Re: [GBlist] Qustn: Rigid Insul above exist'g roof

[Jim Coate <jbc@coate.org>]

Huh - have the foam technologies changed a lot since the information I'm
using came out (1990)? I thought bead board (=expanded?) would absorb a
fair amount of water in case of a small leak, and in turn loose its
effectiveness.

As for eliminating the sleepers, that makes sense structurally and with
long enough screws to fasten the new plywood over the foam, it would be
quick to build. But doesn't venting the roof prolong it's life, reducing
materials used in the long run?

So maybe the first question is: how important is it to vent the roof? 


John Salmen wrote:
> I generally prefer expanded foams to the extruded. Part of the reasoning is
> the friendliness of the technology used the other is about performance. Most
> extruded foams tend to have a lower R value in lower temperatures while
> extruded foams tend to gain in value as the temperature drops, which is what
> you would prefer to have happening. Expanded foams seem to have a longer
> life as well, something to do with cell collapse in extruded. Extruded foams
> (bead board) are available in a wide range of compressive strengths, but for
> a roofing application the typical type 1 offers 10-15 psi rating and higher
> ratings are available.
> 
> I would never really assign a higher value to foam in situ than a R5 per
> inch. Polyisocyanurate has an aged value of 7.2 (which theoretically adds up
> to R21.6 for 3") so there could be some benefit there but at that range the
> additional cost value is dubious and I would like to see those values 20 yrs
> hence - they can be compromised substantially by any thermal bridging which
> can substantially reduce the real value of the assembly. In a situation such
> as Barbara describes I would prefer to see a completely sealed insulative
> layer with all gaps foamed or sealed (complete to the overhangs) with no air
> gap between insulation and roofing. Typically I would use the thickness
> needed in a basic eps rather than considering a thinner profile in a more
> chemically complex foam.
> 
> The elimination of strapping reduces material usage and labour allowing for
> an increase in insulation, a small increase in material which provides some
> actual benefit. The use of a lighter coloured roofing material would
> eliminate the need for a foil facing and an increase in foam thickness would
> further that.


_________
Jim Coate
1992 Chevy S-10
1970s Elec-Trak E20
http://www.eeevee.com

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