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Foil-faced polyiso left exposed longterm - impact to R-Value?

A client's deep energy retrofit got started but stopped about 16 months ago, right after the foil-faced polyiso was installed but before the siding was installed. All polyiso joints and almost all edges were taped and sealed. The tape looks great (pretty amazing after two very hard zone 5 winters).

So what to do about the polyiso? The R-Value has decreased, but by how much? Has the overall durability of the product been compromised? The Dow techline says that exposure up to 45 days is ok, but then the R-Value will change due to water and sun. Depending on location on the house, the reduction will vary. No idea if it will be decreased by 10% or 75%. This is what I am trying to determine.

I see the options as follows:
1. Remove and replace all polyiso ($$$, but clearly the bullet-proof solution)

2. Keep existing, if can determine if r-value of existing install is acceptable (even if compromised) by testing or rule of thumb. Can this be determined without testing samples?

Anything else to be considered? Thanks.

Asked by Kristen Simmons
Posted Jun 25, 2014 2:45 PM ET


2 Answers

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This hasn't been laying on it's side pooling up water (snow), it's always been well drained. If there is any roof overhang at all it's not likely to have been compromised by water migration- vertical foil sheds water pretty well, and dries VERY quickly.

Direct UV degradation only happens at exposed edges- the foil protects the rest by reflecting the incident UV- wouldn't sweat that one here either.

In this scenario I'd be surprised if ANY of it has suffered measurable performance degradation. Best guess is that you're looking at a hit of no greater than a couple percent, and probably less than that.

I'd be curious to know what the Dow tech line believes the degradation process is, for foam that is installed and kept mostly dry, compared to goods that were stacked up outdoors in the weather, or buried in the dirt, or on a flat roof water puddling on it. This stuff just isn't all that sensitive when it has UV-reflective moisture-rejecting foil facers, and mounted vertically so that it can drain.

In climate zone 5 for exterior sheathing applications just derate it to ~R5/inch from a dew-point control & average performance point of view. That is a derating for average outdoor temperature, not a degradation of the material- you would derate it to ~R5/inch for design purposed even if it was blown & installed just yesterday. (If it were installed in a more temperate mid layer of the assembly you wouldn't derate it, an use it's labeled R value.)

If what exposed edges there were are still pale yellow instead of brown, don't worry about those either. If they're looking like deep butterscotch that damage is probably limited to the first half-inch or so from the edge, but still not big deal.

Answered by Dana Dorsett
Posted Jun 25, 2014 3:44 PM ET


Thanks for the response. This seems to be the general consensus - that there is some loss, but it's negligible. An overhang protects about 50% of the insulation, but not all. There's also a low slope roof currently without an overhang, although ice and water shield does wrap from roof to wall. In a few locations, bubbles are apparent on the face of the foam - these are pockets of water that did get into the foam panel. I've attached a pic of the worst case condition, where there was a failure at the sill. A indicates what are most likely water bubbles just underneath the foil face. At B, a previously flush panel is now 1/4-1/2" proud of its neighbors. We'll probably replace the foam between the windows and address the sill issue.

bubble and plane.png
Answered by Kristen Simmons
Posted Jul 10, 2014 3:57 PM ET
Edited Jul 10, 2014 3:58 PM ET.

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