What’s the best way to insulate an attic with 2×4 joists

atticinsulationjoists

My 1938 home currently has zero insulation (aside from a few fiberglass batts haphazardly tossed about). The ceiling joists are 2×4 (spaced 16 inch OC), rather than the more modern standard of 2×6 or 2×8.

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 2×4 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 2×4 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?

Best Answer

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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