1 1/2″ Foam Board Insulation with foil face in walls
I am building a shop with 2″ x 8″ x 16′ tall walls , 2′ on center in Big Lake, Alaska.
Just finished with framing and all dried up. Just starting with electrical in attic. From there I would like to put a vapor barrier or a vapor retarder ( don’t know which one to use or thickness ). I am planning on 20″ of cellulose in attic with 5/8″ sheetrock on lid.
From there I would like to insulate walls with 1 1/2″ foam board with foil face on one side on the inside on each bay ( foil facing inward). I have 2″ x 4″ s on the flat every 4′ in each bay at all ends of sheeting for nailers.
My question is there anything I need to do special after placing foamboard against sheeting ( like sealing). The reason I would like to do this is so I can turn on heat after blowing I insulation in attic and work in a semi insulated building thru the winter while finishing up wiring , plumbing and mezzanine and staircase. After which I would like to use rock wool to finish the cavity out. Then I am confused on which vapor barrier or vapor retarder to use before wall sheetrock goes up?
I can not place foam on outside as this is attached to a existing shop that has a finished exterior of vinyl siding which will continue on to the new part.
Any advice to help me with this build would be appreciated, Joel
The foundation has 2″ of foam on outside below grade and below slab
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Replies
You want your final wall to have a vapor barrier on the interior side and everything towards the exterior to be considerably more vapor permeable. You also want an air barrier on the interior side and preferably another towards the exterior.
There is no way to make a reliable-for-life air seal of the foil facers to the framing using cut'n' cobbled foam- there will (eventually, if not on day 1 ) be breaches in your vapor barrier putting the exterior sheathing at risk.
But a continuous layer of foil faced polyiso on the interior can be air sealed at the seams with purpose made tapes, and even 1/2 - 3/4" of continuous polyiso would outperform a full cavity fill of polyiso due to the R5-ish thermal break it puts over the framing fraction.
If you're installing 1.2" foam just as fill between the 2x4s UN-faced Type VIII (1.25lbs per cubic foot) or Type II (1.5lbs) would outperform most foil faced polyiso at your mid-winter average outdoor temperatures in that stackup, but at ~2 perms (give or take) would not become a major impediment to drying toward the exterior- it won't create a moisture trap with the (absolutely necessary) interior side vapor retarder/vapor barrier.
With 2x8 framing the remaining 5.75" of depth is 1/4" deeper than an R23 rock wool batt, but with some careful installation it should be possible to tease it out enough to make it a compression fit for continuous interior side polyiso or wallboard.
Thanks for all your replies!
This is what it would look like but here in Alaska we are limited on what we can get, so that was my reasoning for the use of the 1 1/2" foam board.
What do you think on this application
If there is no continuous interior moisture source (ie people, cooking, green wood), the interior dew point will be the same as the exterior, you can pretty much do what you want without moisture issues. North of me, which is zone 6, it is very common to have insulated workshops with just fiberglass batts in the walls.
Overall, I think the cut and cobble there is not buying you anything and adding an exterior vapour barrier. For houses, you generally don't want exterior VB in cold climate, but this is a shop so less of an issue. I'm with the posters above. Install batts in the walls than the 1.5" insulation on the interior with the seams taped. This will get you a much higher R value.
Your current assembly (assuming 2x8 16 OC, 1.5" rigid with 2x6 batts) is R23. Moving the 1.5" rigid to the interior and going with R28 batts (or two layers of R14) you get an R30 wall without the problematic exterior VB and way less work. You can hang drywall/OSB directly over the rigid with longer drywall screws.
>"What do you think on this application"
It's risky, and as Akos points out, it buys you nothing.
Splitting R13-R15 batts into two 1.75" thick batts and compressing them into the same place as the foam board would be higher performance, and doesn't introduce the problematic exterior side vapor barrier.
If you can lose the foil facers installing EPS that way lowers the risk but it's still better/safer to install any rigid foam as a continuous layer on the interior side and use only fiber insulation in the cavities.
I’ll add that when using 1.5” rigid foam on the interior of the wall, you can run your electrical using 1.5” deep 4” square boxes in holes cut in the foam board. Use mud rings to bring the devices flush with the finished drywall surface. These are common and readily available electrical items that even the box stores should have in stock.
Since you’re in an area where the air seal is very important, I’d cut squares of the foam board bigger than the holes you cut out for the boxes and then glue those squares to the foamboard behind the hole. This insulated and air seals the back of the box so no leaks, and no risk of the steel box conducting heat (or cold in this case I suppose) and causing condensation in the box and on the devices themselves.
You can run the wire in slits cut in the foam and then seal them with a bit of great stuff, or just do a slit with a knife and press fit NM cable into the slit.
Bill
Thanks for everyone's replies to this , guess its not such a good idea!