I’ll be starting a ranch style home with basement on the Canadian border this fall. It will feature hydronic radiant heat and high performance windows. I’m planning on 2×6 const with fiberglass batt, 1/2in ply sheathing, followed by housewrap, 1″ taped rigid foam and foam backed vinyl siding (Craneboard-with drainage channels built in). Ceilings will get 1in rigid foam under the ceiling drywall as a thermal break, and blown cellulose or fiberglass batt above and between the trusses.
What confuses me is if and where to put a vapor barrier. It seems to me that the taped rigid foam on the outside creates a vapor barrier, and I fear trapping moisture in the wall cavaties if I also put one on the inside walls. Any thoughts? Does the housewrap act as an escape for any wall cavity moisture?
Also, I don’t know if the rigid foam in the ceiling butting against fiberglass batt walls creates issues. Spraying foam into the wall cavities at the ceiling line was one alternative I thought about. I chose putting the wall rigid foam on the outside to get fewer penetrations and a better overall seal. Any suggestions appreciated.
Charlie
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Ive never heard of heated walls.
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keep looking for customers who want to hire YOU.. all the rest are looking for commodities.. are you a commodity ?... if you get sucked into "free estimates" and "soliciting bids"... then you are a commodity... if your operation is set up to compete as a commodity, then have at it..... but be prepared to keep your margins low and your overhead high...."
From the best of TauntonU.
There are many on this forum much more knowledgeable than me on this topic, but for what it's worth. . . foam board is not totally impermeable to water vapor. You have to look at its density and perm rating.
Also different foamboard (e.g. expanded polystyrene, vs extruded polystyrene, vs polyisocyanurate) will have different perm ratings.
The wall systems you're describing do have to be detailed very well to get the maximum theoretical R-value that the engineers say you can get out of them.
If I was considering building where you are I'd consider a Mooney wall type system (many threads on that here on Breaktime) or maybe even better, a spray type urethane foam (Corebond or Icynine) with the Mooney Wall type framing to help minimize thermal bridging from the 2X6's.
As they say "The Devil is in the Details"
Good luck planning your project.
A little off topic but batt fiberglass in the ceiling is by far the last choice when some kind of blown in insulation can be installed.
Vapour barriers always go on the warm side of a wall. Their purpose is to prevent moisture migration through the wall. Walls vary in temperature from warm on one side to cold on the other. If too much moisture migrated, at some point within the wall it would become cold enough to condense, and that would soak the insulation. Vapour barriers are always on the warm side to prevent moisture from moving into the wall.
For this to work, the other side of the wall must let moisture escape faster than moisture can leak through the vapor barrier. Normal plywood and OSB will let moisture out. Housewrap will also, that's one of it's important benefits, it resists letting liquid (rain) water through but lets vapour out.
Some foams will let moisture out, others, not so well. I don't know specifically because I never looked it up, but I think expanded polystyrene (beaded board) is fairly open but other foams are not.
However, if you are putting foam behind the drywall on the ceiling, why not on the walls, too, instead of outside?
I'd recommend upgrading your fiberglass in the walls from R-19 to R-21. It is a denser fiberglass that fits in the same 6" wall. Being denser it resists convection better, too.
For the ceiling cellulose will produce tighter results than blown fiberglass. In Canada you can also get electrical boxes with gasketed fronts that seal against air infiltration.
Another design feature is to use raised heel trusses for your roof. That allows the ceiling insulation to maintain full thickness all the way to the outside of the walls.
One more thing, ICF foundations are wonderful. Also, put 2" of foam under your basement slab whether or not you heat the basement hydronically.