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Community and Q&A

Adding wall R-value in a historic retrofit

akhuntia | Posted in Green Building Techniques on

What options are available for adding r-value to a wall if we can’t remove the exterior siding? I see three options…
1) dense pack cellulose in the existing 4″ wall cavities (~R 14-16)
2) Closed cell spray foam in existing 4″ cavity (~R 26-28)
3) Add a double interior wall filled with 7.5″ of dense pack cellulose (~R 26-28)

I am discouraged from pursuing superinsulation levels of the latter 2 options due to cost and the fact that i would then need to address the thermal bridge caused by the original solid wood double front door. I am planning on replacing windows with R5+ units and taking steps to remove all penetrations in the outer wall like electrical outlets or switches but i don’t think i can do much else without substantial expense. Are there any other options i am missing for wall insulation?

thanks for any comments.

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  1. GBA Editor
    Martin Holladay | | #1

    It's also possible to install a layer of rigid foam on the interior side of your walls -- if you want.

    The fact is, the only one of the listed options that is likely to be cost-effective is dense-packing the wall cavities with cellulose insulation. The cost of the other options (like the cost of window replacement) will be so high that you will never save enough energy to justify the cost of the work based on energy savings.

  2. akhuntia | | #2

    Thanks Martin, I hadn't considered that.

    How would the rigid foam be installed? Screwed with taped seams? Would the drywall then be installed with long screws through the insulation to the studs? If the foam were glued and screwed, would that provide any noticeable rigidity to the structure?

    Could i apply two layers of rigid foam? I am assuming 2 layers of foam would be easier and less expensive than a new stud wall.

    This additional insulation is part of a large retrofit-addition which will be taking a 1600sf home to about 3000sf and there is some budget for energy improvements and modernization. So i would consider some improvements that aren't offset with energy savings. I am just trying to find the right balance of cost vs, benefit.

    This brings up a larger question...
    The addition will be about 40% of the perimeter of the remodeled house. the addition will be completely new from foundation up. I expect that it will be pretty easy to achieve whatever R-value we want in these new walls. If the new walls were a R40 and the old walls were R-16, is that the same or worse than a theoretical R25.6 in all walls? [(.40*R40 +.6*R16) = R25.6 ]

    Will there be air flow problems caused by the imbalance in insulation?

    Regardless of wall insulation, we will be shooting to achieve R60 in all roof areas.

    I attached some of my own concept drawings to provide an idea of the addition.

    Thanks again for your comments and suggestions.

  3. Dana1 | | #3

    Before insulating the wall cavities it's important to know the wall stackup in more intimate detail. If the siding is nailed directly to the studs, without at least plank sheathing (and preferably a layer of tar paper or rosinpaper between the siding & sheathing) filling the wall cavities with ANY type of insulation will cause the paint to fail in very short years, followed by siding rot down the road.

    The window flashing details are also important to consider before insulating the cavities.

    The implied performance benefit of closed cell foam cavity fill over cellulose based on the center-cavity R value is deceptive. Even though the center-cavity R nearly doubles, it only adds about R2 to the "whole wall" performance due to the R1.2/inch thermal bridging of the framing for ~25% of the wall's cross sectional area. (A 25% framing fraction is typical for 2x4 16" on center framing, after you add in the bottom top plates, window framing, jack studs, etc.). Save the high R/inch foam money for continuous insulation that insulates over the framing as well as the center-cavity. It only takes 1/2" of continuous EPS to raise the whole wall performance of a cellulose filled wall to that of a 2lb pour of closed cell foam.

  4. akhuntia | | #4

    IIRC, when i cut out for a bathroom vent last year, the wall consisted of this..
    stud -> plank sheathing -> some kind of paper -> wood lap siding
    So it sounds like dense cellulose might be appropriate for this case.

    When you say continuous insulation, you are referring to an interior installation, right?

    Your guidance on the effective whole wall performance is very helpful, thanks!

  5. akhuntia | | #5

    I was intrigued by Martin's mention of interior rigid insulation for my application. After digging around for a bit, i can't find any examples besides basement retrofits where rigid insulation was used on interior walls. Can anybody point me to some info/examples? I would think it would be ideal for retrofits if the exterior can't be dismantled.

