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Seeking advice on siding and moisture...

I recently noted moisture on the interior of my plywood sheathing in cold winter weather (doing a wiring repair that had me access the interior wall). This is probably not surprising. I am in cold zone 6 and have standard wall structure of about 35 years ago; -sheetrock, 3.5" fiberglass with either kraft or foil facing stapled on to studs, plywood sheathing, a building paper/wrap (Tyvek, I think), and then cedar shingles at 5" exposure, stained. It did not appear the sheathing was much along toward rotting though, fortunately, but that was just one area.

I'm now thinking of residing, in part due to moisture issue but also due to maintenance of keeping shingles stained at my age now and with a two story house. But now I'm stumped as to what would work and be an upgrade. I'd like to use rigid foam outside the sheathing (for moisture control and more R), but per Martin, none is better than not enough in a case like this. And "enough" is R-7.5, which translates to 1.5" of xps, or more, since xps loses R value over time (eps might be better here but more depth is needed for R value). I was considering vinyl for it's low maintenance. Installers usually use half inch of rigid foam below the vinyl, which in my case would seem to be a negative (not enough). If I were to consider more foam, what in the world does one do about the added depth for windows and doors? Some of my windows could be fairly replaced, but some are good vinyl windows only a few years old. No way I would want to discard those.

Has anyone had a similar experience or useful advice?

Thanks so much.

Asked by Howard Gentler
Posted Fri, 01/17/2014 - 11:47


10 Answers

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Interview Vinyl siding companies letting them know the amount of R value you desire. Hire the one that has experience doing as you desire since you are not doing the work yourself.

In my area the installers do know how to retrofit vinyl with added insulation.

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 01/17/2014 - 12:16

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Any good trim carpenter with some flashing experience should be able to install exterior trim for your windows. It's hard to describe every step, but basically your windows need exterior jamb extensions (which are similar to interior jamb extensions).

You also need a sill extension for each window, and the sill extension must be watertight; most installers use metal flashing. It's important to try to get the interior edge of the new metal sill under the existing sill if possible, and to turn up the flashing on the ends so that the installation is watertight.

Use water-resistant materials for your jamb extensions -- perhaps cellular PVC trim.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/17/2014 - 12:33

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Thanks Martin and AJ. Your responses are helpful and a relief. I had thought the whole window thing would be more of a problem than that (not that that won't be considerable work and expense). I thought that the windows worth reusing would have to be removed and put back in to be suitably integrated with the siding. I think this can be done with the windows with vinyl flanges more easily than wood frames, but never an easy thing.

Answered by Howard Gentler
Posted Fri, 01/17/2014 - 16:08

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By the way, what about the R factor? What would I need for depth, given the loss of R factor with xps?

Answered by Howard Gentler
Posted Fri, 01/17/2014 - 16:09

Helpful? 0

Q. "What about the R factor? What would I need for depth, given the loss of R-factor with XPS?"

A. In your original question, you mentioned using R-7.5 foam. That matches the minimum R-value for exterior foam on a 2x4 wall in your climate zone, as recommended in my article (Calculating the Minimum Thickness of Rigid Foam Sheathing).

Unlike polyiso, the thermal performance of XPS does not degrade at low temperatures. In fact, the thermal performance of XPS improves slightly at low temperatures.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/17/2014 - 16:31

Helpful? 0

The R-value of XPS falls with TIME not temp, as the blowing agents escape. At year 50 expect no more than about R4.3/inch @ 75F, making that R7.5 rated foam more like R6.5, barely more than EPS of the same density.

EPS is made of the same polymer, has a stable R over time and offers a higher drying rate toward the exterior.

On 2x4 construction in a zone 6 climate figure on 2" of type-II (1.5lbs density) EPS. At 2" it will run about 1.5 perms which is somewhat higher than the vapor permeance as 1" of XPS. At 2" it'll be R8.4, sufficient R to protect the sheathing from a dew-point considerations, yet sufficiently permeable that it won't matter for the sections of wall that may have foil faced (highly impermeable) batts, avoiding the moisture trap. It would also roughly double the thermal performance of the walls.

If that's too expensive for your budget, the half-inch underlayment approach will probably be fine from a moisture point of view as long as it's 1-perm or greater (which means nothing with vinyl or foil facers.)

Answered by Dana Dorsett
Posted Fri, 01/17/2014 - 16:53

Helpful? 0

Number crunchers verses reality. I insulated a home exactly as being discussed here. Vinyl siding over 1.5" fanfold EPS. The savings in energy use is not as great as my next experiment in using open cell spray foam.

Air sealing.... Must be done well to make progress in reducing energy costs.

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 01/17/2014 - 19:28

Helpful? 0

AJ; at this early stage I was thinking 2" of EPS would be most appropriate. I had to look up the "fanfold". Interesting idea, -I couldn't tell if it came in 2". If I use xps I would likely also go with 2" to cover the loss of R value over time, wanting to wind up with 7.5 if possible.

I did other research regarding Martin's comments about windows, and found much good stuff, including Martin's own article on "innie" and "outie" window installation and retrofit when using foam or other thick walls. It sounds like no great advantage one over the other, -perhaps mostly appearance.

Answered by Howard Gentler
Posted Sat, 01/18/2014 - 11:13

Helpful? 0

Oh, AJ, is the open cell foam idea for a retrofit? -and how would it be applied? It would seem like a challenge to spray on the outside, especially because there seems to be a need for a flat, uniform surface which would be difficult with sprayed foam.

Answered by Howard Gentler
Posted Sat, 01/18/2014 - 11:16

Helpful? 0

Howard, to really notice a reduction in energy use I find much more has to be done besides adding rigid foam to a home. Use the search feature.

Answered by aj builder, Upstate NY Zone 6a
Posted Sat, 01/18/2014 - 13:24

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