Vapor barrier when insulating tongue and groove shop

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Forum topic by cmorss posted 11-07-2018 03:31 PM 1640 views 0 times favorited 8 replies Add to Favorites Watch
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2 posts in 844 days

11-07-2018 03:31 PM

Topic tags/keywords: insulation workshop

I’m insulating my garage that has tongue and groove exterior siding and pretty much nothing else. I need to add some additional studs (the existing studs are about 4 feet apart), insulation and then tongue and groove siding on the inside. I can’t make any modifications to the exterior of the existing tongue and siding.

I’m wondering what I should do about a vapor barrier before I put in insulation. Tyvek before adding studs? After or nothing?

I haven’t nailed down the insulation to use either, but cost is an issue. Any pointers there would be appreciated as well.

Thanks for the help!


8 replies so far

View clin's profile


1128 posts in 2004 days

#1 posted 11-07-2018 03:50 PM

Best practices vary dependening on where you live. Big difference between the dry southwest vs wet and fridgid northeast. A vapor barrier may or may not be best. Barriers can be just as likely to trap moisture as to keep it out. Most of the time it is a trade off and best is what is best for the majority of the time in your climate. So I’d ask a local expert.

My guess as to what would be best would be spray foam. Sounds like you will have uneven and non standard stud spacing. So fiberglass batts will be harder to install properly. The spray foam will also seal everything inside and out. Leaves no place for moisture to build up and rot something. No place for insects to take up residence. However, spray foam is one of the more expensive forms of insulation and wouldn’t seem to fit with the fact that cost is an issue.

-- Clin

View jonah's profile


2136 posts in 4306 days

#2 posted 11-07-2018 04:53 PM

In some climates, the vapor barrier belongs on the inside of the insulation (colder ones, mostly), and some, the outside. It varies based on where you live. I’d ask a local pro if you can.

View cmorss's profile


2 posts in 844 days

#3 posted 11-07-2018 04:59 PM

Thanks for the replies. I live in Seattle so moisture is a real concern. I had considered spray foam and may have to bite the bullet and just use it, but it’s so darn expensive!

View LesB's profile


2860 posts in 4451 days

#4 posted 11-07-2018 05:27 PM

Another alternative is rigid foam insulation. It comes in 4×8 sheets from 1 to 3 inches thick that are easy to cut to size and shape. Some have metallic reflective covering on one side to help deflect the heat back in or in desert areas back out. If you run into odd shapes or loose fittings you can infill with the hand held can of spray foam that is often used around door jams and windows. There is also a tape to use around the edges of the rigid foam to seal small gaps.
I don’t have a cost comparison for you but it is a do-it-yourself type product.

-- Les B, Oregon

View therealSteveN's profile


7226 posts in 1582 days

#5 posted 11-07-2018 05:58 PM

I think foam, either rigid panel, or spray will be your best bet for insulation. If you mentioned the fact that it has T&G siding, I’m impressed you have concerns of it not being tight, so it must have some voids, and the foam will go a long way to fill them from infiltration of air, and dampness. As to a vapor barrier, Tyvek is an exterior housewrap, so if the siding is on, it’s too late. Plus it only decreases air infiltration, and slows water, both can penetrate it, as it is permeable. Plastic wrap vapor barrier can be used inside to keep interior moisture from batt, or blown insulation, not as big a feature on foam.

Another thought if the current wall is similar to a pole barn with uprights every 4’ as you mention is to just build a 2×4 wall inside the outer wall. You could sheath it with CDX exterior ply, you could insulate with the cheaper batts, and use drywall for interior walls, which is your lowest cost wall. The outer shell would be a weather break, and the inner wall one to easily run electric, and add insulation. I’m in Ohio, we have 4 seasons, and in most areas pretty high humidity fluctuating year round, so this type arrangement is frequently used to allow for conditioned air indoors, also year round. Not as expensive as you may think.

-- Think safe, be safe

View jonah's profile


2136 posts in 4306 days

#6 posted 11-07-2018 06:58 PM

Closed cell foam would be my choice if you can come up with the money. It is insulation, air sealing, and vapor barrier all in one. It’s also more efficient than open cell foam, though it costs more. All things being equal, it’s the best insulation you can get, and it’ll fill every void and crack.

View johnstoneb's profile


3167 posts in 3180 days

#7 posted 11-07-2018 07:04 PM

vapor barrier goes to the heated side. This stops the moisture from migrating into the insulation and exterior wall condensing on the cold surface and causing mold and rot. you definitely need a vapor barrier where you live.

-- Bruce, Boise, ID

View jerkylips's profile


495 posts in 3578 days

#8 posted 11-07-2018 07:22 PM

spray in cellulose is a really good product & much cheaper than spray foam. If you do the wet spray version, the cellulose has an adhesive mixed with it, and water is added when it’s sprayed to activate the adhesive. It sticks to itself and prevents settling – the big issue with the “old” cellulose. High R value per inch, since it’s sprayed in it is good for uneven voids.

Also, if you’re using t&g on the inside, it probably would be considered its own vapor barrier. You can get lost in the details of insulation build ups, vapor barriers to the warm side/cold side, etc. A minor variance, like bare drywall vs drywall painted with latex paint will affect the vapor permeability.

If it were me, in a garage space, I’d definitely err on the side of having some air infiltration and not have to worry about mold issues.

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