Kneewall & Foamular over batt questions...

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Old 09-20-12, 06:39 AM
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Kneewall & Foamular over batt questions...

Hello all,
Bought a Cape with major insulation issues. Kneewalls with slant ceiling to flat ceiling. Joists are 5-1/2". For the slants, I'm thinking of Durovents > furr for r-21 batts > then 1" or 1-1/2" Foamular > 1/2" moisture resistant drywall. Unclear on weather or not to use faced or unfaced batts?
Thanks,
Blue
 
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Old 09-21-12, 08:53 AM
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How much R-value do you need in the slants? One trick we've discussed elsewhere in this forum is to install up to 2" of XPS, cut to the width of the framing, on the face of the rafters/joists. This will reduce the bridging effect, where heat is transferred directly to the roof through the framing during heating season, and it increases the depth of the bay by 2" to provide room for more insulation plus an adequate ventilation channel.

Unclear on weather or not to use faced or unfaced batts?
Either faced fiberglass batts or a separate moisture barrier, even with your moisture resistant drywall.

Given that fiberglass insulation is not very dense, it does not have as great an R-value per inch as some other insulation types. It is also susceptible to wind scour, and there should be a fair amount of air movement through the slants if you've set up the ventilation system to be effective. An alternative choice for these areas would be mineral wool batts, which are denser, have a higher R-value per inch, and are more resistant to wind damage. One example, available in most areas, is Roxul. The unfaced mineral wool batts will need a separate moisture applied over them.

I would go with standard 5/8" drywall over that, to help minimize the sagging sometimes seen with thinner rock.
 
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Old 10-01-12, 11:53 AM
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Thanks for the reply. Your suggestion has help me alter my installation a little. So instead on my original thoughts of Ferring up with a 1-1/2 x 1-1/2 will be replaced with strips of XPS. Is this the term for Foamular? So my current plan will look like this for the slant ceiling parts of the second level, with more questions of course...

Shingles > tar paper > t&g boards (original to the house. 1 x 8's, maybe 1x10's) > duravent (I've taped the seams & edges to seal the channels) > ad XPS strips to make the cavity either 6 or 6-1/2? Or should I purchase a 1-1/2" sheet of XPS and knife cut strips to make the 6-1/4 cavity for the insulation? > drywall.
 
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Old 10-01-12, 07:59 PM
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a 1-1/2 x 1-1/2 will be replaced with strips of XPS. Is this the term for Foamular?
Yes. XPS is short for extruded polystyrene. FOAMULARŪ is Owens Corning's brand name for their XPS product.

> add XPS strips to make the cavity either 6 or 6-1/2? Or should I purchase a 1-1/2" sheet of XPS and knife cut strips to make the 6-1/4 cavity for the insulation? > drywall.
How much insulation (R-value) do you need there. To get a relatively precise estimate, I suggest you plug the specific information for your house into the ZIP-Code Insulation Program, a calculator developed for this purpose at the Oak Ridge National Laboratory.
 
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