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Raycore - is it the product for me?

I was pretty much on board with buying Raycore for the 4500 s.f. home I am building in downstate NY, which is steel framed, so I actually don't need any structural value out of the exterior walls other than for supporting the windows and doors (and supporting the walls themselves). I am also going with a wire lathe and stucco on the exterior - Raycore told me I could direct apply the wire lathe and did not have to use any sheathing if I did not need the shear strength, but I am hesitant to do so.

After reading all of these comments, I don't think Raycore is a smart idea as the R-value I would attain for a 5.5" wall doesn't seem worth it to me for the cost of $6/s.f.

I was thinking of just staggering two rows of 2x4 timber stick-built walls back-to-back, with 2' wide, 3 1/2" thick rigid polyurethane foam in between, so that I eliminate the thermal bridging, similar to what Raycore offers (with obviously not as good a seal at the 2x4's, but I would look to spray along the edges by hand). Would this achieve close to R-35?

Would I bond them together with adhesive? Vapor barrier/foil between? Or perhaps go 3" and 3" with 1" furring between to give an air pocket between (would give me a chase for wiring too that way)?

Thanks.

Asked by Joe McCarthy
Posted Mon, 07/14/2014 - 17:10
Edited Tue, 07/15/2014 - 12:39

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3 Answers

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1.
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Joe,
If you want to build a double-stud wall -- and it's not a bad idea -- the best insulation to use is dense-packed cellulose. Skip the rigid foam layer. You'll get better performance at lower cost with cellulose.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 18:21
Edited Tue, 07/15/2014 - 12:39.

2.
Helpful? 0

Joe,
Raycore is real close to dishonest in their r value claims. Polyurethane foam does begin life with an r value of 7/" but as it ages the r value declines. Most reputable polyurethane vendors use r 6/" as an aged value. However it can age below even that value and ultimately be no better than EPS. The second issue with the Raycore claim is that they have wood studs in all their panels and these drop the effective r value as well. using r6/" and 24" OC studs and ignoring top and bottom plates the 5 1/2" Raycore panel is r27 or less not the r35 you mentioned. An 8 foot wall with a top and bottom plate is just over r24, even worse. If you want r35 you'll need a thicker wall and if you are going thicker there are much less expensive site built alternatives. However if your goal is to simply have the performance of the Raycore panels and a 5 1/2" wall a 2x4 24" OC wall with mineral wool bats and 2" of continuous XPS is real close (r22.7 for the 8 foot example) A 6"thick wall again 2x424" OC filled with mineral wool bats with 2 1/2" continuous XPS will outperform the Raycore 5 1/2" wall.

Answered by Jerry Liebler
Posted Mon, 07/14/2014 - 18:46
Edited Mon, 07/14/2014 - 19:32.

3.
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Until and unless the rigid polyurethane (or polyurethane SIP) manufacturers will stipulate that only a low global warming potential blowing agent such as HFO1234yf (with a GWP ~4x CO2 ) was used rather than the far more common (really the current standard) HFC245fa, which has a GWP ~1000x CO2. using it at higher R-values is the antithesis of "green" building, and may do more environmental harm over it's lifecycle than the energy use it is offsetting (which varies by energy source too.)

EPS is far lower impact, blown primarily with pentane (~7x CO2), but it's same-polymer sister XPS is blown primarily with HFC134a (~1400x CO2). The higher R/inch you get out of XPS is due to the HFC blowing agent, but most of it (and the enhanced performance) is gone in 50 years, whereas the performance of EPS is comparatively stable over time. At the same density & thickness the thermal performance of EPS & XPS converges to the same R value over time. Where the thickness isn't a huge issue it's generally lower impact, greener, and cheaper to use EPS.

Polyisocyanurate is a first-cousin to polyurethane, but is blown with pentane like EPS. It has a very non-linear derating over temperature, with performance peaking at about 50F mid-foam temp, but falls off a performance cliff at mid-foam temps below 30F. In a sandwich situation with fiber-R on both the exterior and interior side it's wintertime performance will be pretty good in NY location, averaging between R6-R7/inch, and in summer would still deliver ~R6/inch. In cold-climate applications where it is the exterior (and thus colder in winter) layer its performance can be as low or lower than EPS (depending on climate & thickness, and the amount of interior-side fiber R.)

All R-ratings of insulation-only products are the performance at standard ASTM C518 delta-Ts and mid-insulation temp of 75F. EPS & XPS performance monotonically rises at lower temps within the winter ranges you would expect in NY, outperforming their rated-R when the foam is on the exterior side of the assembly in colder climates. This is probably true of rigid polyurethane blown with HFC245fa too but I'm not sure about the derating curves of PU blown with water or HFO1234yf compare to that.

Bottom line, if you're going to use rigid foam, polyiso & EPS, placed appropriately in the stackup are your greenest options. Some day HFO1234yf and it's low-GWP cousins may be used for closed cell polyurethane & XPS, but SFAIK none of the major manufacturers of rigid foam board have gone there yet.

Answered by Dana Dorsett
Posted Wed, 07/16/2014 - 10:55

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