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My 1938 home currently has zero insulation (aside from a few fiberglass batts haphazardly tossed about). The ceiling joists are 2x4 (spaced 16 inch OC), rather than the more modern standard of 2x6 or 2x8.

This home is in California, with a mild climate even by California standards: temperatures dip below freezing and top triple-digits only a handful of times each year.

My question is: What is the best way to insulate the attic?

Here are my options as I see them:

  1. Put standard fiberglass insulation that's appropriate for the climate (probably R-38) between the joists. Since this insulation will be 12 inches thick, the top will sit above the joists, but will not cover them. Some amount of thermal transfer will still occur through the joists themselves.

  2. Put insulation meant for 2x4 framed walls between the joists (which will be the same height as the joists), then lay a perpendicular layer of attic insulation (maybe R-30) on top of them.

Option 2 has the benefit of preventing any thermal bridging via the ceiling joists. The drawback is that, according to Home Depot, the cost of 2x4 wall insulation is comparable to the thicker, R-38 attic insulation, so I will effectively be doubling my material costs by getting both types of insulation (approximately 700.00 for each layer on my 900 sq. ft. house).

Is it worth it to spend the extra money to prevent thermal bridging via the joists, or will the difference in heating bills be so negligible that I'll never recoup the costs?

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    It kinda depends on the type of venting you have roof vents or soffet vents or gable but blown in is just as good as bats as long as the venting is still good,
    – Ed Beal
    Dec 14 '18 at 2:18
  • Your assertion that modern joists are larger than 2x4 is faulty. Unless the free span is greater than around 30 feet, 2x4 bottom chords are still quite common. Regardless, since most attics are blown, it's irrelevant.
    – isherwood
    Dec 14 '18 at 3:49
  • What are you doing about a vapor barrier? And what amount of insulation is required per latest building codes for your area? Is it really R-38, which seems high to me for a mild climate area?
    – SteveSh
    Jan 14 '20 at 14:00
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Blown in cellulose can be quite effective if all air gaps are located and sealed with expanding foam for this purpose. Look for gaps around light boxes/electrical, fan vents, and those in corners. Unless sealed, you'll lose heating and waste money. If the previous fiberglass batts had dark marks, they generally indicate an air leak. You can lay insulation batts in the directions you mentioned, but I'd go with the most effective insulation at the lowest cost. If your attic has an access panel, insure you've built a "surround" and tightly sealing door, as well.

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I live in a similar climate. I'd go with blow in. Just be sure it's thick enough for R30-36. It'll compress to half the size it is when it's first put in. Option 2 is also a consideration. Just be sure not to put any insulation over any soffet vents whether blown or batts. You should and you want soffet vents. If you don't you should put some in and vent the roof if none as well. You will notice a difference with heating and cooling cost. You may also consider radiant barrier for the triple digit days. Insulation does helps with the heat but if you can keep the heat out of the attic in the first place, then the insulation works even better.

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  • You might clarify specifically how you suggest radiant barrier might be used.
    – isherwood
    Jan 14 '20 at 15:37
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I know this question is a few years old, but another option is to insulate the underside of the roof (and maybe gable ends?) with closed cell spray foam. This is more expensive, but it will convert your attic space into something more comfortable and lessen the importance of air leaks and insulation between the rest of your house and the attic.

I'm in a fairly mild climate, so 3 inches of foam at about R7 per inch was sufficient. Then I put down some plywood on the 2x4 joists (house built in 1906) and have a lot more storage space instead of a small room full of batts or cellulose.

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