Radiant Barriers ad nauseum

[Chad Fabry]

I'm involved in a scenario where there is large builder which is building a massive neighborhood of essentially identical homes.

2x4 walls, R15 Fiberglass- The res-check provided indicates an R3, radiant barrier house wrap as a necessary component of the thermal barrier assembly.

The foil-faced wrap has a 1/16" foam back that looks remarkably similar to the stuff that protected my Dell computer in transit.

I've poured over the documentation provided by the manufacturer. The claimed R3 value depends on R2 or so air layer between the vinyl siding and the house wrap.

What's the likelihood that the radiant wrap and the air layer between the siding and the wrap actually performs to an R3 level?

For those of you who are taking a snow day like me, I attached the "studies". They're brief enough.

[Bill Robinson]

Chad, I doubt the reality will meet the modeling.

That said, attention to detail on the site is , IMO, more critical to performance than the optimistic thinking of the designer.

My approach would be to work with the installation crews to get it right, there is where the gap in construction most often occurs, literally and figuratively.

[NW Architect]

Radiant barriers CAN work and produce about an R-3 WHEN CONDITIONS ARE RIGHT. To wit, there needs to be some radiant heat to be blocked. Also needs to b e an air space - I've always heard 3/4" minimum and your paperwork says 3/8" . . . and unless the vinyl siding is on furring, only about half of the surface area will be 3/8" distant. I also kinda wonder whether the air space is supposed to be dead air space?

Finally, radiant barriers begin to fail as soon as they get dirty.

Then there's the possibly bigger issue of the material. Betcha it is vapor impermeable and, in your area, on the wrong side of the wall assembly

[Chad Fabry]

. and unless the vinyl siding is on furring, only about half of the surface area will be 3/8" distant. I also kinda wonder whether the air space is supposed to be dead air space?

The air space behind vinyl certainly isn't "dead air" and it raised my eyebrows when the manufacturer claimed that it worked as such. Up here in the NE, there isn't much need for a radiant barrier anywhere except maybe on a southern exposure. We cool air condition less than 30 days a year as rule.

The wrap face is perforated and claims made indicate the wrap has a perm rate right around 1. The lack of a listing or rating has me skeptical about any of the claims made in the product literature- especially the R3 part.

[NW Architect]

Perm rate of 1 = vapor retarder

The only reason I didn't unload on the post was that the manufacturer is, at least, not claiming R-22 like some other manufacturers of similar products do!

[worthy]

An independent study of the P2000 system requested by the Alaska Housing Finance Corp. and undertaken by the Cold Climate Research Center "concludes that for well-insulated buildings in cold climates,the use of reflective insulation adds very little to the overall R-value of the building envelope."

The minimal benefit of added R Value has to be offset by the introduction of a strong vapour retarder into the wall and the inevitable reduction of any added R value of the reflective material by contamination and moisture. So in the Moderator's case, he could end up with the added R value of 1/16" of EPS, about .2125.
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Short conclusionfrom the Cold Climate Research Center.