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Community and Q&A

A low cost alternative to “flash and batt” insulation?

Richard Ryker | Posted in Energy Efficiency and Durability on

Hello to all –

I’m trying to find a cost-effective solution to insulating the exterior walls of a 1900 vintage 2-1/2 story house. I’m located in Omaha – climate zone 5. I had originally thought that using flash and batt insulation would be an effective way to form an air infiltration barrier and provide an R-value higher than available using 3-1/2 fiberglass batts. However, the cost of 1-1/2″ of spray closed cell insulation was a concern.

I now wonder if using EcoSeal sprayable caulk and squishing 6″ of fiberglass insulation into a 4″ cavity might not provide most of the benefit of “flash and batt” at a substantially lower cost.

Mark Holladay, in his posting on September 23, 2011, “Air Sealing with Sprayable Caulk”, was quick to point out that sprayable caulk was not the same as flash and batt insulation. Then, too, in his post and Q&A of July 10, 2009, “Installing Fiberglass Right”, Martin explained that squishing fiberglass insulation does, in fact, increase the R-value in a wall cavity.

The house I’m renovating is, quite simply, just old. It would never be considered a historic renovation. So my goal is provide the best insulation I can without the high cost of high-performance insulation. The plaster walls have deteriorated and I am in the process of removing the plaster and lath from the exterior walls.

My plan would be create an air barrier using EcoSeal to seal the edges of the studs AND each gap of the exterior sheathing (nominally 6″ boards), then compress 6″ of unfaced R-22 into the 4″ cavity. Hopefully the compression would yield an R-value of 17 or so.

Since I can apply both the EcoSeal and insulation with out hiring a specialized contractor, it seems to me that this combination would provide an adequate and relatively inexpensive insulation solution for a 110 year-old house.

Your comments and feedback are solicited.

Dick Ryker

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  1. Aaron Vander Meulen | | #1

    "Since I can apply both the EcoSeal and insulation with out hiring a specialized contractor"
    I assume then you are finding a paint contractor with a big enough pump to spray this? Have you considered making the walls thicker? And I assume you're not doing any exterior renovations? How much gap is there b/w your sheating boards?

  2. Robert Wood | | #2

    What would be the ramifications of gluing 1' rigid foam against the exterior sheathing, use low expansion foam to seal the edges. Then press the 2x4 batt against the rigid to compress it slightly. I am not sure if you should use a vapour barrier in this case.

  3. Richard Ryker | | #3

    Aaron - thank you for your comments. The boards are edge to edge, so the gap is less than 1/16" You're also correct in that I'm not doing any exterior renovation as the exterior is aluminum siding over the original clapboards. I'm hoping to rent a piston sprayer for the application.

  4. Richard Ryker | | #4

    Robert - As I mentioned above, I'm not modifying the exterior so adding rigid foam to the exterior is not possible.

    However, your comments do make me wonder if adding 1/2" rigid foam between the batt insulation and drywall wouldn't be a good idea. The rigid foam would provide a much needed thermal break between the wall studs and interior finish drywall. You shouldn't do this with flash and batt, but since the EcoSeal doesn't create a vapor barrier moisture can still escape to the exterior.

    Does this make sense?

  5. David Meiland | | #5

    Is this house currently uninsulated?

  6. GBA Editor
    Martin Holladay | | #6

    There are a lot of ways to skin a cat, and your way is certainly one of them. If that's the approach you want to take, there's no reason not to do it.

    As you point out, a layer of interior rigid foam across the studs (as thick as you are willing to install) will improve the thermal performance of the wall. With thick rigid foam, however, all of the electrical boxes will need to be extended or moved.

    However, you should note that when an R-22 batt is compressed into a 2x4 wall, the resulting compressed batt has an R-value of R-14, not R-17. (See the chart below.) It would make more sense for you to buy and install R-15 batts that are 3 1/2 inches thick. Such batts have been optimized -- they are denser than old-fashioned R-11 batts -- and in a sense can be considered "pre-compressed" batts.

  7. Richard Ryker | | #7

    Martin - Thank you for your response. I had unsuccessfully tried to locate compression chart on the Owens-Corning website. Using 3-1/2 R-15 appears to be a logical choice.

    BTW, since I'm gutting the exterior walls, relocating deep electrical boxes will not be an issue. I'll also consider using 1" rigid foam.

    David and others - the house is uninsulated. I have a hard time imagining the energy dollars this and other uninsulated houses have consumed over 100 years!

  8. David Meiland | | #8

    Richard, since you are opening up all the walls, you will have a good chance to look for any signs of past water leakage. Since you apparently are not doing any work from the exterior, you presumably won't have a chance to fix any areas that may have been leaking water. Areas that have been getting wet have been drying fairly well since there has been no insulation hold moisture. You will be changing that part of the equation, so be aware.

  9. Richard Ryker | | #9

    David - thanks for the advice. I'll inspect for past water damage as I work.

  10. Aj Builder, Upstate NY Zone 6a | | #10

    I like ridgid foam. But have opened up walls recently and in the past that were destroyed internally from moisture intrusion causing rot. In fact every remodel I have done where sheet foam was used near a wetting source contained hidden massive internal rot.

    Option for no foam would be like John Brooks posts, to run horizontal 2x materials. Then insulate this space too and you have a thermal break. Cost is less, labor is the same. Drywall will hang easier. Wall can dry both in and outward. Much less chance of someday finding the hidden rot I have found though must admit moisture was wrongly entering the walls I opened. Missing flashings and way too much love for way too many plants getting lots of water that loved to get into the wall and then not get out past the impassable ridgid foam layer in two or three of the cases.

  11. Richard Ryker | | #11

    "Curiouser and curiouser!," cried Alice. As Martin mentioned above, there are many ways to do this, so I'll have to find a balance between complexity, performance, cost and minimizing use of carbon generating materials.

    Thanks for your input AJ

  12. Aj Builder, Upstate NY Zone 6a | | #12

    I just tossed out two never opened packages of Sharp cheddar cheese. Never opened. Mold growing nicely thanks to Irene.....

    Sort of what Alice and I take note of sharpening are intuition maybe... Just a bit even? Plastic bags with tiny imperfections... Da moisture and critters gets in... But not back out... Not before mommy nature whips up some interesting life forms....

    Building assemblies need to be built not to get wet but when they do best if they can dry out faster than life moves in.

    Aj ( Zen master cryptician)

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