How can I maximize the R-value of floor insulation in a walk-up attic?
tjo6
| Posted in General Questions on
old house with walk up, unfinished attic. some old insulation under floor boards. don’t want to lose floor space which is used for storage. taking up floor to air seal around lights, etc. so I am already in the space. can I put rigid foam in between joists before putting floor planks back down. Not much room there for adding cellulose or fiberglass. foam panels have more r-value. attic has vent at ridge and small windows in gable.
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Replies
Tom,
There are three issues:
The first issue is insulating the stairs leading to your attic. Here is an article that explains what you need to do in this area: Insulating Attic Stairs.
The second issue has to do with air sealing the attic floor. This work is required before you can add any insulation. For more information on this topic, see Air Sealing an Attic.
The third issue is the question you asked: "How can I maximize the R-value of floor insulation in a walk-up attic?" You're going to want to install as much cellulose as you think you need on your attic floor. Most cold-climate builders and homeowners stop at about R-60 (about 17 inches of cellulose), but it's your house -- so maximize away.
To make room for additional insulation, I suggest that you temporarily remove the existing floor boards. Then add new 2x6 or 2x8 joists, installed at 90 degrees to the existing joists, 24 inches on center. That will give you more room for insulation. Then you can re-install the old floor boards above the new joists.
Martin: Thanks for the prompt and thorough answer and the helpful references. I had actually thought about raising the level of the floor as you suggest but thought that would be a lot more work than using the panels. I want to do this the best way regardless of the effort but I am left wondering when the panels have higher R value per inch they are not a better way to go. (I have no background in this; I just go by the R value). I was also thinking of addressing an unfinished basement and using panels down there on the ceiling. Is there some reason why these panels are not appropriate for some situations? Are they a hazard or is there something not readily apparent to someone like me about using them?
Tom,
Cellulose is cheaper and much easier to install than rigid foam in this location.
It's quicker easier & cheaper to raise the R-value via the added cross-joist method than using high R/inch foam boards. At a 15% framing fraction thermally bridging the foam most of the additional performance is undercut by the low R1.2/inch of the framing. With 2x10 joists (9.5") and R3.5/inch cellulose you may have an R33-ish center cavity R, but only an R25ish whole-assembly performance after factoring in the thermal bridging. If you made that 9.5" of R6.5/inch polyiso that becomes a whopping R62 center cavity, but the thermal bridging reduces the performance to about R37- barely more than R10 higher performance than the cellulose fill.
But if you add 2x6 (5.5") joists perpendicular to the original joists and fill it up with cellulose, the center-cavity R would be only R52, about R10 lower than the cut'n'cobbled polyiso, but the whole-assembly performs at better than R40, HIGHER than the foam solution, since the lower joists are now all thermally broken with R19-R20 of cellulose, except for the ~2% of the total area where the upper joists rest on the lower joists. The cost of the cellulose + new joists is less than half the cost of the polyiso, and the installation is easier & quicker too.
In the basement it's fine to use sheet foam on the ceiling, but it's usually less foam and more effective to put it on the foundation walls, and DON'T thermally bridge it with framing (either on the walls or the ceiling).
In either application code demands a thermal barrier between the foam and the space (even an unconditioned attic that is used for storage), such as half-inch gypsum or OSB/Plywood.