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Community and Q&A

Overinsulating or underinsulating?

bren77 | Posted in Energy Efficiency and Durability on

We have a home built in 1986 in Columbus, Ohio (Zone 5). We are replacing our siding due to hail damage and plan to have the following layers on our home:

1. drywall
2. 2×4 walls with unfaced batt fiberglass insulation (R?)
3. polyiso 1/2″? – (old, some gaps, foil faced, says “R3.6” on the face)

4. 3/8″ EPS w double foil face (~R1.5) –
5. Tyvek
6. Cedarmax insulated vinyl siding (R2.4) –

Does this sound like an ok plan? Ideas? Concerns?

For #4 we are also considering 1/2″ or 5/8″ — they only cost about $500-700 more.


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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone, you need at least R-5 of rigid foam for the exterior side of a 2x4 wall. (Here is a link to an article that explains why: Calculating the Minimum Thickness of Rigid Foam Sheathing.) So R-3.6 + R-1.5 is adequate.

    However, thicker foam (and a higher R-value) would always be beneficial.

    Insulated vinyl siding doesn't perform as well from a moisture-management perspective as ordinary vinyl siding, because the molded foam insulation that is adhered to the siding takes away the beneficial air gap between the siding and the WRB or sheathing layer. If I were you, I would specify ordinary vinyl siding, not insulated vinyl siding. If this change in specifications resulted in savings, I would invest the savings in thicker rigid foam.

  2. charlie_sullivan | | #2

    Also note that the "radiant barrier" properties of the foil-faced EPS are only useful if you have an air space facing the foil. You don't have an air space on either side of it, as I understand it. So I'd buy ordinary EPS, which will get you a thicker layer and higher R-value for the same price.

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