Does taped exterior insulation over a self adhered wrb further increase air tightness?
MikeonLI
| Posted in General Questions on
I know taped exterior insulation increases r value and prevents thermal bridging. However for air sealing purposes only, does taped exterior insulation over a self adhered wrb make a building more air tight? Or will the effect on air tightness be minimal if the self adhered wrb is applied carefully and throughly?
Thanks!
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Replies
Are you asking if taping (the insulation) helps with air tightness or if taping and insulating helps with air tightness? Yes and yes.
A self adhered wrb makes an air tight house. Does insulation over that further increase air tightness or is the effect minimal since the self adhered wrb already makes it so air tight that the exterior insulation is redundant
I am only a reader on this site, NOT an expert. So someone else please chime in - isn't there a "rule" that there should not be a double air barrier (meaning the air barrier should only ever be one plane)??? If so, why?
gordy,
In almost every situation, the more air-barriers the better. It’s multiple vapour-barriers that can cause problems by restricting the assembly’s ability to dry.
I like to detail exterior rigid foam as an air barrier (by taping seams), and the exterior sheathing (by taping seams AND sealing the perimeter with polyurethan, AND I detail interior drywall airtight, AND I detail the interior smart vapor retarder airtight. It sounds like a lot of work, but it's really not -- only the vapor retarder detailing really adds much in the way of time and effort to the project, because that membrane is a bit fussy to work with.
The idea is that no air barrier is quite perfect, so if you put them all together, each one cuts down on airflow out by increasing the total restriction to airflow through the wall. If you can easily detail a layer air tight, it's usually worthwhile to do so. As Malcolm mentioned, it's multiple VAPOR barriers that can be a problem, multiple AIR barriers are fine, and are usually helpful. Having multiple air barriers is also nice to have in case any one barrier gets damaged at some point in the future (holes for picture hangers in drywall come to mind, if they later get removed and not patched).
Bill
Bill,
Thank you. Because of lack of overhangs and significant cost doing window bucks, I can only do 1/2 exterior insulation in addition to self adhered wrb or self adhered wrb and a rain screen Mat. I chose the self adhered wrb and the rain screen mat bc water is the top priority. All the sheetrock is up so I can’t do an interior air barrier but I can air seal from the outside by removing all the sheathing. As you said in your other comment it would just be foaming the plates and the corners.
It seems some are suggesting it’s too much expense and effort to remove all the sheathing to do it while it seems you think it might be worth doing. Let me know if I am right about what you are suggesting.
Mike,
When the sheathing or a self-adhered WRB is the primary air-barrier, then a secondary one on the interior that restricts air cycling in and out of the all cavities is useful. Other “back-up" ones are more there to add redundancy if the primary one fails. If it is installed diligently, these secondary ones shouln't improve air-sealing appreciably.
Malcolm, thanks so much. Very helpful as usual. I can’t do an interior air barrier at this point bc all the sheetrock is up but when the siding starts I could remove the exterior sheathing and spray foam the framing to the drywall from the exterior. That might give me an airtight drywall approach from the outside and function like an interior air barrier. Do you think that would be beneficial?
Mike,
I don’t think the effort and expense justify it. Adding a self-adhered WRB and exterior insulation under your new cladding are what will give you the big gains.
I can’t do the exterior insulation bc of lack of overhangs. Most I could get is 1/2 inch of insulation and doing that would prevent a rain screen. So I’m choosing the rain screen instead of the exterior insulation. just doing the self adhered wrb and a rain screen Mat. Is the effort and expense still not justified for the interior air sealing with foam?
Mike,
No I still think you should leave the sheathing alone if you can.
Also, depending on how much spray foam you were to use, you could end up making your sheathing a condensing surface that would also be trapped between two vapour barriers being spray foam and rigid foam, just pay attention to air sealing on the exterior.
Sorry didn’t word it correctly. Meant to foam or caulk the corners and perimeter not the entire drywall or anything
There is no need to sprayfoam the drywall. You only need to seal the perimeter interface, the tiny gap where the drywall is against the framing. If this "tiny" gap isn't so tiny, I'll inject some canned foam in, otherwise a bead of polyurethane sealant does a good job. I only bother with the top and bottom plates and extreme outside studs (i.e. at corners), unless any particular stud bay might be a "special case". A "special case" is usually a stud bay with something in it that I expect will leak more than usual like a big penetration. In my own home, I have a furnace flue in one such place that prevents me from putting rigid foam on the back of the studwall (this is in an attic) and sealing that, so I sealed the studs on either side of the "problem" studbay.
Bill
Right bill. That’s what I meant. I didn’t word it correctly but plan to do exactly what you are saying with the foam. However I’m trying to decide on here if that’s worth the expense and time.
It's belt and suspenders. If you are going to do a good job sealing the exterior sheathing/rigid insulation, then you could skip the step to seal the drywall. I would inject canned foam into any large gaps though, such as holes for electrical wiring (I seal those at EVERY stud in exterior walls), and around any pipes or other mechanicals. The drywall perimeter is nice to have sealed, but not absolutely necassary if you already have a good exterior-side air barrier. I would do a quick eyeball check for any "larger than normal" gaps/voids between the drywall and the perimeter framing though, and I'd seal any extra bad spots that I found.
If you use a "pro" gun for canned foam and one of the narrow tips for it, you can make pretty quick work of sealing in the way I described -- maybe 5-10 minutes to do a decent size single story wall. A cordless power caulk gun makes caulking long gaps a lot easier too. I used to think power caulk guns were a joke until I actually tried one...
Bill