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THERM: Added Material Layer, R-Value decreased

asked 2016-09-06 16:07:08 -0500

updated 2016-09-08 10:58:26 -0500

Hello,

I am fairly new to THERM and am a little bit confused about an analysis I am performing. I took away a layer from a construction to test how much resistance that layer was giving, but the resulting assembly had a higher thermal resistance. I don't understand how this would be possible. See image below describing issue. The image is coming in a bit too small to see, so if you click the attached image you can see a larger, clearer version of it. The purple layer, a fiberglass bullet resistant material, is the what I removed. I've checked that the materials and that the boundary layers are the same for both walls (except the layer removed from the bottom wall).

The layers of the top wall are as follows: Stucco, Exterior Insulation, Gypsum, 3.625 stud with insulation, Fiberglass bullet proofing, Air gap, 3.625 stud with insulation, Gypsum.

The layers of the bottom wall are as follows: Stucco, Exterior Insulation, Gypsum, 3.625 stud with insulation, Air gap, 3.625 stud with insulation, Gypsum.

I've checked that the materials and that the boundary layers are the same for both walls. I've already posted this on the THERM forum and haven't had much luck. Anyone have any ideas on what I may be doing wrong?

image description

THERM.png

Thanks!

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answered 2016-09-08 18:23:23 -0500

curcija gravatar image

Adding a layer in a 1-D construction will always increase thermal resistance. Normally, this should happen in 2-D and 3-D constructions, but there may be some thermal bridging effect that can cause reversal. Can you please send both THERM files, so that we can look at it? Thanks.

Charlie Curcija, LBNL Windows and Daylighting Group

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I am emailing to your lbl email now. Thanks!

Jonathan K gravatar image Jonathan K  ( 2016-09-09 10:35:51 -0500 )edit
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The reason that configuration without fiberglass panel gives better R-value is that in that configuration, both steel studs are exposed to air cavity, while in the case of configuration with fiberglass panel, only one has exposed side to air cavity. Since steel has lower emissivity than fiberglass (0.6 vs 0.9), that makes the small, non-intuitive difference in R-value. If the fiberglass panel was more insulating, this radiation heat transfer effect would be less pronounced and would result in expected behavior (i.e., configuration with fiberglass panel would be more insulating).

curcija gravatar image curcija  ( 2016-09-09 11:24:45 -0500 )edit

Perfect response. Thanks!

Jonathan K gravatar image Jonathan K  ( 2016-09-09 11:32:06 -0500 )edit

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Asked: 2016-09-06 16:07:08 -0500

Seen: 496 times

Last updated: Sep 08 '16