Storing stuff in unconditioned attics isn’t a great idea. Typically attic storage requires access to the unconditioned space via a big hole – usually attic stairs – which are hard to make air-tight or insulate well. Things that are stored in unconditioned attics don’t fare very well. Attics have really big temperature and humidity swings, which is bad for almost everything except insulation. Nevertheless, attic storage is a sometimes necessary evil.
One key design principle for well-insulated buildings is to decouple the insulation from the framing cavity. Therefore, attic joists must be covered with insulation. Using this principle, we’ve tinkered with a few attic storage systems.
System 1
One system we’ve used several times is raise the storage area above the existing joists by running plywood joists perpendicular to the ceiling joists and filling the cavities with insulation. Advantages:
- Low environmental impact of using exclusively cellulose insulation,
- All materials are readily available to anyone running a cellulose crew (plywood, cellulose),
- Possibly low expense if labor costs can be kept down (good access, not too many cuts or craziness),
- Fairly robust.
Disadvantages:
- Laborious. Yeah, holy crap laborious,
- Still has thermally bridging at the plywood joists,
- Cellulose (and all blown insulation) has a lower R value than foam and therefore requires higher platforms to achieve high R value.
System 2
A second system we’ve used a lot is to laminate multiple sheets of foam with plywood on top. We usually use 2 or 3 layers of 2″ polyisocanurate with 1/2″ plywood. Advantages:
- Efficiency. Probably a lot faster than joists,
- Almost no thermal bridging.
Disadvantages:
- Using foam has a higher environmental penalty (but polyiso is the best of the foams).
- High material cost.
System 3
The final system is very similar to the 2nd, but requires purchasing sheets of plywood with polyisocanurate laminted to one side, like these from Atlas Roofing. Interestingly, the bonded sheets can be purchased for about the same as buying equivalent thickness polyiso and plywood from a lumberyard, which is sort of nice. It seems likely that this system would be even faster than the 2nd, depending on access.
Stuff that I often wonder about:
