Replies

1. GBA Editor
   Martin Holladay | May 15, 2017 12:31pm | #1

   Jeremy,
   Your situation seems like a good candidate for DuPont ThermaWrap R5.0.

   Here is a link to an article about the product: Two New Exterior Insulation Products for Walls.

2. 
   JeremyArch | May 15, 2017 01:04pm | #2

   Thanks for the link to the product. I'm always open to an option 6. It doesn't seem to be available at any of my local big box stores but there are a few other supply stores i can get in touch with. The product doesn't seem to get rave reviews and a lot of apprehension regarding its use. The installation of the vinyl siding appears to be the most important detail to minimize the R value loss with compression.
   Any thoughts on the other options if i am unable to get me hands on any of this product?

3. 
   JeremyArch | May 16, 2017 12:44pm | #3

   Martin,
   Just a follow up to the Thermawrap. It appears to be fairly easy to compress this material. If im looking at a none rigid option, would it not make more sense to use the roxul? it looks more dense and likely wouldn't compress as much during installation.

4. Expert Member
   Dana Dorsett | May 16, 2017 02:32pm | #4

   Roxul compresses as much as anything, and doesn't have the WRB air barrier on both sides like ThermaWrap. ThermaWrap looks like it was designed to be compressible so that it can be applied over the furring without large voids where the furring meets the sheathing.

5. GBA Editor
   Martin Holladay | May 16, 2017 02:43pm | #5

   Jeremy,
   The advantage of using ThermaWrap is that DuPont worked with vinyl siding manufacturers (I assume) when they developed the product. That means that DuPont should be able to identify a brand of vinyl siding that can be used with the ThermaWrap -- and I don't think you can say the same for Roxul.

6. 
   JeremyArch | May 16, 2017 03:07pm | #6

   Thank you both for the feedback, if this is the recommended product for my particular situation I will see if i can find a local supplier and check in with my window and siding installers to see if they are familiar with it as well.
   Out of curiosity, is there a reason why Option 3 wouldn't be recommended? would it not allow for outward drying and give the siding a rigid backing?

7. GBA Editor
   Martin Holladay | May 16, 2017 03:53pm | #7

   Jeremy,
   Option 3 is (a) still going to be squishy, and (b) won't be approved by the vinyl siding manufacturer (and will therefore mean the loss of the vinyl siding warranty), but it might work.

8. 
   JeremyArch | May 16, 2017 04:10pm | #8

   Martin,
   Thanks for the feed back. I guess in my head i saw the rigid foam providing more stability that would reduce or cancel out the squishy factor. I guess if i cant get the Thermawrap, ill break some rules and cancel the warranty on vinyl install.

9. 
   JeremyArch | May 23, 2017 03:23pm | #9

   Martin

   Both contractors i am dealing with are not familiar with the Thermawrap so we wont be using this product. My installer and I looked at the house and i can actually get 2 inches on the exterior. We decided on 1.25" continuous roxul, strapped with 1x3 and we are going to add 3/4" rigid foam between the strapping for both a little extra r value and a little backing support for the vinyl. Question, would any rigid foam (ie faced polyiso) between these straps work?

10. GBA Editor
    Martin Holladay | May 23, 2017 03:31pm | #10

    Jeremy,
    Q. "Would any rigid foam (i.e. faced polyiso) between these furring strips work?"

    A. Probably, because if any condensation forms on either side of the rigid foam, it should be able to dry. It's an unusual approach, but it will probably work.

11. 
    JeremyArch | May 23, 2017 03:45pm | #11

    Thanks martin,
    I guess my other approach could be to use 2" of polyiso for an r12 though i think i read here that it performs closer to an r10 which brings me in slightly under the recommended R11.25 for my wall set up.

12. Expert Member
    Dana Dorsett | May 23, 2017 05:34pm | #12

    With the interior side polyethylene still in place being only slightly shy of R11.25 performance isn't very risky at all.

    Over the past three decades something on the order of tens of thousands of houses in zone 6/7 parts of Canada built with interior polyethylene and skimpy R4-R5 foam on the exterior. Even though it's less resilient to bulk water intrusion (make sure you get the window flashing lapped correctly!), if that's a disaster of a stack up we'd know about it by now. But even a tiny bit of air flow provided by a crinkly housewrap between the sheathing and foil faced foam eases things quite a bit.

    Read this: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed

    "Nothing however prevented anyone from using R-5 vapor closed insulation with 6 mil polyethylene on the inside (Figure 5). Yes, but that “creates” the evil double vapor barrier. Yes, but it has been shown to work all over the place. Relax. And again, as noted already, we have been here before (“How Come Double Vapor Barriers Work?”, ASHRAE Journal, January 2016). But, apparently folks are unable to relax. So now what?"

13. 
    Jon_R | May 23, 2017 09:43pm | #13

    My understanding is that more permeability to the exterior provides some offset against less than desired R value. In such cases, I'd prefer no foil facing.

14. 
    JeremyArch | May 23, 2017 09:52pm | #14

    Thanks for that Dana. Good information in the article. It seems there is a constantly revolving door of do's and don'ts in building sciences. It makes it difficult for people like me to make the right decisons and build something that will function well for a long time. It appears i could use either of these options and though not traditional, would still likely work just fine. I appreciate the time you and martin take to answer the questions i throw out there!