Do generous overhangs need venting or insulation?
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Building a 1900 sf house, Berkshires, ma – zone 6. We have an unvented roof assembly – flat roof 2 x12s. Interior to be spray foam, but what about the exterior and carport area? I have attached a some perspective elevations for reference
Lukas Schwartz, Great Barrington MA
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Replies
Lukas,
There is no need to install ventilation or insulation in soffits or carport roofs if your roof assembly is unvented.
Just make sure that you know where your home's air barrier is, and make sure that there aren't any leaks.
Martin,
Should I Insulate 2' roof Overhang for a Vented Roof(24" Heel Truss with R-60 Blown-in Cellulose)? If Insulated then how do i handle the Air Ventilation gap between the Brick and the Sheathing?
Thanks
Naveen
Whether the overhangs are insulated or not the attic insulation needs to extend out to at least the wall sheathing, fully covering the top plate, which is usually how it works with energy heel trusses.
In all cases it's generally better to vent the brick veneer cavity to the outdoors rather than into the roof/attic venting. In my area weep holes in the vertical mortar at the bottom or second course of bricks every third brick, with corresponding vents in the vertical mortar in the top or next-to-top course of brick is fairly common.
@Martin
"There is no need to install ventilation or insulation in soffits or carport roofs if your roof assembly is unvented."
According to Joe Lstiburek, that generalization I beleive is quite wrong. time stamp 14:28 https://www.youtube.com/watch?v=Ld8pzIu45F8&t=262s
Maybe in the case of this persons specific climate and roof that might be correct, but in a high snow load area it sounds like that's not the right answer.
Maybe Joe is wrong? Maybe I'm missing something..
Note: Like all of MA, Berkshire County/Great Barrington is in US DOE climate zone 5, not zone 6.
Thanks for the information- I had been told by a gardener that we are in a microclimate of zone 6 because of higher elevation- not that it would make a huge difference. Would y’all agree the best way to stop interior air into these soffit areas is with a warm side air barrier, even though it will be sprayed?
Lucas,
I've found climate zones to a good starting point too, but they do contain within them a huge variation in climatic conditions. The difference between a fully-treed lot and a cleared one here on Vancouver Island makes a huge difference to the performance of a building and the materials. I can also often watch the thermometer on my truck drop a good 5C as I drive from Victoria along the coast to my home.
Lucas,
Q. "Would y’all agree the best way to stop interior air into these soffit areas is with a warm side air barrier, even though it will be sprayed?"
A. I'm not sure what you mean by a "warm side air barrier," nor am I sure what you mean by "it will be sprayed." What is "it"? What will "it" be sprayed with?
To stop air leaks, here's what you need to do:
1. Identify the main air barrier for the walls. Usually, this is the exterior sheathing, although in some cases it may be the drywall.
2. Identify the main air barrier for the ceiling. Usually, this is the drywall, but in some houses it may be another material.
3. Draw the detail where the ceiling intersects the walls to make sure that you have a continuous air barrier without any leaks or gaps.
Sorry for my brevity- ceiling and walls to be filled with spray foam. I won't be able to use typar and tape on the exterior to address the air barrier where wall and ceiling meet. Just wondering if it's possible to rely on the spray foam for this or if we might need an air barrier before drywall on the interior. My concern with using drywall as an air barrier is all the electric receptacles, which might be easier to seal well with a membrane. The plans call for about 80 recessed LED lights in the ceiling so really my fondest hope is that the spray foam will take care of the air barrier.
New thought: bring typar up walls, across soffits and up the fascia to integrate with the tpo membrane on the flat roof, keeping the air barrier on the exterior. Is this a solid solution?
USDA plant hardiness climate zone numbers bear no relationship with DOE climate zones. The code requirements for buildings are all based on the latter.
