Adequate insulation in unventilated attic in 4A?
I have a chance to create an unvented attic in a cape code style house in 4A climate. For various reasons I can’t insulate above the roof sheathing. I can foam the underside of the roof deck. Roof rafters are real 2x6s. I’m planning to use closed cell SPF that would create an approximate roof assembly R-value of 38.
My biggest concern is whether or not approximate R-value of 38 is enough. Definitely worried about moisture on the underside of rafters. I was also planning to cover all sections of exposed foam with 5/8″ drywall.
I could add another 1 inch layer of polyiso to the underside of the rafters if it would help.
Let me know your thoughts.
Thanks in advance.
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Replies
Cliff,
In your climate zone, the 2012 IRC calls for a minimum of R-49 insulation for roofs. However, as you probably know, it usually isn't necessary to meet code when performing insulation retrofit work.
I don't think you have to worry about any moisture problems in your roof sheathing if you install only R-38 of closed-cell spray foam.
If you decide to install a continuous layer of polyiso on the interior side of your rafters, there will be two benefits and one drawback. The polyiso will improve the R-value of the roof assembly and reduce thermal bridging through the rafters. Those are the benefits.
The polyiso will also reduce any inward drying from the edges of the rafters. However, these rafters should stay dry, so I would definitely say that the benefits outweigh the small drawback.
Is the benefit of adding Polyiso board a marginal improvement or worth the effort?
I'll need to spend a substantial amount of time attaching the polyiso and then furring out the surface for drywall attachment
Cliff,
The answer depends on your performance goals, your budget, and your projections for future energy prices. (There is no single answer to your question.)
If you want to take a stab at figuring this out, you'll need to use energy modeling software to determine the annual energy savings from the extra insulation.
Here is a link to an article that discusses this type of calculation: Payback Calculations for Energy-Efficiency Improvements.
If you plan to stay in your house for a long time; if you have ambitious goals for energy reduction or carbon reduction; if your budget is adequate to perform the work; and if you anticipate that energy costs will rise in the future, then the additional insulation makes sense.
If you may be selling your house in a few years; if you have no particular reasons to reduce your carbon footprint to a bare minimum; if your budget is tight; and if you expect that energy prices will stay low, then you probably wouldn't want to invest in the additional insulation.
Using high R/inch foam between rafters to achieve some center-cavity number is usually a waste of good foam due to the huge hit from the thermal bridging. Even at a 7% framing fraction (yours is probably higher if it's full dimension 2x6. or on spacing tighter than 24" o.c.), a full 6" of R6.5/inch foam (R39) delivers a whole-assembly performance of less than R32. If the framing fraction is higher than that, the benefit of the higher R foam shrinks even further.
If you add 2" polyiso edge strips and 8" of much cheaper ~R4/inch 0.7lb density open cell foam (~$4 per square foot instead of ~$6) you would hit about the same overall performance even at a 7% framing fraction. At 8" the vapor retardency of 0.7lb foam runs about 5 perms, still well into the class-III zone, so a "vapor barrier latex" primer on the gypsum would be in order to bring it down to class-II territory. (Half inch gypsum is fine from a fire code point of view.)
Moisture is never a problem on the underside of rafters in a zone 4 climate- the moisture problems (when they occur) are at the roof deck, which experiences mid-winter temperatures below the dew point of conditioned space air, not the more temperature & moisture stable underside of the rafters. Adding R6 or R12 of impermeable rafter edge insulation isn't going to change that enough to matter in a zone 4 climate, which needs only ~30% of the total R to be on the exterior of the condensing surface as dew point control. A full dimension 6" rafter with the most common species runs ~R7 or a bit higher, so even using a dumb 2D model and R13 polyiso that's still 35% of the total. The rafter edges will be fine, even better with open cell, which would give the rafters better drying capacity toward the interior than with a close cell solution.