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Flash and batt insulation

I have a 100+ year-old home in climate zone 6A (Wisconsin) that needs new sheathing and siding, so I figured I may as well reinsulate the wall cavity as I pull and replace the sheathing (currently has R13 batt in a 2x4 wall cavity). After considering a variety of options, I keep coming back to flash and batt and wonder if this will work for me. Most (all?) flash and batt applications are applied from the interior--that is, 1-2" of closed-cell spray foam is applied to the exterior sheathing, batt insulation is installed, and drywall is put up. I will need to do the opposite--2" of spray foam applied to the drywall side of the cavity, then batt insulation, then plywood sheathing, WRB, 1/4" vented drainage plane, then clapboard siding.

The more I read about water vapor buildup, the more confused I get, but it seems to me that in my climate applying the 2" of spray foam to the drywall side, where it will act as an air and class-II water barrier (since it has low permeability) is the proper practice anyway. Is this correct? The batt insulation and the vented plywood would be the more vapor permeable side, but this is still not the most breathable of buildups. Am I going to create a mold/rot issue?

Also, any concerns about the drywall cracking and popping from applying 2" of closed cell foam to it?

Thanks for the help. This has been stumping me for a couple months, and I'd like to get a move on with the project before it gets too cold.

Asked by Eric Rinehardt
Posted Jul 27, 2012 12:32 PM ET
Edited Dec 21, 2012 2:01 PM ET


6 Answers

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Q. "Am I going to create a mold/rot issue?"

A. Probably not, but you want to be sure that the spray foam is a closed-cell product, not an open-cell product, and that the foam installer does a good job of air sealing.

Q. "Any concerns about the drywall cracking and popping from applying 2" of closed cell foam to it?"


My own opinion is that the thermal value of the batts is low, and the installation hassle of the batts is high. So if you can possibly afford it, just install your spray foam a little thicker.

Answered by Martin Holladay
Posted Jul 27, 2012 12:47 PM ET
Edited Jul 27, 2012 12:48 PM ET.


Thanks Martin,
I will think very seriously about doing the whole cavity with closed-cell foam. This, though, will mean the cavity has very low permeability throughout. Are there still no concerns for condensation buildup and mold/rot on the studs? Again, the exterior side of the plywood sheathing will have a vented air space, so this should assist with drying of the sheathing and studs. Sorry to beat a dead horse, but this is going to be a significant investment of my time and money, so I want to do it right.

Thanks again.

Answered by Eric Rinehardt
Posted Jul 27, 2012 12:57 PM ET


If you fill your stud bays with closed-cell spray foam -- or as close to that goal as you can manage (since closed-cell foam can't be easily trimmed) -- there is no reason to believe that your wall will have any mold or condenation problems.

Answered by Martin Holladay
Posted Jul 27, 2012 1:11 PM ET
Edited Jul 27, 2012 1:12 PM ET.


Fantastic. Somebody mentioned this to me a few days ago, but I ruled it out due to cost. Really, though, it's not much more expensive to do all closed-cell, so that's what I will do. Thanks Martin

Answered by Eric Rinehardt
Posted Jul 27, 2012 1:15 PM ET


Am I wrong in thinking that exterior sheet foam would be the most cost effective solution since siding and sheathing are coming off? Better and cheaper I would think

Answered by Keith Gustafson
Posted Dec 22, 2012 8:38 AM ET


Even 1" of closed cell foam applied to the sheathing is sufficient to eliminate mold hazards in a 2x4 flash'n'batt in climate zone 6A, and is sufficiently permeable to allow the sheathing to dry toward the interior (if you go with un-faced batts), making it a more resilient way to go.

With ~R6 of foam and ~R10 of compressed batt (an R13 compressed to 2.5") means that the interior surface of the foam is the "condensing surface", but in a house with comfortable 40F dew point to the interior air (~35% RH @ 70F) that condensation can only accumulate mold-inducing levels in the wall when the sheathing stays below 20F for long periods of time (weeks without break).

Use weatherspark.com graphs for a nearby city and eyeball the average winter temp in your location using the cursors. If your mean winter temp averages above 20F an inch is plenty. The mean Dec-Feb. temp for Minneapolis is about 22F:

(Use the sliders to view a span greater than December-February, and read the temp of the cursor when the horizontal line is at about the winter midpoint of weather history.)

With 2" of closed cell foam (~R13) and an R11 batt compressed to fill the 1.5" gap (~R6) condensation won't occur on the face of the foam until it's -25F at the sheathing, which is fewer than 1% of the heating season hours in any zone 6 location.

Using 3" of closed cell foam in your stackup yields a whole wall R of about R9.5-10 after thermal bridging through 3" of stud rather than 3.5".

A 1" flash & batt with R13s is almost exactly the same (but a lot cheaper!).

If you put an 2" of rigid EPS (or 1.5" of iso) on the exterior under the siding and used open cell foam or cellulose (or unfaced batts, if you absolutely must) your whole-wall R would rise to the R17-R20 range, for about the same money as a flash'n'batt. Air sealing a plank-sheathed building is a bit easier if you went with open cell in the cavity, but the exterior foam could/should be detailed as an air barrier.

It takes a minimum of R7.5 of exterior foam to meet code without stronger interior vapor retarders in your climate zone. With 2" EPS you have R8 (but it performs at better than R9 when it's below 0F outside). With 1.5" of polyiso you'd have ~R9-10 (but it would perform at about R8 when it's below 0F outside.)

Any time the siding is being replaced it's an opportunity moment to add exterior insulation, which will do FAR more for the thermal and moisture performance of the walls than any foamed magic mouse-milk R10,000/inch insulation you might install in the wall cavities. With a typical 25% framing fraction, even R-infinity per inch only yields R15 after factoring in the R value of the framing. If you put the foam budget on the outside of the sheathing where you only have the thermal bridging of the fasteners you get pretty much the full-R value out of the foam, all of the structural wood stays warmer (=drier), so both the performance and resiliance will soar.

Answered by Dana Dorsett
Posted Dec 24, 2012 6:35 PM ET

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