Vapor barrier on 100 year old home?


Hi, Folks.

I’d like to get your opinion on something.

I am about do what many in the building science world consider a big no-no: insulating the exterior walls of a previous uninsulated, 100 year old home.

The cladding is original cedar clapboard. Sheathing is 1” thick t&g. The interior plaster and lath has been removed due to significant cracking throughout the house. The plaster will be replaced with drywall. Now that the interior of the walls have been striped, the studs and backside of the sheathing are exposed, allowing me to inspect for any signs of water intrusion. There are almost no signs of water coming from the exterior in. No staining, no current moisture. Where there were any gaps/cracks (found exclusively around windows), I have sealed the areas with spray foam.

I am only minorly concerned with potential issues related to water entry from the exterior not being able to dry out once insulation has been added to the cavities. However, due to the inevitability of imperfection, and the simple fact the home does now have an exterior weather barrier/house wrap, I’ve elected to fill the stud cavities with moisture resistant mineral wool batts.

What I’m mostly battling with now is the decision to use a vapor barrier on the interior, just behind the new drywall. Despite the likelihood of vapor permeating through the drywall, into the cavity, and condensing on the backside of the sheathing, I am still hesitant to add a control layer which discourages drying to the interior. My gut says to keep it simple, don’t try to overly seal a building which is not designed to be sealed tight. My brain tells me to use a semi-permeable membrane such as Certain-Teed’s Menbrain product. The reconciliation between my gut and brain tells me to use a vapor-retarding primer on the drywall. Now I’m curious what each of you would tell me.

It should be noted that a small number of the exterior walls were previously insulated with fiberglass batts. In these areas, drywall had been installed over the batts. Apparently one of previous owners did a bit of remodeling. Judging by the branding on the fiberglass batts, the insulation was installed roughly 40 years ago. There was absolutely no moisture damage behind those batts, the studs and sheathing bone dry. I could have simply left the fiberglass batts installed, but I was curious to inspect for damage and see what the internet has been yammering about in terms killing old houses by installing insulation within stud cavities. Anecdotal, for certain, but no issues in this case. Those fiberglass batts will be replaced with R-15 unfaced mineral wool batts, which I intend to use throughout the house.

So, lets hear it. Any suggestions or tips? I’m mostly curious to hear opinions regarding vapor control. As good ol’ Mr. Lstiburek points out, even an unpainted sheet of drywall (no holes, of course) will diffuse only 1/3 quart of water per heating season. Add some latex paint to that, you’re vapor permeability is negligible. Perhaps even vapor-retarding primer is overkill and I’ll have better luck keeping it simple.

Let me know what you think.

Note: I am in climate zone 5. We heat the house with forced hot air.

Dec 8, 21 5:27 pm

I'd say no vapor barrier - not unless you are putting a steam room in somewhere. Sealing the wall from the interior seems counterproductive with the exterior wall you describe

just painted gyp for vapor retarder

Dec 8, 21 6:41 pm  · 

I'd still install a vapor retarder behind the GWB and forgo the special primer. 

Dec 8, 21 6:49 pm  · 
atelier nobody

I wouldn't. I might rethink that foam, too, unless it's vapor permeable. That wall has obviously benefitted from being able to dry to the inside or out for the last 100 years - it it ain't broke, don't fix it.

Fill it with unfaced mineral wool and use regular gyp bd & latex paint on the interior.

Dec 8, 21 7:05 pm  · 
2  · 

you could do Kraft face Batts if you're up north.  Don't do sheet plastic like visqueen unless you're north of the frozen Canadian tundra.  

Dec 8, 21 7:14 pm  · 
Non Sequitur

can't be canadian, the op wrote vapour wrong.

Dec 8, 21 7:40 pm  · 
2  · 

Note that most current building codes (check your local code requirements) generally require a Class I or II vapor retarder on the interior of the wall assembly for Climate Zone 5, unless you have vented cladding or continuous insulation. If you do have vented cladding or continuous insulation, then you can use a Class III vapor retarder (i.e. latex paint). Wood siding is sort of vented but Lstiburek also points out that it "need[s] help" to be considered ventilated.

