Whole wall thermal performance

[Daox]

I came across this paper today by the Oak Ridge National Laboratory. Its quite interesting and shows actual tested whole wall r-values. This means they take into account the thermal bridging of the wall studs and everything. They even show a larsen truss type all, but with fiberglass batting, not cellulose (boo).

Whole Wall Thermal Performance Content Page


Here is the chart that shows the different wall types tested:

[S-F]

That Oak Ridge study is awesome. Recently BSC has been working on something similar.

Air Leakage Degrades the Thermal Performance of Walls | GreenBuildingAdvisor.com

[Daox]

Yeah, I read that too and was going to post info on it later. Its very interesting to see the difference between the insulations when there is in/exfiltration.

[Daox]

I'm just adding some of the pretty charts from the article S-F posted because they are really good information.

•Wall 2: 2x4 studs, 16 inches o.c., with inset-stapled kraft-faced fiberglass batts;
•Wall 3: 2x4 studs, 16 inches o.c., with face-stapled kraft-faced fiberglass batts;
•Wall 4: 2x4 studs, 16 inches o.c., with damp-sprayed cellulose;
•Wall 6: 2x4 studs, 16 inches o.c., with R-13 open-cell spray polyurethane foam;
•Wall 6: 2x4 studs, 16 inches o.c., with R-13 closed-cell spray polyurethane foam;
•Wall 7: 2x4 studs, 16 inches o.c., with R-13 friction-fit fiberglass batts plus 1-inch-thick exterior XPS rigid foam;
•Wall 8: 2x6 studs, 16 inches o.c., with R-21 friction-fit fiberglass batts.




So, this chart is showing us the amount of air transfer in a wall with +10 pascals and -10 pascals of pressure. The first half of the chart labeled [42] is done at 42C or 108F. The second half of the chart labled [-18] is done at -18C or 0F.

As you can see, at colder temperatures a wall is much more leaky for the same pressure. Also note, that fiberglass quite a bit more air than cellulose, and spray foam being the best. Its also interesting to note that a 2x4 wall with cellulose performs better air leakage wise than a 2x6 wall filled with fiberglass, and also better than a 2x4 wall with 1" thick XPS over the exterior in most cases.

[strider3700]

on the fiberglass batt walls do they have a vapour barrier or is it just plywood, 2x4's/fiberglass, drywall?

I'd assume that with a properly sealed vapour barrier, all joints taped you'd be just as good as the foam's.

[Daox]

The article doesn't mention any vapor barrier anywhere which is odd IMO since its a pretty big deal in cold climates. Good point.

[Daox]

Their points at the end of the article are also quite good and summarize the article pretty well.

•“Stopping air leaks is just as important as — maybe more important than — adding insulation. Unless builders prevent air from leaking through walls and ceilings, insulation alone won't do much good.”

•“If you choose an insulation that doesn't stop air flow, it's important to install an adjacent air barrier material.”

•“Fiberglass batts are air-permeable — they do a poor job of resisting airflow — so it is essential that they be installed in continuous contact with an impeccable barrier to air movement. For the very best performance, fiberglass batts should be installed in a sealed cavity (for example, a stud or joist bay) with an air barrier on all six sides.”

•“Dense-packed cellulose resists air infiltration better than fiberglass batts, but it's not an air barrier.”

•“Spray polyurethane foam is better than any other type of insulation at reducing air leakage.”

•“Fiberglass-insulated homes are the leakiest. … Bruce Harley, the Conservation Services Group’s technical director for residential energy services, … assembled airtightness data on Energy Star homes (including single-family and multifamily homes) completed in 2004 in Massachusetts and Rhode Island. All of the homes were blower-door tested after completion. … Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than those houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass.”

[bizboy]

In case you all haven't seen this, it applies to anyone's decision whether to blow in cellulose or fiberglass in cold climates:

"There is a problem with loose-fill fiberglass attic insulation is cold climates. It appears that, as attic temperature
drops below a certain point, air begins to circulate into and within the insulation, forming "convective loops" that
increase heat loss and decrease the effective R-value. At very cold temperatures (-20F), the R-value may
decrease by up to 50%."

In full-scale attic tests at Oak Ridge national Laboratory, the R-value of 6 inches of cubed loose-fill attic insulation
progressively fell as the attic air temperature dropped. At -18 F, the R-value measured only R-9. The problem
seems to occur with any low-density, loose-fill fibrous insulation.

Nisson, J.D. Ned, JLC, "Attic Insulation Problems In Cold Climates" March 1992, pp 42-43