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Community and Q&A

Wall Insulation Retrofit Best Practices

dsummers | Posted in General Questions on

My house is next to multiple houses that burned to the ground in the recent Marshall Fire in Colorado.   My house is a spec home built in 1992.  Near Denver.  Climate zone 5B.  Cold in the winter  (a few days below 0F)  and hot in the summer ( a few days above 100F).

Due to the fires next to my house, the wall insulation is contaminated with soot and ash and needs to be replaced.  My local building department is telling me that if I pull out the smoky fiberglass batts, I have to upgrade to current building code of R21.

I am not 100% sure where they are getting the R21 number, since it seems the the current code for my zone is R20 or R13 + 5.  The building department is very adamant that R21 is their requirement.

My house currently has 2X4 walls, therma-ply (carboard) sheathing, and fiberboard lap siding.   No WRB unless you count the thermaply.   R13 fiberglass batt in the walls.  I have a 6mil poly vapor barrier behind the drywall.

While I am at it, I’d like to upgrade the siding to Hardie, replace the cardboard sheathing with OSB, and add a real WRB.  Therefore it seems best to fix the insulation from the exterior by removing the siding and sheathing.  That way I can achieve my other goals and not disturb the drywall and tile on the interior.

What is the best “bang for the buck” to meet R21 in a 2X4 wall?

1. Closed cell spray foam, sprayed from the exterior?  I realize that this isn’t the optimal solution from an energy point of view.  It does nothing to address thermal bridging of the studs.  I think that it is the cheapest and easiest option.    Are there any caveats to spraying foam from the exterior side?  It seems more common to take down the drywall and spray from the interior.  Do I need to remove the poly vapor barrier, or just have the contractor spray foam onto the poly?

2. R15 fiberglass in the wall cavity with ZIP R6 on the exterior.   I like this system the best, but the price quote I got for this was outrageous, about $10/sq ft installed.  What should ZIP R6 cost in the Denver area per square foot?   This would also require a lot of extra work to work around windows, doors, hose bibbs, etc to accommodate the extra thickness which adds to the cost.

3. R15 fiberglass in the walls, OSB, then R6 foamboard on the exterior of the OSB sheathing.   I’ve read enough to be worried about moisture with this one.  Is that as much of a concern in a place like the Denver area where we have really low humidity?  Is there any advantage to this over the Zip system other than (possibly) reduced cost?

4. Other options?

Since this is a completely unplanned remodel, I am not looking for the absolute best solution.  I really just want to meet the local code while getting the best value out of my money.   Your help would be greatly appreciated.


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  1. Expert Member


    If y0u are replacing the sheathing, I wonder if the simplest thing to do might be fur out the studs with 2"x2"s giving you a 5" cavity. Filled with R-21 high density batts the wall would meet the city's requirements.

  2. wantone | | #2

    Hi David, the easiest way would probably be to blow in cellulose from the exterior not sure if you’d hit 21. You’ve described a larger project so if you are pursuing that I would recommend using rockwool insulation in the studs and CDX plywood sheathing wrapped in a high quality WRB like solitex not sure it’s the “easiest way” but it sounds like you’ve already settled in to replace the siding

  3. Expert Member
    PETER G ENGLE PE | | #3

    David, It is possible that the city has adopted somewhat different energy codes than the I-code series standard. I generally have success with building departments when I ask them to show me the requirement or point to an online document source. Sometimes they point to their own document that shows they're wrong and once in a while, they even admit it. Otherwise, you've learned something.

    If you are considering an exterior insulation approach, #3 is perfectly safe in your climate zone. The exterior insulation will keep the sheathing warmer in winter, which is good. Yes, technically, the sheathing may still get cold enough once in a while to see condensation, but that will be rare in practice, and the very dry conditions will allow it to dry out just fine. Very good air sealing, even on the exterior, will help to minimize air-transported moisture which is often the major actor in condensation. Also, keeping your indoor humidity on the lower side (below 30%) in very cold weather will minimize the potential for condensation.

  4. Expert Member
    BILL WICHERS | | #4

    Option #1 isn't a good one, especially with closed cell spray foam (ccSPF). With ccSPF, you typically underfill the stud cavities to avoid the need for extensive trimming of the spray foam. If applied from the exterior, this means you'll have around a 1/2" gap between the insulation and the sheathing. The gap robs R value, but it's also more of a problem if on the "cold" side in this case due to the need to apply from the exterior. Open cell spray foam (ocSPF) can be completely filled and then trimmed flush, but you'll end up with an R value close to that of batts. I would not use spray foam here.

    Option #2 is a good one, but Zip is a premium product and it's priced accordingly. It's a very good product though if you want to spend the money. Zip is basically a high quality sheet of OSB with a factory laminated sheet of polyiso (in the case of Zip-R), and the green WRB coating on the exterior side of the OSB.

    Option #3 is probably the best cost/performance option here, especially if you use 1.5" EPS to get to your target R value since that will probably be cheapest. 1" polyiso should also be priced out though. Polyiso is sometimes cheaper per unit R value compared with EPS, but not always, at least in my experience, and there have been some availability issues lately too.

    Malcolm's option is also worth considering. You could tack up 2x2s fast with a nail gun, and when you're done, you'd hang your siding in the usual way (no complications from exterior rigid foam), but it doesn't perform as well as options #2 and #3 since it won't get you a layer of continuous insulation to help with thermal bridging.

    I would probably remove the interior layer of poly while doing this project. A utility knife will make fast work of it, and it doesn't have to be perfect -- just remove the bulk of the poly and you eliminate a lot of the moisture trap issues you would otherwise have if you use something like foil faced polyiso on the exterior.


    1. dsummers | | #10


      Could you elaborate on "The gap robs R value, but it's also more of a problem if on the "cold" side in this case due to the need to apply from the exterior."

      Why is it worse if the air gap is on the cold side? Reduction of R value, or moisture issues?

      1. Expert Member
        BILL WICHERS | | #11

        Mositure is more likely to accumulate on the cold side between the spray foam and the sheathing. Moisture is less of an issue if the gap was on the warm side of the assembly. There is also more chance of issues with insects building nests in the gap if it's on the exterior of the assembly.


  5. walta100 | | #5

    “Due to the fires next to my house, the wall insulation is contaminated with soot and ash and needs to be replaced”

    Is this a real thing or someone’s wild thought?

    If it is a real thing of course your home owner’s insurance is going to cover the cost.

    From a dollars and cents point of view it seems unlikely the old insulation could be comprised enough that replacing it would make financial sense.

    I suspect that if you modeled a home with R15 and R21 wall insulation the fuel costs saved per month would be less than $5. Generally, you need to double the R value to have a measurable affect.

    Can you smell the smoky insulation in the house?

    After such a close call with fire I am surprised, you would be considering any flammable insulation products like any flavor of foam.


  6. huey_ce | | #6

    Has your jurisdiction adopted the International Existing Building Code? Provisions in that code typically allow you to repair damage with similar materials rather than forcing code upgrades as long as the materials are not creating a dangerous situation. In theory, you should be able to repair your home without incurring prohibitive expenses because codes have changed.

    That being said, a stubborn building official can stamp their feet and demand what they want, I've been there before. Also, if you were thinking of making a similar upgrade in the future, this would be the time to do it.

    As a thought experiment, say your neighbor came home drunk and parked their car in your living room. You should be able to fix the wall and reinsulate with similar material and get on with your life. I don't think its right to tell the homeowner that well because energy codes have changed you need to upgrade all of your insulation as part of the repair. In your case, the damaged area is just larger.

    1. dsummers | | #13

      I have had meeting with the building department and they have refused to give an exception. My town recently adopted IECC2021 + NetZero. I think that the R21 requirement is already a waiver in that they are not holding me to the zone 5 requirements of R20+5 or R13+10 in the 2021 code. And yes, this is a good opportunity to improve the efficiency of my house. The original builder did some questionable things like using an exterior wall stud cavity as an HVAC supply duct. Adding exterior foamboard will at leas provide some insulation for this duct.

  7. dsummers | | #7

    Thanks for all of the suggestions.

