Sorry – More Poly-Iso Questions
Thinking of using Poly-Iso Foil Foam on BOTH sides of 2×4 stud walls with F’Glass in wall cavities. Marine Zone 4 (Seattle). Spray Foam probably the ultimate but its not DIY friendly and too expensive. Will this work? Regular 1/2″ Drywall over 1″ or 1/2″ Foil Foam, 2×4 studs with Unfaced F’Glass Batts, 1″ Foil Foam Exterior, Tyvek, & Finally LP Smartside 4×9 panel siding.
Reasons for: 1/2″ or 1″ foam on inside is easy to put under drywall using adjustable electric boxes. 1″ foam on outside is good w/o making trim details too much PITA. Reasons against: “Dreaded double vapor barrier”. Won’t the foam on both sides keep the interior from gettiing cold enough to cause condensation? Should I forget the interior foam or do you think its OK in Zone 4?
Is there some other product that could be used on interior? Only thing I can think of is “sound board” which I think is made from cane fiber but it doesn’t do much for sound OR insulation in my experience but would be very “breathable”.
Electric BB heat is just so expensive. Wanting to try to insulate better since replacing all the old single pane windows. Old cellulose (ground up newspaper?) worked great but again its not something that can be DIY.
thanks for opinions/advice.
Replies
Foam is pretty forgiving allowing a lot of options while solving condensation problems - I wouldn't hesitate using foam on both sides of fiberglass. With normal construction (short of sealing the foam with caulking around every stud bay) there is quite a bit of air movement between the subfloor and attic through the stud bays so it's not really as sealed up as you'd think.
Having said that I would personally go with a thicker layer of foam on just one side - often installed horizontal with horizontal nailers every 2' is used if the foam is to the inside - but whatever you want to do is probably ok - I just know it's easier to finish out a wall if your not fighting foam to find nailers and I"d rather deal with interior foam than exterior.
As for electrical boxes I'd install normal boxes on plywood scraps no taller than the height of the box and screwed to the studs to bring them out the 2" or whatever the interior foam is.
What about using EPS foam instead of polyiso? It actually does breath, and may not give hime the double VB
You can get
polyiso with felt facing instead of foil, and its a lot more permeable... I just put a bunch of it on interior walls... but I would just use 2" on the exterior instead of making both sides of the wall a PITA.
Why do both sides ... I'd only do one ... likely the exterior. I colleague of mine would skip the exterior sheathing in favor of 1/2" polyiso ... no changes in details and he adds a thermal break to the studs. Shear is via 1x4 inset or the simpson metal which is evern easier to apply.
Using only polyiso on exterior walls -- no sheathing????
In my recent research about this, the poly iso is never intended for structural support. Ommitting sheathing from the exterior is weakening the wall structure. Instead of a true box structural cavity, the poly iso is to malleable to really create an integral box that will reinforce the wall. Much additional internal bracing would be required to even come near the lost strenght of the missing sheathing. Something about wind loads. The bigger the exposed surface, and higher the gust possibility, well I guess over time, it would weaken, and in a high wind episode, might rack the wall.
Comments by a "hobbyist" -- Polyiso or any foam insulation should be applied over the normal sheathing to preserve the full structural integrity.
This is what I understand. Have I learned right???
oldtom67