Hello,
I am building a new house, well….. sub contracting it out myself, in upstate NY. The framers are just applying the shingles and was considering using the Dow or Owens Corning pink “Foamular”insulation on the exterior of the wall sheating and doing a flash and batt system on the interior stud cavities with 1 -1.5” sprayed polyurethane and fiber glass batts. I would like to end up with a high R-Value wall system. My question is “How will this type of wall system operate, and do I need a vapor barrier on the inside of the wall?” The location of the house is at the top of a hill and very exposed to wind, so I don’t want to skimp on the opportunity to do it right the first time since there aint no redo on insulation. If anyone has any information on this type of wall sysytem, please pass it on, Thank you.
Replies
Are you "doing it right" or "doing it cheap?" Batt insulation is cheap but arguably not right if you are looking for a high R wall assembly. At least the foam will do an air sealing of the cavity. However, you also need to use something like an acoustic sealant (eg. Tremco) to seal between framing members (like between bottom plate & sill) to stop infiltration through places that the foam won't seal.
As for vapor retarder, if you have 1 to 1.5" of closed cell foam on the inside of the sheathing for air sealing and 1" Foamular (XPS) outside, that arguably constitutes a vapor retarder. Then you don't want inside of the foam any more than half of the total R provided by the foam on out. That way you don't have a vapor retarder more than 1/3 the way from warm to cold, and the temperature drop from warm to vapor retarder won't be more than 1/3 the total temperature drop to outside. That will keep the VR location above the dew point, and no VR at the drywall is necessary. If you have more R than that inside the VR, then you would need a second VR at the drywall (even latex paint will do), and the drywall edges must be sealed to prevent convective air movement into the cavity from inside. Double VRs generally are not a good thing, because moisture can be trapped inside the two.
If the sprayed foam is open cell, then is is permeable, and you would need the VR to be on the inside (MemBrain or latex paint).
What thickness "Foamular" is on the outside? That is XPS, and of limited permeability by itself. If more than 1" then I have to wonder if you would be better off just spraying all closed cell foam inside. Expensive, though. Total R (assuming 2x6 framing @16"oc) would be 6x5.5=33, degraded to about 24 by thermal bridging, plus R5 for 1" Foamular outside and maybe another 1 for sheating, siding, and drywall, to bring total real R to 30. If the air sealing of framing joints has been done also, then you have an excellent wall.
But with such a tight house you MUST have a HRV to provide fresh air, or else you'll have either stale air or energy efficiency compromised by having to open windows.
Hey, shouldn't all of this have been worked out before the first nail was fired?
My thoughts. Others will provide theirs.
Thank you very much for your reply, I am astounded by your level of knowledge on the subject! There is currently no Foamular on the outside yet. I have been overwhelmed by the amount of conflicting information on what to do to create a quality wall system. I have read every article in the archives, and online that I could find. From what I can tell there are a number of ways to insulate a house using one or a combination of materials. FG Batts, blown in wet/dry cellulose, exterior foam over sheathing, thin layer of closed cell interior with batts, full stud cavity of closed cell foam sprayed 5", hay, newspaper, soda bottles, shredded tires. Each has pro's and cons.... Arrrrgh, and of course every insulation contractor I talk to has the best system for the cost.
My question to the forum should have been. "If you were building two houses side by side, one with a budget and the other with out a budget, what High R Value wall system would you design, and why? Keep in mind that budget is certainly an issue in my case, however, I'll be living with it for a long time so it better work well. Yes, this should have been figured out before the first nail was fired, but .....
Thank you
Go for a complete spray foam job... it makes for a lot of confusing and scientific reading if you want to study the subject, but I'd say Corbond is the right choice.
It should be mentioned that sealing cracks is as important as choosing the right insulation. Even if an air leak isn't big enough to make a substantial difference in heat loss, it can cause cold spots in the home that greatly impact comfort level.
builder253
You're too late,, the best system would have been either SIP ofr ICF construction.
dramatically lower energy costs with either system.
At this stage spray foam is your next best choice..
your absolute worst choice would be fiberglas..
fiberglas is a lousy air barrier.. that's why they make furnace filters from it.. In addition when insulation is tested it's tested at 70 degrees.. things change as the temp differance increases.. there is what is called the wash effect.. cold air on the outside wall sinks while the warm air on the inside rises. each bay is a mini little wind chamber warm air rising, cold air falling etc.. scrubbing the heat you've paid for outside..
Lots of research on the subject if you care to look.
in addition fiberglas and celluliose when wet lose R value dramatically, it doesn't take much moisture for it to drop to near zero rating..
It's not a perfect world.. sooner or later moisture will enter a stud bay. It may be an improperly sealed moisture barrier, heck maybe an electricain needed to get in or a plumber or whatever.. maybe it's a careless sheetrock screw or a poorly sealed window or doorway. whatever!
Evan blower door testing or thermal immagining won't always detect the problem. But there it is, a bay where the R value amounts to zero.
Spray foam isn't affected by moisture nor does it allow air to inflitrate.. In addition it will strengthen the house, make it far more quiet and rigid. It's really the third best way to insulate a house while celluliose and fiberglas are the fourth and 5th.
" fiberglas is a lousy air barrier.. that's why they make furnace filters from it.."They also make things like boats and RV's out of it.And and there is not wind blowing through the side of my boat..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Billhartmann,
this will shock you so sit down.. when they use fiberglas to make boats and RV's they add something called resin. Think of it as glue.. then they crunch that fiberglas into the glue.
Since they don't use resin in the same fashion when making insulation air flows thru it..
You may still doubt me but if you grab a fibgerglass batt you can blow right through it.. That should prove to your satisfaction that what I am saying is true..
Now I don't know where you can link to a fiberglass batt or any links that demonstrate what I'm saying but the test isn't really hard.. I'm sure if you ask them the people at your nearest store that sells fiberglass batts will allow you to blow thru the batts to see if air flows easily.
Try it and let me know if it works..
> this will shock you so sit down.. when they use fiberglas to make boats and RV's they add something called resin. Think of it as glue.. then they crunch that fiberglas into the glue.Yeah, now try that with styrofoam and see what happens.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
DanH
I can't blow thru styrofoam.. coffe cups made with styrofoam won't allow moisture out and if held underwater will trap air in.
At some point water pressure would most likely infiltrate strofoam but at what depth and over what period of time I don't know.. I do know I've found styrofoam water coolers that have been held under water for years and the water stain only went into the foam slightly..
DarnI did not set down before I read this and fell into my keyboard.You owe me a new keyboard.Just as fiberglass used in boat is different product from fiberglass insulation, fiberglass furace filters are a different product from fiberglass insulation..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
BillHartmann,
You're kidding right? You weren't aware that filters are differant from insulation? Hopefully you're just yanking my chain. Or are you confused and think they use the same resin impregnated fiberglas to insulate houses with that they use to build boats.. (and Corvettes etc).
Yep, Martha! those walls contain a Corvette, two boats,and a Canoe!
Actually while the porocess of making fiberglas is similar in all cases the final treatment is radically differant..
Just so you aren't confused any further fiberglas batts used for insulation allow air to flow thru them freely..