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Does the housewrap go over or under the rigid Foam insulation siding?

Hotsauce1183 | Posted in Green Building Techniques on

This website is awesome and I have spent many hours browsing through articles. None of my builder friends, myself included, have ever used rigid foam panels under the siding of a home under retrofit. Consequently i have kept my eyes open for the answer to the above question but the solution has still alluded me. I am planning on replacing all of the siding and windows in a circa 1970’s home that I recently acquired and I thought that this will be an excellent opportunity to get my feet wet with applying rigid foam panels under the siding. After much study, and all things considered, I have decided to use two layers of 1-1/2″ EPS. As a greenhorn who has never employed this method before here is what I and thinking of using:
a. Before proceeding, the current siding is also the shear panel. So I will necessarily have to install a new layer of ~1/2″ OSB.
1. Over the top of the OSB will go 2 layers of 1-1/2″ EPS IF I can find a foil faced version. I was also considering Foil-Faced Polyiso but here in zone 5 with a nominal 15 days of 10 degree weather per winter I am concerned about the cold weather performance of Polyiso. Plus there is also the cost factor.
2. I will either tape all of the seams using a quality tape like Dow Weathermate or one of the Venture products. Any other recommendations would be greatly appreciated.
3. Alternatively, many of the articles I have read on this site recommend using one of the many canned spray foams to seal the seams. So, What is the current thinking: Tape, Foam, or a combination of the two.
4. On top of the rigid foam panels I will install 1×4 furring strips using long washer-head screws. They are kind of expensive but it seems to me that this isnt the place for plastic capped nails or screws.
5. Attached to the furring will be something on the order of SmartSide… I think that is a PPG or OC product.

The one big question still remains: if I use housewrap where does it go and is it even necessary?

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  1. user-2310254 | | #1
  2. PAUL KUENN | | #2

    Good day Roger!

    See my GBA blog from 2016 "One man's quest for energy independence". That should answer many of your questions. If you do innie windows (where they belong if you want to have warm windows in winter and cooler in summer), then keep that WRB (house wrap) on the inside of the insulation. You'll find ripping plywood cheaper than buying pine 1x4s and they won't crack.

    Good luck, PK

  3. user-2310254 | | #3

    Where are you located? Advice is climate-specific. You also might want to look into DensElement. It has an integrated WRB and air barrier.

  4. Hotsauce1183 | | #4

    Sorry, I should have made that clear. We are around 5000 feet in zone 5. The climate is high mountain desert so it leans more toward dry and arid than wet and humid. We do get rain and snow so my planning should anticipate 4 seasons.

  5. Hotsauce1183 | | #5

    PK - thanks for the reference to a very informative blog. And, yes, I will most certainly take your advice on ripping plywood.

  6. user-2310254 | | #6


    You are shooting for R-20 of exterior insulation, correct? That is what you need for moisture control in Zone 5.

  7. GBA Editor
    Martin Holladay | | #7

    As Steve Knapp pointed out, the answer to your last question can be found here: Where Does the Housewrap Go?

    Most brands of EPS have an R-value of about R-4 per inch, so 3 inches of EPS has an R-value of R-12. That's plenty of R-value for 2x6 walls in Zone 5. Steve Knapp is wrong about the need for R-20 foam; all you need is a minimum of R-7.5 for 2x6 walls in Climate Zone 5. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    As you read GBA articles on topics that interest you, don't forget to check out the links in the "Related Articles" sidebar that appears in most articles. Here are links to two other articles you may want to read:

    How to Install Rigid Foam Sheathing

    Installing Windows In a Foam-Sheathed Wall

  8. user-2310254 | | #8


    Thank you for catching that error. Apparently, I thought Roger was working on his roof, which he was not.

  9. Hotsauce1183 | | #9

    Thanks Martin - great information. One thing though... my 1975 house only has 2x4 exterior walls with R11 fiberglass bats loosely stuffed between the studs.

    My plan, which has been evolving, is to remove the 40 year old hardiboard siding which has reached the end of its life here in the high Nevada desert climate and replace it with the following composition in the order shown:

    [Original] Fiberglass Bats R11
    1" Foil Faced Polyiso R6
    3/4" Air Gap created with 3/4"x1x4 furring (R2) (I have read on this site and elsewhere that when an airgap is combined with a foil faced something on one side and siding on the other the resulting R-value is equivalent to ~R2.)
    LP SmartSide
    Note that I did not specify any housewrap as I am proposing to use the polyiso with all of the seams taped and/or glued and all of the edges properly flashed as the WRB.

    The above plan will result in a combined R-value of R19 and should yield a strong and air-tight WRB. I wish that I could get foil-faced EPS but it is simply not available here in Northern Nevada, hence the Polyiso choice.

    I am still struggling with the decision of 1" polyiso at ~R6 or 2" rated at R13. I would be very interested in hearing the recommendations from the experts on this site.

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