Reference that approves unvented soffit without insulation
Sullyson3
| Posted in GBA Pro Help on
I’m currently arguing with the designer of my house about closed cell insulation in unvented soffits. I have read on this website that unvented soffits with unvented attics do not need to be filled with insulation, but I have not seen this in building code. House is in CA and unvented soffit is required for fire protection. I am looking for a convincing reference that I can send to designer which states that insulation is not required in unvented soffit. Please advise.
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Sorry, my question must have been poorly worded… I am asking about insulation in the soffit under the eaves. My plans call for an unvented attic with closed cell foam insulation. I understand the need for that. My question is in regard to the soffit which is also unvented and also calls for closed cell foam insulation. I don’t understand why the soffit needs insulation given that it is thermally separated from the house. I have seen details on this website where the soffit cavity is empty with an insulated unvented attic. True?
Sullyson,
As is often the case - the misunderstanding was mine.
Like you and FrankD I don't see any practical or code reason to insulate the soffits.
When this has come up here in the past, neither insulation or venting of soffits not connected to the roof system were seen as necessary by Martin.
If you are talking about the roof overhang, I have only ever heard of that being insulated in cold regions to reduce the risk of ice damming. The building code doesn't state explicitly that overhangs don't need to be insulated, but neither does it say they do.
Your designer may be under the impression that any empty roof space must be vented. However, IRC R806.1 only refers to "enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters". An attic is defined as the space between a ceiling and a roof. So, venting is only required for enclosed spaces above a ceiling. An overhang is obviously not above a ceiling and therefore is not required to be vented.
Thank you Frank… that is exactly what I am asking. The designer is worried about moisture accumulation. I can cite 806.1 and I agree with your argument. I don’t know if it will convince him, but I’ll give it a try.
Ask them how much insulation is required on a woodshed roof
Maybe ask why they are concerned about moisture accumulation. Do they perceive a weakness in the air or vapor control layers? Is their concern based on actual problems or is it just theoretical?
He says that any void will accumulate moisture and thus the void in the soffit needs to be filled with foam. He says this is the code.
Well, if he knows it's code then he should be a able to give you a code reference. I'm not aware of any, and when the building code doesn't address something, you can do it however you please.
I agree with frankd's interpretation but I've lost that argument multiple times with plancheckers in California. If it's an enclosed rafter then they want it vented or insulated... Even in wui areas.
I've gotten that feedback from designers and builders too that they get all hinky about the enclosed spaces without venting and other 'interstitial' voids.
I just think it's thoroughly considered but I generally go with the flow and use it as a teachable moment
There is no point in insulating the soffit since there is no "conditioned space" to insulate against -- the top (roof) AND bottom (soffit) are exposed to outdoor, unconditioned, air. Just thinking about this logically should avoid any issues with design people.
Rergarding post #2, I suppose some insulation might help with ice damming if the soffit is trapping some warm air from the exterior of the home. This would be similar to the need for the soffit vents to be out at the far edge of the soffit as much as possible, so that they bring in cold air. This wouldn't be a concern in warmer climates though, and even in cold climates, I think it's of limited benefit.
What you want to do is make sure you have insulation over the top plates of the exterior walls. There is no point in insulating the roof out past the top plate into the soffit area. The easiest thing to do is to put a baffle against the exterior of the top plate running up to the roof deck to keep insulation out of the soffit as it's installed. If you use something like 1/2" polyiso for this, you get the added bonus of a bit of insulation over the exterior of the top plate too, to cut down a little on thermal bridging. I see no need to do anything further than this regarding insulation in a soffit though.
Bill
Yes, that is the scenario Lstiburek described, in particular dark cladding in combination with large overhangs.
Thanks… I agree, but I can’t get my designer to agree. I will attempt to get the local building department to agree instead of the designer, but they have already dismissed the question once.
You have an enclosed void space in your overhang. This void space can indeed accumulate a small amount of moisture. The better solution than filling it with foam would be to ensure that either the soffit or the roof deck has the necessary permeability to allow this moisture to evaporate.
A parallel situation is a roof parapet over 12 inches high. Typically these are given vent holes to permit evaporation.
I think this is what your designer is thinking of.
If your soffit is simply enclosed with plywood or something similar there should be plenty of permeability there. If you are using something more fire resistant such as 3-coat cement stucco then that needs to be understood. Such a stucco assembly on a wall with the usual 2 layers of WRB and a layer of plywood may well be approaching a low-perm condition. But you don't need to do all that for a soffit. Some California jurisdictions are fine with a plywood soffit that's enclosed. If that doesn't satisfy yours you could look into using something like a cement board, Hardie Panel, Fermacell or the like.
Here's a link to Marin County's information page on roof overhangs for wildfire zones. This material and what I mentioned above may be enough to satisfy the qualms of your designer.
https://firesafemarin.org/harden-your-home/fire-resistant-soffits-eaves/
You can even do a traditional 3-coat stucco soffit by using 1 layer of backing paper, a heavy duty wire lath capable of spanning between the rafters, and your 3 coats of stucco. You could cut down to 2 coats of stucco to reduce weight. It used to be applied like this a lot back in the first half of the 20th century. A 5/8" layer of stucco with a vapor permeable paint would be fine for permeability while giving you plenty of fire resistance as well. The key is to use a heavy duty wire lath, not just the light stuff normally used on walls.
Thanks for the info. The design calls for a fly-ash fascia board and a 1/4” cement fiber soffit. Although the space is not vented, I wasn’t planning to seal the edges with caulk, so that could add to permeability. I will need to keep cracks small to keep embers out, but it won’t be air tight.
I'm sure a 1/4" cement board is permeable. Best to check with the manufacturer though. I know the Hardie Panels are fine. And consider the paint. A water based latex wouldn't make much difference. An oil based product would reduce permeability.
In a pinch you could also do a vent strip. There are metal vents designed to keep embers out. These are approved by Marin County. https://www.brandguardvents.com/vent-products/soffit-and-undereave-vents/
Thanks again… I believe the cement board, acrylic paint, and cracks will be permeable enough to keep moisture from building up. I don’t think the vents are needed given the attic is unvented… just need to convince the county.
I'm always curious as to where the moisture is coming from in these situations. For parapets, I can fathom plenty of sources. For the enclosed eaves... It seems more of a stretch, but colorable to claim that interior moisture could find its way into there.
Outside of that I think of roof leaks, and venting will be insufficient to accommodate that amount of water in a reasonable amount of time.
That said, when in doubt, poke holes in it.
That what I think about all my theories!!