how many taped layers in a retrofit?

[Kristen Simmons]

As the subject says...

I'm working on a retrofit to the EnerPHit standard, and there's a lack of clear consensus (surprise!) on how much taping is necessary. The current plan, which has Building Science Corp's approval,  starting from stud out: original board siding, SIGA building wrap lapped shingle style and taped (primary air barrier), 2 layers of polyiso, joints staggered, both layers taped (secondary air barriers), 1/2" air space/furring strips, and fiber cement board. The roof is similar, with 3 taped layers.

Is this overkill (and additional cost), having the 3 taped layers, or a prudent belt and suspender approach to air sealing? Can the case be made for taping only the building wrap and the outer layer of polyiso? 

What do you think?

[Hayden Robinson]

I don’t think the insulation taping spec is aimed at reducing in/exfiltration. BSC typically calls for installing insulation in multiple taped layers. My understanding is that they are looking to prevent heat loss due to air movement at panel edges. When I heard Joe Lstiburek on the topic, he showed a slide of a building with exterior foam roof insulation on a frosty morning. The panel edges (the insulation had been installed in a single thick layer) were clearly telegraphed by the absence of frost.  Joe explained that convective loops were occurring at panel edges. I also remember shiplap insulation edges being mentioned as an alternate to taping.

 

I know I’m going out on a limb contradicting Joe, and I don’t have any empirical back up, but with tight, staggered joints, I wouldn’t lose much sleep over skipping the tape on interior layers of insulation in your assembly.

 

Hayden

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[Rob Harrison AIA]

The way Kat explained it in our class, if you've got a bucket with a leak in it, putting that bucket in a larger bucket that also has a leak won't stop the leak. I thought that was a pretty good metaphor.

Rob

Rob Harrison AIA

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lyrical sustainable design  ::  passive house plus

[Adam Cohen]

Kristen, where is the air barrier layer, that is the key.  Close attention at the air barrier layer point is what is needed, no matter how many bandaids go on if the would is not stiched it will bleed through - EWWW!

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[Albert Rooks]

Kristen,

In all fairness to the subject and NOT to push more tape. Haydens point is more important than given credit for a cold climate. The problem is that Polyiso shrinks... A lot. Take the point from Dr Joe's Fine Home Buildings article this year where he notes the fact, and this was EPS (more stable than PI) and that the movement caused some problems. The gaps that open up will fill with air that will carry vapor and cause heat loss. Taping each layer is the best that you can do for sheet goods to break up the 3D air networks.

Here is a rather funny rambling GBA thread that predated the FHB article. Note the 2nd entry and the pic of Dr Joe's barn roof. http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/22751/heat-bridge-or-3-dimensional-pressure-boundar

Wish I could agree that it's not important, but...

albert.

[Albert Rooks]

So perhaps that GBA rambled farther than I thought. Here is a link to the full FHB article. Might need to be GBA pro to open it. I'm not sure...

http://www.greenbuildingadvisor.com/system/files/sites/default/files/foam-shrinks.pdf

albert.

[Skylar Swinford]

Albert,

Don't forget about "Foam Shrinks, and Other Lessons:Correction". Good explanation of how thermal cycling "smushes" the edges of rigid insulation together to cause "shrinkage". 

Skylar

[Kristen Simmons]

Thanks everyone for the articles and links. It's much appreciated!

The decision was made to tape just the housewrap (primary air barrier) and the outer layer of the polyiso (primary water barrier), eliminating the tape on the inner polyiso layer.

Because the house is part of a Deep Energy Retrofit incentive program, Building Science Corporation has been reviewing the project, and I need approval on the envelope details like this one. I've been working with Phil Kerrigan there, who ok'ed the change. He notes that that BSC tends to be conservative on the DER projects, which is why they recommend taping all layers as best practice, but that he feels comfortable with both the designer and the contractor on this project. He also said that this continues to be discussed at BSC, so there really doesn't seem to be a clear cut agreement, even there.

He noted that his opinion does not represent that of BSC.