    As for the smoke damage, yes you can smell the odor coming in through the electrical boxes on the exterior walls. It is also me being cautious. If it was just wood ash I might be willing to do some air sealing leave it there, but since it is full of burned plastic, car tires, etc I'd rather not have it in my walls at all. Insurance should pay for the insulation too, but I did not have a "building code" rider on my policy so I don't know if they are going to pay for the code upgrade. They may only pay for the cost of R13 fiberglass batts. (This is still being negotiated 3 moths after the fire)

    I had a meeting with my local building department and they are not giving me a waiver. I would also just like to make my house better while I have the walls open.

    My local lumberyard can get ZIP R6 for $146 per sheet (March, 2022) but it is out of stock at all of the distributors.

    At my local big blue box store, regular sheet of OSB is $55 and a sheet of 1" (R6) ISO foam is $24 for a total of $79 per 4X8 sheet. That is a half the cost of the ZIP system!!

    I am leaning towards foam board exterior to the OSB. It sounds like that will be a better solution than closed cell spray foam and half the cost of the ZIP-R.

    As for furring out the studs to 5.5", I think the extra two inches would start o give me problems in a few spots where the siding meets some brick veneer details. I think that it will still look OK with 1" extra thickness (really 1.375" with the extra sheathing thickness), but 2" will be a problem.

    I have a couple of follow on questions:

    Is it OK to install Hardie Plank lap siding right on top of the exterior foam, or would I need to add furring strips and further increase the wall thickness?

    Would I still want to put house-wrap under the siding?

    I am also replacing all of my windows during this project. All of the vinyl windows are showing some signs of heat damage from the fire (warped frames, cracked glass, broken seals). Is the best way to mount the windows to fur out the window framing to match the thickness of the foam? Then mount the windows flush with the foam and weather seal the window to the foam? That seems better to me than mounting the windows to the OSB and then adding foam and using flashing/trim to cover up the foam.

    Thanks again for all of your help.


    1. Expert Member
      BILL WICHERS | | #8

      Remember that Zip also includes a WRB, and the OSB used in the Zip panels is widely regarded to be superior quality compared to the regular OSB you find in the box stores. To really have an apples to apples comparison, the closest you can do is to use a fluid applied WRB with your OSB/polyiso stackup. Note that plywood has been cheaper than OSB at times during the past few years of supply chain weirdness, so it's worth checking plywood prices prior to committing to using OSB. Plywood is better than OSB for exterior siding, OSB is only used due to it usually being lower cost.

      You can save some money by taping the polyiso seams (I like to use Nashua 324A tape for this purpose), and letting the foil facing double as your WRB. There are both pros and cons to this, but it is doable.

      You need furring strips to install the hardiplank siding. Hardi's documentation specifies what you need, but typically that means 1x4s or ripped strips of plywood. I prefer ripped strips of 3/4" plywood.


      1. dsummers | | #9

        This tech bulletin from James Hardie says that you can skip the furring strip if the foam is 1" or less. Of course "allowed" and "good idea" are sometimes not the same thing. Any problems with this as long as the manufacturer says that it is OK?

        1. Expert Member
          BILL WICHERS | | #12

          The issue with the manufacturer is mostly to keep their warranty. If they say it's OK, then they should honor their warranty. I don't think anyone on GBA would recommend installing right over the foam though. I think you'll have a lot more issues with cracking and blowouts on the fiber cement planks installing it right over a relatively squishy material like rigid foam. I would recommend the use of furring strips.


  8. pico_project | | #14

    If fire was a known issue, I would be wrapping my house in Rockwool.

    1. paulmagnuscalabro | | #17

      One more vote for this; we're seeing it used more and more up here in Montana for both its fire resistance and for it being just a bit less environmentally damaging to produce than foam boards (at least that's my understanding - please feel free to fact-check or correct me if I'm off-base).

      That said, it is usually a bit more expensive, and lately has been a little tougher to source.

    2. dsummers | | #18

      Isn't polyiso foam relatively fire resistant, at least compared to other foams? I agree that the rockwool would be better. I'll look into it.

      This fire was so intense that I don't think anything short of a concrete bunker would have survived. I just had a lucky combination of wind direction, and the fire crew showing up just in time.

