[Greenbuilding] P2000 insulation

[David Delaney]

At 01:52 PM 17/04/2007, Shawna Henderson wrote:
>At the risk of sounding slightly behind the times, because this has  
>been around for a couple of years at least: does this product  
>actually have the merit the mfr gives it? A 3/8" EPS core wrapped in  
>foil outperforms 6" f/g batt insulation is the major claim on the  
>website. I understand the theory of how it can work (ie, using a  
>reflective barrier to bounce energy back into the room (heating mode)  
>and keeping it out of the house (cooling mode)), but does it work? 

The claim is not credible.  

Let's do a calculation that is very favorably to the foil faced EPS.

EPS has a thermal resistance of about R4/inch (4 ft^2.hr.F/Btu.inch).

Fiberglass batts have a thermal resistance of about R3/inch. 
Six inches of fiber glass bats would give a total of R18, by 
a simple minded calculation. Even if we deeply discount this 
to allow for thermal bridging or small air channels by, say, 
a pessimistic 50%, that still leaves a total of at least R9.

To give a two sided foil faced layer of EPS its best 
circumstance, it would be placed so that there was about a 
1/2 inch air gap on both sides. In this circumstance, and 
assuming an effective emittance for both air gaps of 0.05 
(optimistically low) (See Table 2, F24.2, ASHRAE 
Fundamentals, SI, 1997), the effective thermal resistance of 
each of the two air gaps, taking the radiation inhibition of 
the low emittance into account, would be RSI = 0.48 K.m^2/W 
(See Table 3, F24.3)   This ignores thermal 
bridging through the strapping that would be needed to hold 
the foil faced EPS in the middle of the air gap, which could
be minimized by placing it vertically on one side and horizontally
on the other.

So the R value of one air gap would be RSI 0.48 /0.1761 = 
R2.7.  Two of those gaps plus the 3/8" EPS would give a 
total R value of 2 x 2.7 + 3/8 x 4 = R6.9.

If there is an air gap on only one side, then it does not 
matter whether of not that side is foil faced, and the R 
value is only R5.2.

If there is an air gap on neither side, then only the EPS 
itself counts, R=3/8 in x R4/in = R1.5

David Delaney, Ottawa