Attic Underfloor Insulation
My 1950s colonial in Rhode Island has a full attic with a plank floor over the joists. It is unconditioned. It has cellulose insulation under the planks between the joists. I would like to increase the R value so I thought the easiest way forward would be to lay down some rigid insulation then place 1/4 or 3/8″ plywood directly over it. We use if for storage only, and while people do walk up there it is only occassionally.
I was thinking either some sort of polyiso (which I know has enough compression resistence) or a Rockwool product which I find more appealing.
Rockwool allegedly makes a product called Rockfloor designed for this purpose, but it does not seem to be readily available.
Would Rockwool Comfortboard 110 be an appropriate alternative?
Also wondering if I could just use some R12 Zip Sheathing.
I am certainly open other insulation suggestions
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Loose cellulose is so cheap, and so easy to do in attics, that you might find it better to try and raise the floor and pour in more cellulose. To add R12: take up the planks, put 2x4's on edge on top of the joists, fill with cellulose, put the planks back on top.
A possible issue with foam board: generally in an unconditioned attic you want the insulation to be able to dry toward the outside so that any moisture that gets in can escape. Foam could trap moisture and cause problems
Agreed. Also, if there's an air gap between the boards and the existing insulation, there could be air flow bypassing the new insulation.
Interesting idea I hadn't considered. More labor for sure (is 24x40') prying up those boards, but way less in materials. I take your point about the drying to the outside. Would it be desirable to place a vapor permeable membrane on top of the cellulose to prevent air infiltration?
Any attic work, the most important part is to air seal the existing ceiling. If you are pulling up the boards, moving the insulation out of the way it is pretty easy to access the ceiling bellow. You want to seal up any electrical boxes and around any wall top plates. One of the smaller 2 part spray foam kits work well for this, just make sure to vacuum up the dust in the area.
Once the ceiling is air sealed, you can install lumber as suggest above. You can also go with 2x4 or 2x6 on edge for more R value. Cellulose is the cheapest, for a 1000sqft of attic, batts also don't cost all that much more.
If your house is already pretty well air sealed, you can also keep the existing boards (provided the insulation is fully filling the cavity bellow) and install the 2x on edge above the existing flooring, insulate and add a new subfloor.
Would it be okay to
Pull up boards
As suggested attach sideways 2x4 to joists
Fill with cellulose
Lay vapor permeable membrane across top
Reinstall boards
Are there membranes that allow sufficient vapor passage yet block air infiltration?
Thank you all so much for help with this.
Air sealing should happen at the ceiling, it is very hard to air seal at the attic floor as you now have joists and rafters poking through your air barrier. Focus on the ceiling and keep the attic floor as open as possible. To allow for drying, you do need a fairly permeable attic floor, so only lumber or plywood, no OSB.
You get higher assembly R value by running the 2x at 90deg to your existing joists. This avoids most of the thermal bridging of the lumber but would be hard to re-install the existing floor boards.
Make sure you are not blocking any of the soffit intakes with the new insulation and flooring. The attic space needs to stay vented to the outside.
Have you had an energy audit (National Grid - free). RISE will do a lot of insulation and air sealing for next to nothing. Work with them to develop a reasonable plan.
Another thought, is your electrical system elements in the attic ok. Not a good time to think about adding a light or switch after the wiring is buried in a foot of blowing in insulation.