What insulation/vapor system will work on my remodel?
I am doing an addition on to my house and am looking for an efficient system that will create both an energy efficient and breathable home. I live two hours south of Chicago in zone 5.
I am putting a 20x 28 addition onto my existing wood-frame cottage (28×32). The existing structure has a cathedral ceiling (4/12 pitch) with collar ties approximately 5 foot from the ridge. I have not opened it up yet, but assume the rafters are only 2×6. ( I base this assumption on other lousy design features that I have come across during this remodel).
Given the ceiling needs more insulation, how is this best achieved with a cathedral ceiling and shallow cavities for insulation? Closed cell is an expensive option but how do I maintain an airspace between the sheathing and the spray foam? i thought of using a rigid foam in the cavities, allowing an air gap (how much?), and then furring out the exposed rafters enough to add some batt insulation, then drywall.
I have read enough to know I don’t want to make a sandwich with the foam. My question is, how do I get the biggest insulation value in the cathedral ceiling while still allowing for the system to breathe? I will have 2 foot overhangs with continuous soffit vents and a ridge vent..
Next problem. The existing basement is a walkout with block walls. I currently have a moisture problem in the basement from a cracked block wall and poor exterior drainage. I will be tearing out and redoing the cracked block wall and will be addressing drainage issues with a 4-inch tile around the footing, a sticky membrane on the new block, and waffle grid plastic over the wall from grade to the tile at the footing.
I will be regrading to increase water flow away from the house. I will be painting the new block with a UGL type sealer. The basement walls will be furred with 1×3 and drywalled with the moisture/mold resistant type drywall.
Question regarding the existing basement: Should I spray a closed cell foam over the block or use a rigid foam? I have read pros and cons regarding vapor barriers/retarders, but am planning on forgoing any vapor barrier between either the closed cell or rigid foam. Would you recommend something else?
Now for the new addition. The basement has poured concrete in ICF blocks giving me 10 inches of concrete sandwiched between 2inch foam. The walls on the main floor will be 2x4studs and from the outside in will be James Hardie cement lap siding , Tyvek housewrap, 1/2 inch rigid foam, 5/8 plywood sheathing, unfaced batts of R-13 insulation, 1/2 inch drywall, paint.
Again I have read conflicting information on moisture barriers/retarders, but have gathered that I should forgo any interior moisture barrier including kraft faced insulation. Is this an appropriate wall with regards to breathability? If I used the closed cell foam and sprayed the wall cavities could I forgo the 1/2 inch exterior foam?
The attic in the addition will be unfaced batt insulation to the recommended R-value for our area. I am also installing a whole house fan that will vent into the attic in the addition. Any recommendations would be greatly appreciated.
Jon
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Replies
Jon,
You stress "breatheability", but don't clarify either what you mean by that or what your concern is.
If you mean you want vapor-open assemblies that can dry in either direction as seasons and weather conditions change, then you need to avoid foam of any kind and use hygroscopic materials. If what you're aiming for is wall and ceiling/roof assemblies that can dry in one direction, then you have more options but those options become more vulnerable to less-than perfect installation and aging.
It sounds as if you've done your homework. Roof venting makes a building more durable. A minimum 1" air gap is required and that can be accomplished by installing blocks of rigid foil-faced polyiso foamed in place (XPS would also work but the foil facing will create a radiant barrier against summertime heat gain). Leave an additional gap below those blocks and install continuous foil-faced polyiso under the rafters, taped at seams and foamed at edges, to create a thermal break and air barrier. With 4" installed between the rafters and 2" below, you can meet the IECC standard of R-38 for zone 5.
For the block wall, you're doing all the right things for moisture control. I would not spray foam on the block, but rather install XPS foam inside of the strapping, leaving an air gap to allow evaporation and diffusion of moisture. 2" would meet the IECC R-10 foundation standard.
For the new construction, why 2x4 studs? Current practice and current insulation standards require 2x6 walls unless you're using lots of exterior foam (R-20 minimum for walls). ½" of exterior foam is nearly useless unless its purpose is as a WRB/air barrier only.
The IECC considers a wall condensation protected (class III vapor retarder allowed) if it's 2x4 with R-5 exterior foam or 2x6 with R-7.5. I would recommend 2x6 walls with either dense-pack cellulose or cotton (blue jean) batts and 2" of XPS, either exterior or interior. My preference for a cold-climate wall is to use foam board on the interior as an air/vapor barrier and thermal break, but many builders prefer exterior foam. For a more breatheable wall assembly, you could forgo the foam and use interior cross-hatched 2x3s for more wall depth and dense-pack cellulose (which is superior to fiberglass in every way).
I would also recommend loose-fill cellulose in the attic rather than fiberglass batts. Better insulation value, fire resistance, acoustic dampening, insect and rodent resistance, and far better hygric redistribution to protect framing (as well as much lower ecological footprint).
Why the whole-house fan? A near-perfect air barrier at the ceiling is essential for an efficient and durable house. What you need is whole house ventilation, either exhaust-only or HRV to control indoor humidity and maintain a healthy indoor environment. But don't break the ceiling plane.
Jon,
1. Your cathedral ceiling doesn't have to breathe, but it is wise to ventilate between the top of the insulation and the roof sheathing. Install sticks in the corners of your rafter bays to create air channels -- 1.5 inches is good -- and then install stiff sheet goods in each rafter bay to make site-built ventilation channels. Your plan to fur down your rafters for more depth will work. It's fine to install foil-faced polyiso under your rafters, secured to the rafters with strapping screwed through the foam. Then you can install dense-packed cellulose through the rigid foam. Patch the holes and install drywall.
2. If you are excavating the exterior of your foundation down to the footing, the best way to insulate the foundation wall is from the exterior. I suggest you install 4 inches of XPS on the exterior. Protect the above-grade portion of the insulation with a durable barrier; there are several proprietary products for this application, or you can install stucco over wire lath secured through the foam to the foundation.
3. The plan to insulate the walls of your addition is poor. Fiberglass batts are the worst available insulation. I would go with thicker foam -- 1.5 inches polyiso or 2 inches of XPS -- over 2x6 studs. Insulate between the studs with dense-packed cellulose. Don't use any interior poly; vapor-retarder paint is all you need.
4. Your plan to insulate your attic is also weak. Again, fiberglass batts are the worst available option. Cellulose insulation probably costs about the same as fiberglass batts -- especially if your cellulose contractor is already there to do the walls -- and performs far better. If possible, exceed code minimum requirements for attic R-value.