Seattle is in US Dept of Energy Zone 4 (California is all Zone 3), where the recommended R-values are included here:
| Area | DoE recommendation | Target | Notes |
|---|---|---|---|
| 2 x 4 Walls | R-13 to R-15 | R-13 | |
| 2 x 6 Walls | R-19 to R-21 | R-19 | |
| Attic | R-38 to R-60 | R-49 | |
| Low-slope Roofs | R-30 to R-38 | R-38 | |
| Wood Floors | R-25 to R-30 | R-25 |
The target values reflect the observation that this house is sheltered from temperature extremes. Heat loss governs, which places emphasis on the roof, less on the walls or floor. The target of R-49 comes from the Clean Crawls bidder who casually mentioned that goal as typical in this area.
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| Helpful label: "4R per 1 inch" |
Fiberglass batts are cheapest and bulkiest. They yield about R-3 per inch. So, to achieve R-49, we need a 17" thick blanket above the ceiling.
EPS polystyrene foam board, 2" thick, is $0.68/SF in 4x8 sheets from Home Depot. That gives R-7.7, which can augment batts. No one seems to pile on more than 2" of foam board. They have R-max Thermasheath (PolyIso), R-13, for $1/SF.
Closed-cell spray foam is considered best, because it also seals against air flow, but it is the most expensive. Corey Binford recommends "poor man's spray foam"--you fill each void with a 2" foam board, then seal around it with Great Stuff spray foam. He uses Dow Tuff-R, which gets R-12 from a 2" foam board. (Note that that still doesn't achieve the target R-13.)
Low-slope Roofs
One problem I notice right away is that thick insulation kills the architectural lines of a simple flat roof. MCM roofs with T&G decking on beams are replaced by architecture wearing adult diapers.
Roofs that have no attics should not be ventilated to the outside, but must be sealed against airflow through the ceiling. If the roof is cold, moisture in the warm air from the living space will condense in the plenum, rotting wood and breeding mold. They recommend track lights instead of cans, special drywall paint, and careful attention to sealing the wall-to-ceiling joint.
Closed-cell spray foam works best in this situation, but it is more expensive. It can achieve R-7 per inch, but we still need that R-12, 2" foam board and 3.7" of spray foam (or 5.5" of spray foam) to get R-38. Because it is so expensive, hybrid methods have been developed. 1" of closed cell spray is used to seal against airflow, then the voids are filled with cheaper open-cell spray foam or fiberglas batts (the latter is called "flash and batt"). It's all about the airflow, but 1" of spray is R-7, so you need 9"-11" of something else--that's a thick roof. Throw money at the problem, and you still need 5.5" of insulation--the 2x6s need to be 2x8s.
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| SIP project in Park City, UT |
Existing Attics
The main house has about 5" of rock wool insulation between the 2x6 ceiling joists. There is a fair amount of ventilation toward the peak, but none in the soffits. For R-49, we want a pile of about 16" of fiberglass--11" above the top of the ceiling joists. That thickness will taper at the edges--there will be a 36" border with less than 16".
The issue with the attics is ventilation. We need 1 SF of ventilation for every 300 SF of attic.
For the main house (1230 SF attic), we need 4.1 SF (590 SQ IN) of vents--divided between the soffit and the peak. The rafters are 2x4 @ 24", so we can imagine 2" x 22" soffit vents. We need (7) of these on the north, east, and south (west is the Trees Room). Then we need 295 SQ IN near the peak. Perhaps we leave the gable vent on the south; perhaps we add a 14" turbine.
Over the garage (658 SF including garage, 253 SF without it), we need 316 SQ IN of vents--(4) 2x22 soffit vents and 158 SQ IN near the peak.
Walls
Existing walls have 2" of balsam wool insulation, about an R-6. We could blow in cellulose for a dense pack R-12 that also helps fire resistance, but that's not cheap.
New walls can use fiberglass batts: R-13 for 2x4, R-19 for 2x6. Or, we can use the poor man's foam board and Great Stuff. In the 2x6 walls, we can add R-13 fiberglass batts and achieve R-25.
Wood Floors
Assuming that we use hydronic Pex tubing under the floor sheathing, with a metal heat reflector, we'll want to fill the 2x8 space with batts--we can achieve R-26.
Concrete Slab
Put foam board between the ground and the new slab; put foam board around the perimeter of the slab, protected with cement board. Consider doing this around the existing basement, as well.
Update 3/26/15: SIPs might be too expensive (you don't see them much). Instead, they make shallow, flat trusses, like this:
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| It helps to spy on local construction projects |
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