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What is a correct way of fastening two layers of 2″ rigid insulation to the exterior of a 2×6 wall?

jumanji | Posted in Green Building Techniques on

I would like to know the correct way to fasten two layers of rigid foam insulation where the total thickness is 4″. I am concerned that when hanging this thickness of foam it will be difficult to be sure that the siding application is secure. We will be using HardiePlank horizontal lap siding. This is a form of cement board and heavy on it’s own. Any advice.


  1. GBA Editor
    Martin Holladay | | #1

    The insulation should be attached with cap nails.

    The siding should be attached to vertical furring strips, either 16 inches o.c. or 24 inches o.c., with each furring strip located directly over a stud. The furring strips are screwed through the foam to the studs with long screws.

    For more information, see these articles:

    Fastening Furring Strips to a Foam-Sheathed Wall

    How to Install Rigid Foam Sheathing

  2. fitchplate | | #2

    Go to Wind-Lock and look at the Wind Devil product, the various installation methods, materials and tools. They have great (stainless) cap screws (don't use nails, you will destroy the XPS) and the cordless driver chucks that make installation a breeze. But don't forget you also need a seat for the foam that allows bulk water to drain out, keeps out critters, and lasts: Look at Wind Locks "universal starter track". They also have the seam tape (avoid asphalt based tapes) you need to make that rigid insulation assembly airtight at all edges, openings, etc. Don't use wooden furring, it is conduction bridge; use T&G edges and offset the layers to cover seams. The cap screws and starter strip base will keep things in line and tight. Use quality and flexible seam tapes: most builders tapes will not stay on rigid foam over time. Use a roller to seal the tape contact. Good but very expensive (but very good) tape can be found at FourSevenFive.

  3. jumanji | | #3

    Thanks so much for the information. I will follow up and check into Wind-Lock products. I have a fear of hanging this depth of foam on the exterior of the home.

  4. jumanji | | #4

    Thanks for the help and information. i am concerned about hanging this much rigid foam on our new home. We are going to build in southeastern Pa. I am thinking that maybe two inches may be sufficient.

  5. GBA Editor
    Martin Holladay | | #5

    If you are trying to determine how many inches of rigid foam you need to install on the exterior side of your wall sheathing, then you should read the following article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

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