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I have a vented attic space with poor insulation in the attic floor and rafters, so I'm going to button it up.

My energy auditor suggested I insulate the floor as follows: cut blocks of rigid foam for the ends of the joist bays, and tack them in place with spray foam. Then, fill the joist bays with loose cellulose, and top that with rigid foam (ISO) seam-sealed with spray foam. This should create the air and vapor barrier between the conditioned and unconditioned spaces. Finally, since I'd want to use it as storage space, I'd put plywood or OSB decking over the top.

The rafters will be fiberglass bats (there are some in place that were improperly installed, but in good shape, so I want to reuse them) with the foil side facing towards the roof about 1" off the sheathing. I'll then staple FSK across the rafters with the foil facing in.

I think I'm on board with everything, but I'm wondering about the decking and the FSK. Since the rigid will act as the vapor and air barrier between the conditioned and unconditioned space, do I have to worry about condensation forming on the decking? Is it really worth spending money on the FSK if the attic space is vented? I do have the ability to open and close the gable-end vents (the only active vents in the space).

This house is in Seattle, built in 1925.

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    You normally insulate the attic floor (vented cold roof) or the rafters (unvented hot roof). Not both. If you're fully insulating the attic floor, there's not much to gain (if anything) insulating the rafters.
    – DA01
    Oct 29, 2014 at 3:02
  • Thanks @DA01 That was my thought, too. It's a vented attic, so I didn't follow why I'd be insulating the rafters - other than perhaps to minimize the attic becoming a sauna in the summer and more comfortable digging out Christmas ornaments in the winter!
    – the_meter413
    Oct 29, 2014 at 4:00

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I am sorry but this sounds like horrible advice. Rigid or spray foam should not be used in large attics unless you just have money to burn. For your $ fiberglass blown in insulation is perfect for your installation plus plywood. (Fiberglass over cellulose since insects may love the coziness of the "wood boxes").

Also in large attics you ONLY insulate near the top floor ceiling. Insulating the roofline is useless.

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    I agree, this isn't good advice. It makes no sense to insulate the rafters unless you want to use this as a finished/conditioned space. Typically attics are not a good place for storage. They have extreme temperatures, and insects and rodents hang out there. In my home, I had existing cellulose under the floorboards and that was it. The insulation had shifted and in some places there wasn't anything there at all. I got some more cellulose and packed it in where it was low and then got rolls of R-48 insulation and covered the entire floor like a blanket. It has worked great so far.
    – Jason Hutchinson
    Oct 29, 2014 at 13:01
  • I agree that insulating the floor and the rafters is not warranted. But I still have a question about filling an attic with a blanket of loose cellulose or fiberglass. If there's no air barrier on the vented side of the insulation, heat is still going to move through it when air moves through the vented space. Insulation only works when there's no draft. I think this is why my auditor proposed rigid foam over cellulose-filled joist bays: the rigid would provide the air barrier as well as providing additional R-value. His method seems sound, but I agree the cost is high: $800 for 700 sqft.
    – the_meter413
    Oct 29, 2014 at 21:21
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What is your budget for the entire project and does it near the quote from a company willing to spray-foam the rafters?

Once the rafters are completely sealed with foam, the ceiling joists become less of an issue. Shove all the old insulation at hand into the floor cavities. Deck over it ASAP, so the workers don't damage your drywall by falling into a cavity. The gable vents could be sealed and the attic can now be considered (somewhat) conditioned space.

Insulation does not go in backwards. The vapor barrier goes on the heated side. You should remove the face if you're going to fill behind with it.

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  • Budget is "whatever I can do myself for not a lot of money." I didn't get bids for any work since I can do this myself. If I need roughly 28 sheets of 2" polyiso, I'm at $900ish ($33/sheet). Add to that the cellulose and decking, and I'm at about $1500 for the job, give or take.
    – the_meter413
    Oct 29, 2014 at 4:02
  • @the_meter413 Yea, no where near the cost. Check that link for some good info.
    – Mazura
    Oct 29, 2014 at 4:43
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I did some work in my cousins home, one project was improving his insulation in his attic. He was already using it for storage and only had the 1960's version of insulation in place. (R-8)

The plan included making a 8X12 storage platform and catwalk to his gable vents and HVAC unit. Using a number of 2X8s I crossed them over the existing joists edgewise at 24" on center, the same way I would run the R-30 insulation that I added in the next step. This is what I attached the 1/2" plywood to. The 2X8s kept the deck from compressing the insulation. I also attached the 2X8s with screws, because the hammering would otherwise knock loose the drywall on the ceiling, may be plaster in your case.

23" X 40' +/- unfaced rolls were brought in to do the job, since the existing had foil facing on it. I set them on one end and sent them on their way to the other end unrolling as they went, using a 1X4 to adjust them in place. With the rolls run the way they were, there is a complete blanket of insulation over the entire floor of the attic, only interrupted by the 2X8s in the area of storage.

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