Thread: Eave Vent Replacing Rafter Blocking

I currently have no venting for the attic. I'll have a ridge line exhaust vent installed with the new roofing. For intake vents I'm considering replacing the 2x4 blocking between some of the rafters at the eaves with 3 1/2 x 22" screen vents plus a rafter vent at each eave screen vent to provide a air channel through the insulation batts. Does this present a structural problem? If so, is there still a structural problem if I add 2x4 blocking further up the roof between the rafters that no longer have a block at the eves? The added block would have to be sufficiently up the roof to not block the rafter vent channel.

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A picture would help. In general, the 2X4 blocking between rafters at the eaves is just blocking. It should be no problem structurally to remove them. I sometimes see holes drilled through them, 3 or 4 holes per block, covered with screen or hardware cloth to keep birds and rodents out. The vents you describe will work fine.
You will need to be careful not to damage the roof sheathing. You can clip off some of the roofing nails to get in there safely. It sounds like you have open soffits and can work from the outside?

Thanks.
I've attached a picture of the vent. It provides a lot more ventilation than a two or three holes.
Good thought about not damaging the sheathing. A roofer suggested drilling a hole and then sawing the blocking in half works good when the blocking doesn't knock out easily with a hammer. Maybe in some cases I'll end up slipping in the saw blade to cut a nail or two.
I do have open soffits. I'm still trying to find out if the nailed flange is intended to be toward the attic or toward the outside.

I've since discovered removing the blocking isn't going to be quite so simple. I drilled a hole and discovered I was drilling through 1x material and there is something solid behind the 1x.

Don,
Be careful. Some builders are constructing green buildings. The house (including the attic) is sealed to reduce energy costs, put simply. There are a couple of things to look for. 1) the attic insulation is a blown in insulation in a net attached to the rafters. 2) The attic temperature will typically be within 8 degrees of the habitable space. 3) The air returns will have two ducts, one for the return and another from a single roof vent for fresh air. Do not compromise this seal. This is intentional by the builder.

Don,
Pictures of the blocking from the outside and from inside the attic, as well as pictures of the attic interior will help us to help you.

Originally Posted by Don Parda

I've since discovered removing the blocking isn't going to be quite so simple. I drilled a hole and discovered I was drilling through 1x material and there is something solid behind the 1x.

Your drill probably is hitting the double plate, top of the wall. The rafters are notched down over the plate in older construction. Drill the upper half of the 1X and you should hit air.

If your roof is supported by manufactured trusses, the builder may have installed 2 X 4 spacers, and added a trim plank, but that is not typical.

We assume this is older construction, because you are having shingles replaced, therefore, not a Green building.

Pictures need to be downsized to 640 X 480 pixels, then they are easy to add to your post.

Originally Posted by Lee Taylor

Don,
Be careful. Some builders are constructing green buildings. The house (including the attic) is sealed to reduce energy costs, put simply. There are a couple of things to look for. 1) the attic insulation is a blown in insulation in a net attached to the rafters. 2) The attic temperature will typically be within 8 degrees of the habitable space. 3) The air returns will have two ducts, one for the return and another from a single roof vent for fresh air. Do not compromise this seal. This is intentional by the builder.

Um, I disagree.

Attics should never be sealed. If it is sealed it is not an attic. The only time I've seen no vents of any kind has been with skip sheathed roof deck and rigid (slate) shingles. If your attic is within 8 degrees of the conditioned space in winter or summer, something is seriously wrong.

The only time the insulation should be in the rafters is if it is a roof/ceiling configuration. Insulation must be directly adjacent to the conditioned space to be effective.

#3 is possible in a very new house, but more likely in a multi-family (something under IBC vs. IRC, perhaps?). Hopefully there is also a heat-recovery unit included to temper the make-up air.

Mark,

Disagree if you like, there are apparently regional construction differences in green building design. This technique is the latest design by several builders in this area in order to provide a sealed building envelope. These are energy efficient homes. Their plans are approved by the city and county.

Furthermore, Nevada has implemented an energy efficiency form that is required for each sale, unless both the buyer and the agree in writing on that form it is not required. This is to track all utility use for the previous 12 months, in short.

Originally Posted by Mark Fisher

Um, I disagree.

Attics should never be sealed. If it is sealed it is not an attic.

Mark,

You are years behind the times thinking that attic should never be sealed. Sealed attic is a big thing in many parts of the country.