    If i went with interior rigid insulation, i have two questions...
    1) I think EPS is best for me because i want to allow drying to both interior and exterior, correct?
    2) (repeat of earlier question) is there a problem if half of the homes exterior walls are r40 and the other half are < r20? Is it all about the average R or does an imbalance cause other problems?

    thanks again.

  6. GBA Editor
    Martin Holladay | | #6

    The main reason that you are having trouble finding examples of above-grade walls with interior rigid foam is that exterior rigid foam is preferable in almost all respects to interior rigid foam. That said, the use of interior rigid foam sometimes makes sense for a retrofit project like yours.

    Q. "I think EPS is best for me because I want to allow drying to both interior and exterior, correct?"

    A. EPS will work, but so will other types of rigid foam. This type of wall assembly is designed to dry to the exterior, which is the traditional approach for cold-climate walls.

    Q. "Is there a problem if half of the home's exterior walls are R-40 and the other half are less than R-20? Is it all about the average R, or does an imbalance cause other problems?"

    A. There won't be any problems, other than the obvious one (your R-20 walls will lose heat at twice the rate of your R-40 walls).

    You shouldn't think in terms of an "average R-value." If you want to perform a heat loss calculation for this house, you should consider the R-20 walls separately from the R-40 walls.

    There is at least one GBA article on this topic, as well as dozens of Q&A threads. Here is a link to the article: Second Guessing an Insulation Upgrade.

    Here is a sampling of some of the countless Q&A threads on this topic:

    Rigid foam on the interior face?

    Rigid foam on inside of stud cavity?

    Can I add foam insulation to the inside of a wall instead of exterior?

    Renovating kitchen: Interior rigid foam an option?

    Interior rigid foam installation

    Interior rigid foam

    Benefits of exterior foam vs interior foam

    Interior Foam Insulation vs exterior

  7. akhuntia | | #7

    Thanks Martin! Clearly I was too impatient in my searching....

  8. Expert Member
    Dana Dorsett | | #8

    For interior side foam with a stackup that dries reasonably to the exterior you get a lot more bang per inch with polyisocyanurate (usually foil-faced) than with EPS or XPS.

    If there is plank sheathing and some sort of weather-resistant felt/paper behind the siding it should indeed be dense-packable. You may want to carefully inspect for window flashing though, especially if the roof overhangs aren't very deep, since many old houses built with empty wall cavities still tolerate occasional bulk-water leaking in at the windows due to the high drying rate of the empty cavity. If you fill those cavities the drying air-leak that has been saving it slows to a trickle, which raises the importance of proper window flashing to direct any liquid incursions out past your sheathing.

  9. akhuntia | | #9

    After reviewing the links mentioned (thanks again Martin) I feel quite confident in this interior rigid insulation for my application.

    Would a dimpled plastic sheet like those used on foundation walls be helpful if applied to the interior side of the plank sheathing In the joist bays? I am very nervous of trapping water in the walls and think this might be helpful to allow the sheathing to dry.

  10. GBA Editor
    Martin Holladay | | #10

    You don't want to put any kind of plastic sheet on the exterior side of your wall assembly. To keep your stud bays dry, you want moisture to be able to dry to the exterior. The exterior layers of your wall assembly should be as vapor-permeable as possible.

  11. Expert Member
    Dana Dorsett | | #11

    If you have a lot of knot holes and open seams in the plank sheathing to contend with, take a roll of vapor permeable housewrap and wrap the whole interior side, wrapping over the studs and stapling it to the planks. It doesn't have to be super-flat and taut as a drum and doesn't need a lot of staples- the dense-packing will deal with the voids just fine, but it forms an exterior air barrier and restrains the cellulose from contact with the siding layers. The cellulose will still be able to share the moisture burden with the sheathing, but won't wick as quickly as if it were in direct contact with the wood.

  12. akhuntia | | #12

    Thanks guys for both comments. I will make sure to keep them in mind when we open the walls.

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