Foam-Free Unvented Attic
I’d like to transition my attic from vented to unvented without the use of foam. I know some people feel like if a house is built with a vented attic it’s really always a vented attic and if it’s an unvented attic that it’s really always an unvented attic. In Texas Gov. Abbot is putting rewards in place for homeowners that try to change the way their attics are vented…thankfully, I’m in Tennessee.
All joking aside. All the articles/information I have seen on this require tearing off the shingles and building up layers of insulation on the outside or the use of foam on the underside of the roof. I am not crazy about foam and my shingles are in great shape (around 10 yrs old).
I’ll be putting on new siding and soffit soon…is there any way to make it an unvented attic (without, humidity/moisture issues) without using foam?
Thank you
Andrew
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Replies
Andrew,
You have put three conditions - no foam, no exterior insulation, and an un-vented assembly - which together unfortunately preclude any cathedral ceiling I can think of.
So if I did foam I could transition it to unvented or as Walter below suggests conditioned attic?
Andrew,
Yes. The two articles Martin linked to will help, and to choose an appropriate assembly I'd use one of these: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
I'll probably use option 1. Won't I need some sort of barrier like siga on the interior and tyvek on the "exterior" (the air gap side under the roof decking). So the layers would be shingles, underlayment, roof decking, air gap, tyvek, Rockwool insulation, siga. Does that sound right?
Andrew,
You need a warm-side air-barrier. That can be a membrane like Siga, a layer of OSB, or just the drywall detailed correctly.
To create the air gap you can make baffles from Tyvek, foam, OSB or any other rigid material that can stand a bit of moisture.
Andrew,
You will be creating cathedral ceiling assemblies. If these are vented, you can use fiberglass, cellulose, or mineral wool for insulation -- as long as your roof configuration is ventilation-friendly.
More information in these two articles:
"Creating a Conditioned Attic"
"A New Look at Conditioned Attics"
Thank you, I'll take a look.
I have to ask why you want to change your attic.
It seems to me when people call it an “unvented attic” they are looking for a free lunch and have no intention heat and cool the attic and then they act surprised when mold starts growing in the newly unvented attic.
If you can bring yourself to call it a “conditioned attic” it seems more likely you will install the supply and return ductwork needed to keep the attic at more or less the same temp and humidity as the rest of the conditioned spaces.
In my opinion the only reason to condition an attic is if you are trying to mitigate the damage from poor design choices like half stories and or HVAC equipment in the attic. Even in those cases the benefits are marginal at bests.
Walta
I am definitely trying to do it so I won't have mold or moisture problems. Our current house is 1800 +/- sq ft whereas our old house was about 3200 sq ft. I would be beneficial to have storage in the attic....climate controlled storage at that. So you are saying if we did heat/cool it then it's possible?
Sure, it can be done mold free.
Just do not expect a free lunch you will pay to heat and cool every cubic foot of space just like the rest of your conditioned space.
If you take foam off the table understand your insulation will take up twice as much room and reduce your already limited headroom. Read the articles about cathedral ceilings and do not cut any corners understand unvented assemblies are risky.
Is the juice worth the squeeze? 3-4K to remove the old insulation and install the new insulation plus a 30% increase in utility costs for hard to access storage with very little head room so you can keep a bunch of old junk you are unlikely to ever use again.
Walta
I have developed three (3) rules in making the call for a "conditioned/sealed" attic. At least one of them has to be checked for it to make sense to me.
(1) HVAC duct work in the attic (although ducts frequently can be super insulated to reduce duct loads.)
(2) You want premium attic storage.
(3) The floor of the attic is impractical or impossible to air seal.
Absent a good reason, a properly vented attic is the most proven "lid" assembly in homebuilding.
as per my comments to Walter above we went from 3200 sq ft house to about 1800 sq ft and there's minimal storage. So yes, having some climate controlled storage is what this is about.
Alll three are highly desirable in almost all house, more storage, all hvac within conditioned space, and ability to use can light on third floor HIGHLY sought after these days.
See Figure 5 here:
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned
It's essentially the same as rigid foam on top approach just instead using rigid mineral wool.
Which climate zone? In warmer climates and with the addition of a vapor diffusion port at the ridge, you can use dense packed cellulose on the inside in combination with a class 2 smart vapor retarder. Technically would be 'unvented' depending on your definition.
I am in East Tn, just out side of Knoxville. I don't know what zone that it. We have pretty mild winters, plenty of humidity in summer, spring, fall.