This is from the 2009 IRC, and it has been in the code prior to this, but many places are using the 2006 to 2009 ICC codes or codes based on those codes:
- R806.4 Unvented attic assemblies. Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) shall be permitted if all the following conditions are met:
- - 1. The unvented attic space is completely contained within the building thermal envelope.
- - 2. No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly.
- - 3. Where wood shingles or shakes are used, a minimum 1/4 inch (6 mm) vented air space separates the shingles or shakes and the roofing underlayment above the structural sheathing.
- - 4. In climate zones 5, 6, 7 and 8, any air-impermeable insulation shall be a vapor retarder, or shall have a vapor retarder coating or covering in direct contact with the underside of the insulation.
- - 5. Either Items 5.1, 5.2 or 5.3 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.
- - - 5.1. Air-impermeable insulation only. Insulation shall be applied in direct contact with the underside of the structural roof sheathing.
- - - 5.2. Air-permeable insulation only. In addition to the air-permeable installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.
- - - 5.3. Air-impermeable and air-permeable insulation. The air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing as specified in Table R806.4 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

I stand corrected. Thank you, gentlemen. That's what happens when I see more houses built in the 19th Century than the 21st

Anyway, I'd researched this issue a few years ago, using my primary moisture resource (Building Science Corp.) and this article . While I can see that it is a viable technique in some instances, I'd be very careful about stock housing (where construction defects are the rule, not the exception), especially in some climates.

From the article:

Unvented attic design falls into two categories: systems where condensing surface temperatures are not controlled and systems where condensing surface temperatures are controlled. The two categories essentially are the demarcation between regions where cold weather conditions occur with sufficient frequency and intensity that sufficient moisture accumulation from interior sources can occur on an uninsulated roof deck to risk mold, corrosion and decay problems.

Originally Posted by Mark Fisher

Anyway, I'd researched this issue a few years ago, using my primary moisture resource (Building Science Corp.) and this article .

Don't stop there, from the same source:

RR-9801: Vented and Sealed Attics in Hot Climates — Building Science Information

RR-0301: Unvented Roof Summary Article — Building Science Information

RR-9701: Measurement of Attic Temperatures and Cooling Energy Use in Vented and Sealed Attics in Las Vegas, Nevada — Building Science Information

And he has a few others too.

I know. I just cited a general one. Everyone here needs to be familiar with BSC - a GREAT resource that I've used to support my findings in court with 100% success. Understanding what they are talking about is the difference between 'Inspection' and 'Diagnostics'.

Most of my 'design' work has been in retrofit situations; I managed the Weatherization Assistance Program here for many years. We routinely changed conditions in attics and I can't think of any that could have been made 'unvented'. This needs to be part of the initial design, hence my initial statement about never sealing attics.

Besides, it goes against my nature to include non-living space within the thermal envelope (R806.4.1 quoted above) since that would then have to be considered in the heat-load as 'unintentionally heated space'. It does make some sense (to me) if the HVAC is located in this space, and I've been advocating perimeter insulation/air sealing in crawlspaces in such instances (and waterline protection from freezing is a factor), but attics behave far differently than crawlspaces. And are usually a much larger volume.

Anyway, none of this is (probably) relevant to the OP.

Thank you very much for the all the information in the many replys and for any further information and recommendations you may provide.

It's an old house built in stages during the early 1900s - definitely not a modern unvented green design. I think our location here in Seattle is zone 4 Marine or 5.

The house has survived all these years without attic venting. Associated with a long-ago 4 foot wide addition to the house, there are a couple of dead-space dormers - not open to the main attic and without windows.

In the process of getting bids to tear off the three layers of old roofing, add plywood sheathing, and install composition roofing I find myself planning to add permeable insulation (about R30?) on top of the existing 3" of loose rockwool (no vapor barrier). I'm thinking with the added insulation I might create a vapor condensation problem in the attic if I don't add venting. I'm also thinking adding a coat of vapor barrier paint on ceilings would be a good idea to reduce the amount of the moisture making it from the living space into the attic.

For exhaust venting I'm planning to go with the $125 option for ridge venting instead of mushroom vents on the suggestion that it's a more effective venting system.

For intake venting a $600 option for THE EDGE ventilation product has been offered. I'm wondering if eave vents replacing rafter blocking would be more effective, less expensive, and minimize the risk of leaks caused by roof penetrations.