"My gut says to keep it simple, don’t try to overly seal a building which is not designed to be sealed tight." It also sounds like you've already added a weather barrier ("the home does now have an exterior weather barrier/house wrap"). If you did, you've already sealed the house up. This is reducing the ability for it to dry with air movement. You're also adding insulation which is reducing the ability for it to dry with thermal movement. Best to design the assembly accordingly and control for vapor.

You didn't mention if the fiberglass you found from previous remodels was faced or unfaced (sounds like it might have been faced if there was branding that you could read). Kraft facer on the batts would be a Class II vapor retarder (so would the CertainTeed MemBrain product).

"My brain tells me to use a semi-permeable membrane such as Certain-Teed’s Menbrain [sic] product." Listen to your brain. It sounds like it more closely matches code requirements than your gut.

Dec 8, 21 8:56 pm  · 
1  · 
Wood Guy

I'd definitely use a variable permeance membrane on the interior--it costs a few bucks but is made for just this kind of situation. Certainteed Membrain is ok but it's very flimsy and more vapor-open than I prefer. I spec and use Siga Majrex or Pro Clima Intello. I want to try Rothoblaas' version as well.

Dec 8, 21 9:29 pm  · 
3  · 

This is my pinned tweet. I have no advice for you.

Dec 8, 21 9:30 pm  · 
3  · 

use a wood fibre insulation and wood fibre plastering board - they are fully vapor permeable so will always dry and regulate. use a chalk plaster and chalk based paints inside and you'll have extra protection against mold.

but use pro clima intello if you're gonna keep the buildup youre mentioning above...

Dec 9, 21 4:35 am  · 
1  · 

This is the correct answer. Better even is to not even insulate the exterior and just use a low-cost/ low-energy radiant geothermal system. My 120 year old masonry house is meant to breathe. I use felt weatherstripping, mineral wool insulation around frames and under floors, and true plaster on lathe. The radiators are so hilariously oversized you could keep it at 70+ degrees with the windows open in the winter.

Dec 9, 21 10:59 am  · 

Geothermal heating is not low cost to install and rather site specific for it's feasibility. 

Also - with a mineral fiber cavity insulation and the right vapor retarder wouldn't you'll still have vapor permeable wall assembly that can handle wetting without causing mold problems?

Dec 9, 21 12:13 pm  · 
Wood Guy

Your 120 year old house is meant to "breathe" (i.e., allow water vapor to pass through) but it's not meant to be kept at 70°F and 50% humidity all winter or to have air conditioning keep it cool and dry in the summer. Modern living is detrimental to most old homes. One approach is to not insulate, and use excessive amounts of fossil fuels to force-dry the walls in winter. Another is to use modern building science products made for the situation.

Dec 10, 21 10:40 am  · 
1  · 

@ Chad - sure, but every layer is just more space for water vapor to hang out, even with vapor-permeable products. I think the link WG posted below is a pretty decent summary on it.

Dec 10, 21 10:53 am  · 

Louder for the people in the back Wood Guy: 

Modern living is detrimental to most old homes. One approach is to not insulate, and use excessive amounts of fossil fuels to force-dry the walls in winter. Another is to use modern building science products made for the situation.

Dec 10, 21 12:25 pm  · 
2  · 

all good comments, thx all

[is this our designated professional discussion for the month? we get HSW's for this, right?]

Dec 9, 21 12:53 pm  · 
1  · 

Archinect should become a registered provider and send out certificates to participants.

Dec 9, 21 1:07 pm  · 
Wood Guy

We had Christine Williamson (aka Building Science Fight Club) and Doug Horgan on The BS + Beer Show to talk about vapor retarders--you might find it helpful:

Dec 9, 21 2:58 pm  · 
6  · 

good discussion

Dec 9, 21 8:02 pm  · 
2  · 

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