I'm using some 6.5" EPS core wall panels. They advertise them as having an insulation value of R-29. The stamp on the foam is for PerformGuard Termite Resistant, ESR 1006 Type 1, manufactured by AFM Corporation. This companies' website has an ICC-ES evaluation report which states that the R-value of type 1 is R-3.5/inch. This would make a 6.5" panel equal R-22.75. Anyone else encountered this discrepancy? Can 6.5" of EPS equal R-29?

Can 6.5" of EPS equal R-29?

Send it down to the marketing department.

I'll bet they can do it.

They can do darn near anything.

Depends on the temperature ..........

6.5" =

Temperature	at 40 °	at 75 °
1 lb Density EPS	27.11	25.03
1.5 lb Density EPS	29.58	27.11
2 lb Density EPS	30.94	28.28

.

The OSB skins have a small amount of R-value that you would include.

Here's how they got R-29 for a 6.5" OSB SIP

EPS Foam thickness to fit a 2x6 with an 1/8" tolerance = 6.625 inches
Cold temperature R-Value for Type I Foam = R-4.17 per inch
R-value of 7/16" thick OSB sheathing = R-0.51

6.625 in x R-4.17 per in = R-27.63 + R-0.51 + R-0.51 = R-28.65 (glue may have some R-value too, so round up)

The company isn't deceiving you, they just don't tell you that R-Value varies with temperature.

(glue may have some R-value too, so round up)

Don't forget the air film approximations.

Using the air film approximations allows you to get rid of that extra 1/8" of foam, so it is

6.5 X 4.17 = 27.10 + 0.51 + 0.51 + 0.68(inside) + 0.17(outside) = 28.97 (+ the material interfaces - "glue", of course)

I'm going to have to go recalculate my ICF block now... :-)

it is just pennies more to use Type XIII foam to accomplish higher R-Values

Simon, i beleive you are making a mistake in your calculation, the eps nécessary for a 2x6 is not 6,625 but 5,625, because the wight of a 2x6 is 51/2 inches, plus 2 OSB makes it 6,1/2'' for a R value of about R 24 +-, sorry, unless your 2x6 are wider than 6''

Pierre Desjardins
Banerpan

Posted By cmkavala on 04 Feb 2011 08:29 PM
Depends on the temperature ..........

6.5" =

Temperature	at 40 °	at 75 °
1 lb Density EPS	27.11	25.03
1.5 lb Density EPS	29.58	27.11
2 lb Density EPS	30.94	28.28

And in cooling dominated climates, it's also useful to know the 100F performance (which is lower still)  to come up with the right model (especially if using SIPs for low angle roofs, which can be considerably hotter.)

Clearly R29 would be heating-climate number, but a valid one.  The colder it is outside, the higher the R of the outer portions of the SIP.  When it's 0F outside, the average temp of the EPS will be a bit under that 40 degrees, and the R-value slightly higher.

Unlike EPS, the R-value of iso (polyisocyanurate) falls slightly with temp- that R6-6.5/inch @75F falls to ~R5.5 @ 25F. Modeling iso with a single K-value in cold/very-cold climates I usually assume R5.5/inch for an average, or R4.5/inch for type-II EPS.  For use in more moderate climes I use R6.0 for iso, R4.0 for EPS.  At 0F average temps across the material their performance is remarkably comparable, with EPS having the edge. (So, if it hovers around -50 to -65F regularly at your house, polystyrene insulation is the ticket, eh? )

Oops, I did make a mistake. 5.625". Guess I have to count the air films now!:-)

The assembly may test higher than it's component parts because of some synergy or something, but that's a guess.

The R-value for the OSB sheathing I found didn't specify at what temperature it achieved R-0.51, maybe it's higher at 40F

When I worked for a panel manufacturer, we always quoted the average of the two numbers, so R-4 per inch.

Does anyone know why R-value goes up when temperature goes down?

>Does anyone know why R-value goes up when temperature goes down?

I'm sure SOMEBODY has studied how/why the K-value of closed cell polystyrene has mostly a negative temperature coefficient across a relevant range for space heating/cooling temps, but I haven't seen published documents on the web. There are a lot of factors, the ratio of interstitial volume between beads to cell volume may change with temp, but that would only be part of it (XPS has no interstitial volume and has similiar by not identical temperature coefficient.) Polyisocyanurate has a lower proportion of closed cells an no interstitial space, with a positive slope with temperature across relevant heating climate temps. It wouldn't surprise me if there were numerous industry and academic papers on the physics involved, but good arm-waving web-forum explanations might be tough to come up with.

Foam is funny. We use it because its macro behavior reduces large scale convection, but when you change the temperature, the thermal characteristics of foam might change based on the convective behavior of the gas trapped in all those little cells.

Some of you will appreciate this with polyurethane foam, for example;

http://www2.dupont.com/Formacel/en_US/assets/downloads/h54876.pdf

In EPS foam, the gas is air, so I suppose the increase in R-value represents the slowed convective heat transfer within the cells.

There could also be effects on the temerature curve for EPS related to the moisture content of the air in EPS cells, boosting the R at temps below the dew point of the encapsulated air (which will probably be near the average seasonal dew point of the local atmosphere in above-grade apps.) It's not a simple problem amenable to simple arithmetic models, to be sure.

Thanks for the polyurethane blowing agent analysis paper!

Thank you for your answers. It appears the answer is not so simple and almost counter-intuitive. Regarding EPS foam, my thoughts are it relates to the moisture content in the air trapped in the foam. Warmer air contains more moisture and water is more heat conductive/convective than air, so the R-value is lower. Colder air is dryer and better at preventing heat transfer because it doesn't have the water in it creating a thermal bridge across the voids in the foam. With solid materials the colder they get the denser they get and the more heat conductive they get, so it is the opposite. I guess one needs to think of EPS foam as a composite material of plastic, air and water vapor to understand it properly.