      1. Expert Member
        BILL WICHERS | | #19

        Polyiso doesn't support fire spread in the same way as EPS and XPS can. Rockwool, however, is fireproof -- it just doesn't burn.


  9. dsummers | | #15

    Thank you all for your suggestions. This is a lot of good advice to process.

    Could someone tell me (or point me to a reference) the effective R value of the following assemblies including heat loss through the framing?

    All studs are on 16" centers, and all assemblies have 7/16" OSB or plywood

    1. 2X4 wall with 3" of closed cell spray foam.
    2. 2X4 wall furred out to 2X6 with R21 fiberglass batts
    3. 2X4 wall with R15 batts and R6 exterior insulation (foam or rockwool boards)

    I know that the exterior insulation is going to be the best, probably the furred out 2X4 wall next, and the spray foam last... in terms of just R value.

    1. Expert Member
      BILL WICHERS | | #16

      Option #1 will give about R16.5 whole-wall performance, assuming R6/inch for the spray foam.
      Option #2 will give about R19.4 whole-wall performance
      Option #3 will give about R19.8 whole-wall performance

      I'm using R1 per inch for the wood in the studs, and I'm using a simple average for the thermal bridging of the studs, which will slightly OVERestimate the whole-wall R value, but that doesn't matter much for comparison purposes since the error is the same in option 1 and 3, and slightly worse in option 2. Note also that "R6 per inch" for spray foam is a bit optimistic in the real world, since you'll never get a perfectly even application.

      The reason the exterior insulation does so well is that it's continous insulation, so it's R value isn't reduced by thermal bridging.


      1. dsummers | | #20

        Thanks again Bill. I really appreciate your help (and everyone's help).

        How would you rank these three options in terms of moisture (and mold growth) risk? We do run a whole house humidifier in the winter and keep indoor humidity at 30%. Denver, zone 5B.

        3" of spray foam is by far the simplest solution since it doesn't require the walls to get thicker. It has a marginal increase in R value for my house, but will greatly tighten up the house by air sealing. I have not been able to get a price quote for spray foam yet, so I can't really compare costs. I'm guessing that it will be about $3/sq ft but that is just a guess based on google results. You had said above that the spray foam solution has condensation risk due to the 0.5" air gap between the insulation and the sheathing. Isn't this OK since the water can dry to the outside through the OSB and housewrap (currently spec'd as HardieWrap by my siding contractor)? Is this assembly any more risky than an old school 2X6 wall filled with fiberglass batts with no exterior insulation?

        The external foam board solution is going to be the cheapest in terms of materials, but there is a lot of extra work just accommodating the increased wall thickness which will probably make the cost close to the spray foam at the end. I do like the higher R value, and the fact that it will add some insulation to the exterior stud cavity that the original builder used as an HVAC supply duct (yes, they really did that). I am very worried about moisture with this solution but also don't want to go thicker on the foam since the siding has to match up with some brick work on the front of my house. Going to more than 7" total wall thickness will be hard to accommodate on the exterior.

        I like the simplicity of just furring out the walls to 5.5" and putting in batts. The current cost of lumber makes this solution a bit more expensive than adding iso foam to the outside, but if it is a safer choice in terms of moisture, then I might go this way. This has the same downsides of the thicker wall, but I wouldn't need furring strips under the siding, so furring the wall out 2" would only be 1/4" thicker than adding 1" foam board plus 3/4" furring strips.

        The original builder put in a poly vapor barrier, R13 of poorly installed unfaced fiberglass batts, then foil faced Thermaply (cardboard sheathing), then lap board siding. No house-wrap. No air sealing. Thermaply seams are overlapped but not taped. Big holes in the poly where electric boxes are. This seems like exactly the wrong way to build a wall since there is a double vapor barrier (poly on the interior and foil face on the sheathing, and the sheathing is cold in the winter. Is it just the air leakage that is keeping my house from being a wet, moldy wreck? I haven't opened up all the walls yet, so technically I don't KNOW that it isn't a moldy mess. I've done a lot of DIY remodeling in the 16 years I've owned the house and never noticed any moisture or mold.

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