Interior strapping and rigid foam
I am climate zone 5A (pittsburgh, pa) and have a 1970s house with aluminum siding, buffalo board (a foil faced fiberboard), 2×4 studs, I will be removing all the drywall and replacing the batt insulation as part of a remodel. I want to do two things at this time. increase the insulation and possibly reduce outside sound transmission. What I am thinking is r15 batt insulation in the bays, 6 mil vapor barrier?, and then add 1″ horizontal strapping and 1″ rigid foam between the strapping. That should get me up to r21 (up from a current r7!!!)
in my zone will I run into condensation issues inside the wall? Will the vapor barrier cause problems? can I do anything more for sound?
scott
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Replies
Don't use a full vapor barrier like polyethylene. Use a smart vapor retarder like Certainteed's MemBrain, which is much safer.
The cheapest upgrade you could do for sound would be to replace any old batts (probably fiberglass) with mineral wool, then use 5/8" drywall instead of the 1/2" you probably have now. A double layer of 5/8" would help even more with sound. Add green glue between the two layers for a little better sound deadening. After those steps, you get into fancy stuff like resilient channel which changes how you have to hang the drywall.
Adding some rigid foam won't hurt, and will probably help a little with sound buy I don't know how much since I've never seen a study and I've never tried it myself. The horizontal strapping MIGHT help a little by slightly decoupling the drywall from the studs, but probably not as much as resilient channel would since the strapping is still a rigid coupling. From the R6 value you imply here, I'm guessing you're planning on 1" of polyiso? Keep in mind that typical "1 inch" lumber (1x3, 1x4) is actually only 3/4" thick, so you'd be limited to 3/4" rigid foam here (a bit over R4 if using polyiso). I'd use a kraft or fiberglass faced polyiso here so that it's a bit vapor open. I would NOT use foil faced polyiso here. You could also use EPS, but you'd have lower R value in the space you have available if you go that route.
If I was doing this as an expieriment, I'd screw the drywall in the middle of the strapping between the studs, and not try to hit the studs through the drywall and the strapping. The reason is to maximize any decoupling effect you get from the strapping to help you minimize sound transmission as much as possible.
Bill
> I want to do two things
I strongly suggest you make it three things, with "air sealing" being the most important.
Scott,
You will get a bigger bang for for your buck by installing a continuous layer of foam and fastening the strapping though it.
If you do go with your original plan, remember that 1" nominal dimension lumber is really only 3/4" actual depth.
if I used craft faced fiberglass would I need to worry about multiple vapor barriers (the craft and the foam)?
I agree with the "three things"....so should I put a smart vapor retarder beneath the foam and make that be my air seal or should I tape and spray foam the rigid foam board to air seal at that point?
I will look into continuous foam and then strapping over it to see if my interior room will get to small, everything is a trade off.
If you are only using 1" thick foam you could also just forgo the strapping and fasten the drywall and trim right through the continuous foam.
You may find this article useful: https://www.greenbuildingadvisor.com/article/walls-with-interior-rigid-foam
I am not a member so was only able to read the lead in of that article. But, I get the idea of what you are saying, my first question is do you get more nail pops with nothing behind the drywall for support except foam?
The answer is a bit squishy on this.
You get more pops if you try to attach a shelving unit or something that really pulls the drywall tight over the foam. Part of my ceiling is directly over 2.5" foam and the only time I got any pops is when I went to attach some built in storage.
It would be good to put some horizontal strapping where you would need to hold weight.
Screws are much less likely to pop than nails, almost no one hangs drywall with nails anymore.
I have in the past used wood discs made with a holesaw as “strapping dots”. They act to provide compressive strength for fasteners to avoid squishing. I was hanging pegboard, which tends to flex when loaded so this was important. These things are a HUGE PAIN to put up, but they do work. I’d recommend not doing this unless you really need to (off something like shelving as Akos suggested), but it is an option.
Bill
Scott,
Here is the relevant excerpt from the article:
Is horizontal strapping necessary?
Many builders have installed gypsum drywall directly over rigid foam that is 1 or 2 inches thick. The drywall is secured with long screws that extend through the foam to the studs.
This method works, although it may result in more screw pops (small defects in the drywall compound that need to be repaired) than drywall that is attached directly to studs or strapping. Moreover, many drywall contractors hate installing drywall over rigid foam. The foam can be a little bit squishy — not enough to cause major problems, but enough to make contractors grumpy — and it’s harder to hit the studs when using 3 1/2-inch screws than when using 1 5/8-inch screws.
If you want to keep your drywall contractor happy, you may want to install horizontal 1×3 or 1×4 strapping (usually installed 16 inches on center) on the interior side of your rigid foam. This makes your wall thicker, of course, so you’ll lose some interior area. However, if you specified foil-faced foam, the 3/4-inch-deep air space will give you a small R-value boost (about R-2) — and that fact softens the blow of the lost interior area.