I'm wondering if its important to make sure the dead-space dormers are vented.

I'll look into the suggestion that I was drilling into the double plate at the top of the wall and will find air space if I drill higher. Now that I've acquired some breathing masks, I can venture a bit further into the lung-unfriendly attic and maybe get some pictures.

Thanks again for all the help.

Originally Posted by John Kogel

Your drill probably is hitting the double plate, top of the wall. The rafters are notched down over the plate in older construction. Drill the upper half of the 1X and you should hit air.
...

Thanks John. As you suggested, the drill was hitting the double plate at the top of the wall. I was able to confirm from outside the attic by inspection with a flashlight and by probing with a piece of wire.

Originally Posted by Don Parda

Thank you very much for the all the information in
It's an old house built in stages during the early 1900s

My kind of house!

The house has survived all these years without attic venting.

And you are about to change everything, so it is good you are thinking this through. You have probably had a lot of 'unintentional ventilation'.

In the process of getting bids to tear off the three layers of old roofing, add plywood sheathing, and install composition roofing I find myself planning to add permeable insulation (about R30?) on top of the existing 3" of loose rockwool (no vapor barrier).

I prefer cellulose when adding over rockwool since it pretty much eliminates gaps. Just make sure that you don't bury any old wiring and all splices are in junction boxes. THIS IS CRITICAL!!!!

I'm thinking with the added insulation I might create a vapor condensation problem in the attic if I don't add venting.

Quite possibly since the attic will be colder since there will be less heat through the ceiling.

I'm also thinking adding a coat of vapor barrier paint on ceilings would be a good idea to reduce the amount of the moisture making it from the living space into the attic.

Probably wouldn't hurt, but most moisture, in my experience, enters the attic through direct air infiltration. Look for open wall cavities (balloon framing), drop closets, ill-fitting hatches, and the like. Get a blower door test if you can.

For exhaust venting I'm planning to go with the $125 option for ridge venting instead of mushroom vents on the suggestion that it's a more effective venting system.

For intake venting a $600 option for THE EDGE ventilation product has been offered. I'm wondering if eave vents replacing rafter blocking would be more effective, less expensive, and minimize the risk of leaks caused by roof penetrations.

Low/high (soffit/ridge) is optimum. I also don't like 'mushroom' vents.

I'm wondering if its important to make sure the dead-space dormers are vented.

Should be, but I'm not sure what you are referring to exactly. If it is the space behind a kneewall, don't forget to address the floor joists as well.

Good luck!

Originally Posted by Don Parda

Thank you very much for the all the information in the many replys and for any further information and recommendations you may provide.

It's an old house built in stages during the early 1900s - definitely not a modern unvented green design. I think our location here in Seattle is zone 4 Marine or 5.

The house has survived all these years without attic venting. Associated with a long-ago 4 foot wide addition to the house, there are a couple of dead-space dormers - not open to the main attic and without windows.

In the process of getting bids to tear off the three layers of old roofing, add plywood sheathing, and install composition roofing I find myself planning to add permeable insulation (about R30?) on top of the existing 3" of loose rockwool (no vapor barrier). I'm thinking with the added insulation I might create a vapor condensation problem in the attic if I don't add venting. I'm also thinking adding a coat of vapor barrier paint on ceilings would be a good idea to reduce the amount of the moisture making it from the living space into the attic.

For exhaust venting I'm planning to go with the $125 option for ridge venting instead of mushroom vents on the suggestion that it's a more effective venting system.

For intake venting a $600 option for THE EDGE ventilation product has been offered. I'm wondering if eave vents replacing rafter blocking would be more effective, less expensive, and minimize the risk of leaks caused by roof penetrations.

I'm wondering if its important to make sure the dead-space dormers are vented.

I'll look into the suggestion that I was drilling into the double plate at the top of the wall and will find air space if I drill higher. Now that I've acquired some breathing masks, I can venture a bit further into the lung-unfriendly attic and maybe get some pictures.

Thanks again for all the help.

After seeing the video of the THE EDGE vent product, I am convinced it would out perform and be cheaper (if your labor was counted) in the long run than soffit vents.

There is another standard being used as well, primarily in the western region. I know the RESNET HERS rating method is migrating eastward. The link is Home Energy Efficiency | RESNET