All spray foam (including open cell foam) meets the building science definitions of "air barrier", including open cell foam. Open cell foam will need an interior side vapor retarder to be protective of the roof in DOE climate zone 5A, closed cell would not. For unvented flat roofs it's easier/cheaper/better to us rigid foam on the exterior of the roof deck for at least 40% of the ~R50-ish code minimum, since that is protective of the roof deck without needing an interior side vapor retarder. By dispensing with the interior side vapor retarder the roof deck will have more than 3x the capacity to dry (toward the interior, through standard latex ceiling paint), and by having the thermal break of continuous insulation over the rafters thermal performance is higher.
A code-min assembly could be 4" of rigid polyisocyanurate above the roof deck and 6.5-7" of half-pound open cell foam on the underside of the roof deck. In a 11.25" deep 2x12 rafter bay that would leave more than 3" of depth to accomodate electrical boxes and light fixtures. The insulation would be cheaper (and higher performance) than 7" of HFO blown closed cell foam on the under side of the roof deck (or 8" of cheaper but more environmentally damaging HFC blown closed cell foam.)
Another possibility is to use 6-7" of polyiso above the roof deck and no spray foam in the rafter bays, meeting code min on a U-factor basis, but that may be less architecturally appealing.
Typar is not an air barrier unless meticulously detailed as an air barrier. If the siding isn't back-ventilated the interior side of the walls would also need an interior side vapor barrier, and be air tight. Half-pound foam would be fine as the air barrier, but using a "vapor barrier latex" primer paint on the wallboard would also be on order. A 2-mil nylon "smart" vapor retarder (eg Certainteed Membrain) under the wallboard would also work, and would give the assembly better drying capacity than vapor barrier latex primer.
I assume you've already assessed just how much snow load that flat roof is will need to accommodate in your location/climate? (Flat roofs can be a real liability snowier parts of MA, not that they can't be done well.)
Great information Dana. Thanks. Exterior foam is out of the question, aside from the tapered foam tpo substrate. Given the perametars you have laid out, sounds like the best solution is to have open cell under the roof with a vapor retarder, or closed cell for at least 41% of the total r-value.
I'm outside Boston. I also have overhang that's not vented nor foamed/insulated. However, I am getting some serious condensation under the overhang. Water is condensing both inside the structure (dripping down the beams in the overhang area), as well as on the soffit (looks like water got sprayed on there). I know it is not supposed to condensate like that, but mine does! How should I go about fixing it?
A bit more detail:
-exterior walls are foamed with closed-cell insulation
-roof over heated area are foamed with closed-cell insulation
-roof overhang not foamed
-roof overhang not vented
-white roof (! - is this causing the condensation?)
-condensation appears to be coming from moisture in the air - I see condensation on the bottom soffit surface of the garage overhang
I thought I'd chime in given similarities to the original poster's house. I seek advice of the group, in the hopes of solving my problem as well as helping the original poster avoid my issues.
Alex,
A full analysis of your problem would require a site visit. But it's worth pointing out that there have been several reports of condensation on roof sheathing after white membrane roofing has been installed. White membrane roofing gets cold at night due to night sky radiation -- and it never warms up on sunny days, unlike black membrane roofing. That sets up a little condensation factory.
For a full explanation of the phenomenon, see Night Sky Radiation.
Thanks Martin. Yes, I do agree that the white membrane roof is likely the key factor here.
I plan to have a contractor take the soffit down this summer and take a look behind it. I think the most likely answer is to spray foam under the overhang, just like the heated portion of the house. This should bring the sheathing above dewpoint and hopefully stop condensation.
Lukas - please make sure you don't run into the same issue.
not totally convinced that insulating solves this issue. We have around 1500sq ft of soffit and overhang area = a substantial cost to insulate! also - we definitely want to get this right. Thanks for the feedback
To follow Martin's advise of 4"or more of rigid above the roof - have your engineer check to see if the roof framing can be reduced to 2x8 or 2x6 with a reduced spacing , also check if engineered lumber can give the thinner profile to add the exterior insulation. It seems the 2x12's were just a conventional framing guess at the roof needs with out iteration from the insulation needs or engineering analysis input.