Icynene

[Richard Stanley]

This am inspection. Town house. New. Attic roof decking/framing covered with icynene. Nothing on attic floor (joists, ceiling gypsum, etc.) My question is that the firewalls between the units were not covered with the icynene, should they also be covered to accomplish the sealed attic space??

[Jim Luttrall]

Good question, never thought about it in that situation.

The firewall though has little to nothing to do with the thermal envelope... but it would be a problem if units on either side of the firewall were insulated differently say one had insulation on the roof decking and the other unit had the insulation on the attic floor.
It seems to me that this would take a "big picture" type approach.
Stand back and follow the thermal envelope path similar to following the load path from attic to foundation.

Assuming (big risk, I know) that both sides of the firewall were of similar construction, then I don't see a problem anymore than the common walls not being insulated.

[Bruce King]

Another thing to think about when the roof has the insulation instead of the ceiling....

On a pre-drywall, large house with this, I commented that the planned (unknown) HVAC sizing may need an adjustment since major amounts of living space are only seperated from the attic by sheetrock.

The buyer said he has seen those type attics at 95 deg in the hot summer and thought that was great. But you have 72-76 deg. living space on the opposite side of 1/2 or 5/8 sheetrock so the heat transfer is going to occur which would increase the cooling load more than conventional blown in insulation.

[Richard Stanley]

Jim, That makes sense. This unit is sandwiched between others. There are 7 in one continuous building and all constructed the same. 3 stories. Probably the only attic walls needing it would be the end units - 1 and 7.

Bruce, They are actually using smaller than normal units. 2.5 ton ac for about 2k sq ft. I noticed the end units were 1/2 ton larger than the middle ones. Probably for the additional exposure - 3 sides versus 2.

I found some good stuff @ www.icynene.com (http://www.icynene.com)

Another note - The super was telling me that using rough figures, it would take some one between 15 and 20 years of energy savings (?) to pay for that icynene!!!!

[Jerry Peck]

No, Icynene is not required on the fire wall.

No, the attic space does not have a 'significant' effect on HVAC sizing (some, yes, just not 'significant' ... so I've been advised by many HVAC contractors who sized the systems for the houses).

As most are installed, the attic space becomes 'semi-conditioned' space as it is not really "conditioned" with a supply.

Some are installed with a supply in the attic, making this actually "conditioned" space.

I was told that typically this does not even add a 1/2 ton to the unit size.

It is one big open space, no supply (the ones with no supply) and the thermal exchange through the gypsum board ceiling is slow and limited, reducing the effect of heat gain / heat loss as once the difference has stabilized (i.e., as in new house), then there is little difference to heat and cool - it's not like the attic is taking on or losing lots of heat which needs to be compensate for.

Now, though, put a supply in there and it will make some difference, but my concern then becomes that of moisture in the attic on surfaces the supply is blowing onto and cooling those surfaces down. I can imagine mildew growing on the top side of the ceiling under the right ('wrong') conditions.

[Jim Luttrall]

Jerry, On the subject of conditioned attics, why would mold be of any more concern there than the rest of the conditioned house? That is one of the beauties of moving the thermal barrier to the outside of the structure, it becomes "conditioned" space by default even if there is no supply register simply because it is inside the thermal boundary. I could see a concern if the vapor barrier was not located at the same place as the thermal boundary, but that is a horse of a different color when discussing IC.
Your thoughts please?

[Jerry Peck]

Jerry, On the subject of conditioned attics, why would mold be of any more concern there than the rest of the conditioned house?

Jim,

Lack of air movement allowing the moisture to accumulate and not dry out as readily.

Everything to those attic are sealed (supposed to be anyway for firestopping and draftstopping reasons), so there should not be any air flow in the attic.

[Jim Luttrall]

Jerry, have you seen / read anything that indicates this would be a problem? Or are you just thinking out loud? ;)

[Jerry Peck]

Or are you just thinking out loud? ;)

Cold air blowing onto a cellulose material reducing its temperature in a warmer and moister (is that a word? :) ) environment.

"just thinking out loud"