I am converting about 900 sq.ft. of attic space to habitable and I have to insulate. What would be the best and most economical solution?

Asked by Mihai Beffa
Cupertino, CA

The roof has typical vent holes on the sides, but when I sheet-rock the habitable area, about half of the roof will not have ventilation. From what I gathered so far, closed cell spray foam is rather expensive. At about $1/bd.ft., to achieve R30 I need about 4.5". The house has about 2,500 sq.ft. footprint, probably about 4,000 sq.ft. of roof area, so I am looking at a whooping $18,000. Way to expensive. The roof is made out of 2x6 joists, but I furred the habitable area with 2x2's. I can think of several ways to cut down the bill. One would be to spray 1" closed cell, then fill up the rest with open cell. Another would be to use batt insulation on the ventilated areas. I live next to San Jose, CA. Weather is quite dry, it rains only a few months in the winter and even when it rains it's still dry. What would be the best and most economical solution?


Lucas Johnson

Answered by Lucas Johnson

Seattle, WA

Cascadia Consulting Group

August 19, 2013

Yes, you are correct that closed cell spray foam can be expensive, but the $18,000 you quoted is out of this world, especially for converting 900 square feet of attic space.

  • The installed cost is typically $2-3 per sf, regardless of thickness applied.
  • When I was running a home performance division for a major green builder in Southern California, we typically quoted at $3/sf and the sf used was the attic floor area.
  • Thus, my estimate is more like $7,500 for 5" of closed-cell spray foam in your attic rafter bays.

That being said, $7,500 is a fair price, certainly expensive, but probably a good value since this would likely be the optimal insulation strategy.

In order to control for air leakage and moisture, you will need at least 1" of closed-cell spray foam applied to the underside of the roof deck (or if you need to re-roof, then you could put foam board insulation on top of the roof deck as well). You could then fill the rest of the rafter bay with R-30+ batt insulation, but with the additional labor required for two insulation styles, this less-than-optimal solution may end up costing almost the same amount.

I would recommend working with a BPI Accredited Contractor, or other building science/home performance certified contractor, to develop bids for:

  • 5-6" of closed-cell spray foam at the underside of the roof deck (given the rafter bays have 6" depth)
  • 1-2" of closed-cell spray foam at the underside of the roof deck with the remaining depth of the bay filled with Eco-Batts (or another lower impact and healthier batt option)

I grew up in Palo Alto, so I am very familiar with the ambient environmental conditions, and I am 95% sure you will be happiest with 100% closed-cell since it'll only cost a little more, it will be more durable, it will minimize sound transmission, it will automatically air seal, and it won't transfer moisture.

Batt insulation could work well, but there would be a higher failure risk, and likely lower performance from the first day you finish the job.

Best of luck and please let me know if you have other questions.


For more information:

Read "I am putting spray foam insulation in my attic. What is the better option: open or closed cell?" a Q&A answered by Polly Osborne.

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