Tyvek over foil-faced polyiso
E247
| Posted in General Questions on
I was looking a the spec sheets for foil faced polyisocyanurate and it gives an R value for the foam and then a R value of the foam with an air space. For example, the 3/4″ foam has an R value of 5, but the R value of the foam with 3/4″ reflective air space is 7.5. If I’m installing foam, covered in Tyvek then 3/4″ furring strips with siding, do I have the reflective air space to get the R of 7.5 since the Tyvek is directly over the foil facing?
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Replies
E247,
No. To get the added value from the air space, the foil has to face the air space, without any covering that changes its emissivity. If you cover the foil facing, you lose the added R-value.
Is there an issue with using the Tyvek over the foil faced foam? I'm trying to decide if I should use XPS or the Polyiso in my application. I'm in Climate Zone 2 (near Houston, TX) and so I was planning on using 3/4" foam.
E247,
Can you tell us your name?
For more information on choosing a type of rigid foam, see this article: Choosing Rigid Foam. In general, green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential.
It's perfectly possible to install a housewrap like Tyvek over foil-faced rigid foam. Install the housewrap with long cap nails.
At it's HFC blowing agents slowly diffuse out of the foam, the performance of XPS drops to about R4.2/inch (R3.15 @ 3/4") after a few decades, the same performance as EPS of similar density.
In Houston's humid cooling dominated climate it's worth putting the Tyvek between the polyiso and sheathing and take advantage of both the enhanced thermal performance and the extremely high vapor retardency of the foil facer (a true vapor barrier.) With a 3/4" air gap and the foil facer it's performance of 3/4" foil faced polyiso over time would be about twice that of 3/4" XPS.