Comments on: Big News: The R-Value of Insulation Is Not a Constant

By: Allison Bailes

Allison Bailes — Tue, 01 Jul 2014 20:09:30 +0000

Shipley:
Shipley: That's a good question, and I should have addressed that in the article. If you click over to the original article, you'll find that that they actually did compare oranges to oranges. The graph above shows the R-value per inch. They tested polyiso at 2" and 4" thicknesses. The interesting part of this research wasn't what the actual R-value for polyiso is. It's that the R-value takes a big dip when the temperature drops.

By: Shipley

Shipley — Tue, 01 Jul 2014 19:01:36 +0000

Why was the XPS tested at R
Why was the XPS tested at R-20 and the Polyiso tested at R-6? This seems like bad science. Though it may be true that Polyiso loses effectiveness at low temp, it has also been shown that the rate of loss decreases with thickness - so the XPS was given an advantage. Apples to oranges and bad science. I like XPS, but for serious comparison you have to test the products in the same way (including same or similar thicknesses.)

By: David Butler

David Butler — Tue, 22 Apr 2014 20:49:43 +0000

@Christian, there are often
@Christian, there are often reasons other than R-value that dictate one type insulation over another. For example, polyiso is often used in crawl spaces because it comes with aluminum facing. Code prohibits exposed EPS or XPS in that application. Sometimes there are structural reasons for using XPS over EPS.

Most importantly, the normalized price per unit of R-value for a given product may still be lower after accounting for the degradation.

For all of those reasons, I would never assume one type of insulation is always the best choice.

By: Christian

Christian — Tue, 22 Apr 2014 16:59:23 +0000

I think for me Eps is the
I think for me Eps is the best choice, is R value is at 100% and will not decrease on time. QUESTION:WE INSULAT A BUILDING FOR A SHORT PERIODE OF TIME ?? AND WE ACCEPT SOME INSULATION THAT WILL DECREASE ON TIME WHY ??

By: steve

steve — Sat, 17 Aug 2013 21:08:50 +0000

I am curious about sips with
I am curious about sips with a polyurethane core. There seems to be some chemical relationship between polyiso and polyurethane so does the r value of polyurethane in sips go up or down as the temperature decreases.

By: David Butler

David Butler — Sat, 27 Apr 2013 20:47:39 +0000

@Allsion, it's important to
@Allsion, it's important to note that the polyiso graph you reproduced does not account for aging. Also, that graph is based on 2" samples. If the assumed mechanism at play is correct, then a 1" layer would show greater relative degradation, no?

@Mark, it is easy to estimate the amount of XPS needed to keep polyiso above it's peak zone (35F). Just sketch out a temperature gradient across the wall section (don't forget to include inside and outside air films). But if we take the above graphs as face value, why not just use 6" of XPS?

By: Paul Szymkiewicz

Paul Szymkiewicz — Sat, 27 Apr 2013 19:48:40 +0000

Allison,

Allison,
I was waiting for you to mention moisture and you did in the comments. I was surprised that the graphs, especially for fiberglass, did not account for RH.

By: Martin Holladay

Martin Holladay — Sat, 27 Apr 2013 00:38:44 +0000

Allison,

Allison,
Good blog — but as you admitted toward the end, not really news.

Two years ago (http://www.greenbuildingadvisor.com/blogs/dept/musings/bold-attempt-slay-r-value), I wrote, “Building scientists have known for years that the rate of heat flow through insulation materials varies at different temperatures. … Polyiso doesn’t perform as well at low temperatures. That’s because the trapped blowing-agent gases start to condense at cold temperatures.”

By: Frank Resso

Frank Resso — Thu, 25 Apr 2013 12:20:29 +0000
John N: High density (11.0
John N: High density (11.0 pcf) rock wool boards do increase slightly in R-Value at lower temperatures. Something like R-3.9 per inch at 75 deg F mean temp to R-4.2 F per inch at 40 deg F mean temp. I think those values have been confirmed by BSC.

By: Allison Bailes

Allison Bailes — Thu, 25 Apr 2013 00:13:54 +0000
John N.:
John N.: Good question. I haven't found data yet on other types of insulation but I'll let you know if I do.

Peter M.: Yes, when air can move through the insulation, that can change the performance significantly. Oak Ridge showed this for blown fiberglass in attics in the early '90s. What I'm reporting on here is what happens only because of changing temperature.

Bill S.: Yes, indeed. BSC is doing a lot of great work and making their info available to everyone.

Mark F.: No, we didn't talk about that, but you can look at the charts in their report and see how much the R-value diminishes. The ideal thickness of insulation is subjective and is a choice for the parties involved in building the house (once you've met code, that is). Some people use the 20-40-60 rule for cold climates: R-20 floors, R-40 walls, and R-60 ceilings. In warmer climates, you don't need that much.

Ryan S.: You're going to get your wish...eventually. One of the things that makes BSC's hot box so special is that they can carefully control the introduction of air and moisture and study the effects. So, wait a bit, and they'll be putting those results out, too.