail. Better be checking those flasing details!

Originally Posted by Keith Gipe

... the only real concern is concealed roof leaks will be harder to detect. But I think they can be detected with observation.

To those who have infrared cameras - a good add-on is an annual scan of the attic for abnormalities such as hidden roof leaks, those hidden roof leaks should show up on an infrared scan.

Originally Posted by Harold Doughty

Donal,
You can control the thermal bridging by wrapping the underside edges of the rafters with the closed cell foam, or by installing a continuous layer of rigid foam insulation to the underside of the rafters.

Wrapping foam around the rafters is not only difficult to control, it's also only going to give you a minimal energy savings. Yes, closed cell foam insulates better than the rafters, but when you consider:

• the overall area impacted by joists/rafters
• the cost of closed cell foam,
• the impact of construction timeline
• the insulating properties of the joists themselves

and you're in for a long term ROI.

Better solution is aerogel tape on the top side of the joists - this creates the thermal break without causing too much impact on time (1 guy for 1 day instead of an insulating crew for a day), and i believe costs less than the foam too. Drawback is it must go on before the sheathing, so I think you missed the boat on that.

Your best option may be to not worry about the thermal break. If you've got shingles, water barrier, 9.25" of closed cell foam, and rafters at 9.25" o.c., then you're passing and then some. First, ask the contractor for the cost for the aerogel tape and if their insulation guy can do 1" of foam on the inside face of rafters (this is tough to do). Tell the architect you want to see the u factor for:

1. the assembly with the studs uninsulated
2. the assembly with the tape
3. the assembly with 1" of foam

Note that the International Energy Conservation Code (which may or may not be in effect in your location), requires a U factor of 0.030 for roof assemblies in your area. Then you can look at the amount of heat loss though each, and make an educated guess as to exactly how much energy is lost though the rafters, and then help the owner decide if they want to spend the money or not.

Quote:

2) why would you want to Condition unlivable space,


Well, you are not really conditioning that space in the sense of providing supply and return, you are simply removing the heat/cold from the attic by stopping the heat/cold at the roof sheathing, the 'conditioning' effect happens automatically.

Jerry
I disagree, In Arizona when I Inspect attics with the outside temp around 110 deg the Attics are comfortable. We see attics temp well in the 140 rang here.. There might not be a supply air duct up there but 5/8 drywall just cant stop the transfer of Heat to Cool - Condition air in the Attic.

Nick Alati

Originally Posted by nick alati

I disagree, In Arizona when I Inspect attics with the outside temp around 110 deg the Attics are comfortable. We see attics temp well in the 140 rang here.

If you are seeing 140 degrees in a sealed, unvented attic, with insulation on the underside of the roof sheathing ... then someone is not doing something correctly!

There might not be a supply air duct up there but 5/8 drywall just cant stop the transfer of Heat to Cool ...

The drywall is not intended to stop the transfer of heat/moisture/etc.

- Condition air in the Attic.

The result is that the attic remains within a few degrees of the living space temperature, and that is why it is called a 'conditioned attic' instead of a 'semi-conditioned attic', which I noted elsewhere when someone referred to those attics as 'semi-conditioned' - I also said that there are *no* supplies or returns to the attic.

If you are finding 140 degrees in those attics ... SOMETHING IS WRONG!