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Johns Manville Spider fiberglass insulation – San Francisco Area – storage attic insulation

severaltypesofnerd | Posted in Green Products and Materials on

I’m looking for an insulation for a storage attic in California Zone 3a (Marine influence).  It’s a metal roof, with 2×6 rough sawn rafters at 6:12 pitch.

Spray foam (SPF) is certainly an option.  But it’s fairly heavy, has to be installed well to avoid toxic problems, and it burns.

What about Spider for this?  I’ve read about it, but been unable find any installers or any local examples.   Is it still viable?

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  1. b_coplin | | #1

    Bay area is IECC cz3c, I believe. That means means some (but not much) foam for dew point control.

    1" of polyiso above the roof deck would do it, but you mentioned a metal roof. I assume you are looking for interior solutions. That leaves 1" of ccspf applied to the roof deck. Spider would be a good option to fill the remainder of the rafter bay. Cellulose may be more widely available in your area, and is carbon negative.

    You could cut the framing losses in half by strapping the ceiling with 2x's on the flat. This would be inexpensive and get you pretty close to the u factor alternative performance in cz3.

  2. Expert Member
    Dana Dorsett | | #2

    If the metal roof isn't flat to the roof deck (or even if it is, if the ridge can be reworked), in IECC climate zone 3C you can do just fine with unvented Spider (or other fiber insulation) using a diffusion vent along the ridge, (which is possible to build to IRC code- not sure if CA Title 24 is on board with that yet. See section R806.5 subsection 5.2:

    Without the diffusion vent or a standard soffit-to-ridge venting scheme Bryan's 1" flash of closed cell foam or R5 foam above the rood deck, with the rest of the insulation being fiber works.

    +1 on the recommendation for cellulose, which WILL work without the cc foam if the metal roof isn't a "cool roof" or light in color. See the 3C San Fransisco row, cellulose column in Table 3 of this document:

    With 1" of closed cell foam the remaining depth is now 5", in which case R23 rock wool batts could be made to fit nicely, at a slightly higher R/inch and lower price point than Spider. Compressed to 5" from the nominal manufactured 5.5" loft it would still perform a bit better than R21 (+ the R7 for HFO blown closed cell foam would make ~R28). If going with Bryan's perpendicular 2x4 approach, with R15 rock wool batts the center-cavity R would be north of R40, and with the reduced thermal bridging would exceed IRC code performance on a both an R-value & U-factor basis. I believe CA Title 24 code isn't as stringent as the IRC's R-value prescriptives in your CA climate zone (but I'm not a Title 24 expert by any means, and am not up the changes that came with Title 24 2019).

  3. severaltypesofnerd | | #3 (an areogel) is a possible thermal break for the rafters.

    What's the price point of JM Spider installed?

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