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I am removing the original barely-still-there insulation in my 1950 attic (S. Cal.) and then sealing and then placing R 30 fiberglass batting. A problem I have is that about 5%-10% of the attic is covered with plywood nailed to the joists.

I have been advised by a professional to not lift up the plywood for fear that tension between the joists and plywood would be released potentially cracking the drywall ceiling below it. To take a stab at insulating the plywood areas I'm thinking of adhering panels of rigid foam to the top of the plywood. It would probably be only R-7. Is it worth it and am I asking for any problems like condensation between the layers?

Assuming any insulation under the plywood is the same as the exposed areas it is so thin it almost just paper. The smallest plywood area is about 3' x 4'. And I do not think condensation is a big issue here.

I have thought about pushing blown insulation under the plywood by hand and also sliding in 14" wide slices of rigid foam.

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    How big are the sections that are covered with plywood? Can you pull the old insulation out, then push new batt insulation or rigid foam board under the plywood?
    – SteveSh
    Commented Aug 3, 2020 at 22:17
  • Also, I'm guessing that since this is So Calif, that humidity is not an issue. Is that a good assumption?
    – SteveSh
    Commented Aug 3, 2020 at 22:18
  • Do you not want to walk on the plywood? I don't think you're supposed to walk on the rigid foam.
    – JACK
    Commented Aug 3, 2020 at 22:28
  • You can walk on rigid foam if it has a thin layer of plywood or hardboard over the top. 1/4" would do. George, do you plan to retain that storage space?
    – isherwood
    Commented Aug 4, 2020 at 15:17
  • I'm going use about 1/3 for storage and it other parts make it easier to go from one end of the attic to another.
    – George White
    Commented Aug 4, 2020 at 15:26

3 Answers 3

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Condensation should not be a problem in SoCal. IS there insulation under the plywood now? If so, but is has compressed or something, or if not, you could drill holes in it and use blown-in insulation under there.If you put the foam on top, you eliminate the option of walking on it, in which case you may have well as removed it.

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  • How big would the holes need to be? And is it feasible to set up for blown in insulation for the 100 -200 sq feet?
    – George White
    Commented Aug 3, 2020 at 23:36
  • Most of the time involved with blowing insulation is driving to the store to get the blower. It's easy. As I said in my answer, though, I'd be doing the whole thing. Doesn't make sense to do two types of insulation if you will have a blower on hand. The hose is probably 4" in diameter or less, so a 6x6 hole would do.
    – isherwood
    Commented Aug 4, 2020 at 15:23
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For any changes or construction concerning the hull of buildings there is a site providing an easy tool for calculating thermal losses, dew points and more.

Each layer can be edited and switched on/off to instantly see any changes. But it is mainly based on European materials and standards. It is free for private use. www.ubakus.de

Fastest way is to choose an adequate example (menu) and to edit the layers.

Switching to R value can be done by clicking on the tool icon next to the U value output.

Disclaimer: I do not have any association with that web site, I do not benefit/profit in any way from my recommendation/mentioning of that site. It is just by far the most professional and sophisticated and easy-to-use- site to calculate and visualize the relevant insulation parameters that I have found up to now.

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  • It's hard to believe your disclaimer when your answer doesn't address the actual question.
    – isherwood
    Commented Aug 4, 2020 at 15:10
  • As in many cases, the given facts in this question are not sufficient to give a qualified answer. In those cases a site which helps to enter all relevant layers may help much more then an answer based on assumptions. I do confirm again that I do not benefit directly or indirectly or in any way from mentioning the site ubakus.de.
    – xeeka
    Commented Aug 4, 2020 at 15:21
  • I'm not sure a person can confirm his or her integrity with a personal statement. :P I'll give you the benefit of the doubt, though ,and not flag it.
    – isherwood
    Commented Aug 4, 2020 at 15:22
  • This answer does address my question in that it gives me a tool to help answer the "Is it worth it" part of my question.
    – George White
    Commented Aug 4, 2020 at 15:28
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I'd be blowing cellulose insulation in the entire attic. Batts are a pain to move and fit well. Then you could just fill the voids completely by poking the hose under, cutting small holes where necessary, and fill over the top. It would probably take half the time, including picking up the blower (which is often free to rent), cost less, and you'd have as more consistent insulation blanket.

Otherwise, just cut out larger portions of the plywood with a circular saw set to depth. You could leave strips or a frame to tie the ceiling joists together (though I don't have the same concerns as your "professional"--that sounds like a liability thing to me).

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