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Is R-1 fan-fold insulation cost-effective as vinyl siding underlayment?

RCon54 | Posted in Energy Efficiency and Durability on

I am installing vinyl siding on a home in Pittsburgh PA as part of a HUD financed project. As such, HUD has supplied the specs. The requirement is for R1 fanfold followed by Tyvec. I think that the principal function of fan fold has traditionally, pre-house wrap, been to limit air infiltration (not provide R value), a function that is in this case served, redundantly, by the by the Tyvec.
There is almost no wall insulation and no plans to install any soon but there is adequate attic floor insulation.
I think that the money for the fan fold (.50/sq.ft +/- for 3500 sq.ft.) could be better spent.

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  1. dvaut | | #1

    Fan fold insulation on a reside functions to smooth out a wall when siding over wooden siding or rough surfaces. I does have minimal R-value but that is not it's main benefit. I guess it helps air sealing slightly but that would also depend on the taping detail and how you seal the top, bottom, and sides of where the foam ends. Again I think providing a smooth flat surface for the siding is it's main function.

  2. GBA Editor
    Martin Holladay | | #2

    If this is a HUD-financed project, and if HUD requires that you follow specs, then our opinions are irrelevant. If you want the HUD financing, you follow the specs. I can't imagine that your opinion or my opinion is going to sway the federal government. I've heard of worse government requirements than this one.

    I'm intrigued by your statement, "There is almost no wall insulation and no plans to install any soon." It sounds as if the owner is missing an opportunity to improve the performance of the walls -- but since we know little about the project, we don't really know who is making these decisions, or anything about the budget or purpose of the project.

    Are you the siding installer? General contractor? Architect? Owner?

  3. Chaubenee | | #3

    You may gain a bit of noise reduction as well.

  4. Expert Member
    Dana Dorsett | | #4

    With fan-fold underlayment in a climate zone 5 location you have to be careful to use perforated types to have sufficient drying capacity for the sheathing. Most unperforated XPS or EPS products have a thin plastic or foil facer, which lowers the vapor permeance considerably. If it's ASTM E96 specification is less than 1 perm it has the potential to become a problem.

    But in a zone 5 location any R5 insulation retrofitted onto 2x4 framed building is pretty safe.

    Re-siding is an opportunity moment for fixing the deficiencies of the wall insulation. The stated condition & planning of "...almost no wall insulation and no plans to install any soon..." could be passing up the lion's share of thermal improvements that could still be economic. In a wood framed building filling the cavites with cellulose (even low density, if the interior walls are too fragile to dense-pack) drilling from the exterior is usually dirt-cheap, and would be more effective at infiltration reduction than adding perforated fan-fold or a sheet of Tyvek.

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