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Community and Q&A

Correcting wall insulation on old home (limited access)

techbiker | Posted in Energy Efficiency and Durability on

I recently purchased an older pier and beam home without exterior sheathing in north Texas (zone 3a). The house was mostly gutted and flipped about a decade ago. The flippers left the shiplap sheathing on the inside of the studs, blew cellulose into the exterior wall cavities and caulked the horizontal joints on the #117 wood siding. The cellulose I have examined seems to stay very damp. The result has been rapid exterior paint peeling, insect problems, and wood moisture damage.

Here is the current composition of the exterior walls:

inside|sheetrock|1×8 shiplap sheathing|studs|cellulose|#117 wood siding|outside

The 100+ year old wood siding is very brittle and the shiplap and newish drywall make interior access prohibitive (I don’t plan to gut the house again soon). I’m quite certain the moisture problem will resolve if I simply remove the old cellulose from the wall cavities. That would provide an air gap for the wood siding. Unfortunately that also gets rid of most of the wall’s R-value.

After much thinking and time spent on your forum, I can only imagine one sensible solution. That is to carefully remove the bottom couple rows of wood siding, remove all of the old damp cellulose, and push ~4′ sections of ~2″ rigid foam board up into the empty wall cavity. The foam would be cut to be a snug fit. That should give me an ~R-10 wall assembly with a 1.5″ air gap, which shouldn’t be too bad for our warm climate.

Given these constraints, can you think of any better solutions? Does the permeability of the foam matter in this situation? Would something like Insulfoam work?

P.S. Should I calk between the bottom of the interior shiplap sheathing/drywall and wall baseplate or leave a gap there (behind the baseboard)?

Thank you very much!

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  1. GBA Editor
    Martin Holladay | | #1

    The moisture is entering your wall cavity from the exterior, due to wind-driven rain and inward vapor diffusion during the summer.

    If this were my house, I would correct the problem by installing three new layers on the exterior: a WRB (housewrap with taped seams), furring strips to create a rainscreen gap, and new siding.

    1. techbiker | | #2

      Mr. Holladay,

      I agree in that the best solution is to modernize the walls. Unfortunately in this case it would be a big project.

      1. I believe vinyl siding is banned where I am.
      2. The windows are lower end aluminum units barely protruding from the current walls.

      It would probably be easier to remove the old siding completely and create a new assembly over the studs vs creating a flat surface, installing wrap, furring strips, and new wood siding back over the old siding. If I build up the surface I will also either have to replace the newish windows or find some way to modify and greatly extend the window trim.

      The old wood siding is very brittle old-growth douglas fir and I may damage 50% of the old siding removing it. Douglas fir #117 isn't available here and any replacement pine must be meticulously back painted. Imo removing the old siding may impact the historic character of the house.

      I understand that some wind-driven rain may seep behind the old siding as-is. The question is how to deal with it until I can allocate the $10k's necessary to completely resolve the condition. The house apparently performed well without a dedicated drainage plane for close to 100 years. Any thoughts on the following stop-gap solutions?

      1. Leave the cellulose and budget for extensive rot repair, deal with tons of bugs.
      2. Remove the old cellulose and leave empty wall cavities. Focus on attic insulation.
      3. Remove the cellulose near the bottoms of the walls and install Rockwool batts. At least these won't absorb/hold water.
      4. Remove the cellulose and push rigid foam into the cavities.

      Thank you!

      1. GBA Editor
        Martin Holladay | | #3

        I'm sorry to hear about your financial problems, but I stand by my advice. There are many times that homeowners wish that the proper solution to a problem was cheap. But sometimes there isn't a cheap fix.

        It may be time to talk to a bank to see if you can secure a home equity loan to repair your walls.

      2. Expert Member
        Akos | | #4

        Unfortunately Martin is correct in his recommendations. There is no easy fix for you. The problem is the insulation is doing its job (limiting air flow and heat loss) and your walls with bad water management details can no longer dry.

        This is a common problem when people just blow in insulation into older homes with sub-par/non existent water management. Leaky walls hide all sorts of water issues, the reason it wasn't an issue for 100 years is that all those leaks had lots of airflow to dry it quickly.

        The two options are, remove all the insulation so the siding can dry or remove the siding add a rain screen/proper water management and re-side. Extending windows with a couple of pieces of custom made flashing is pretty straight forward, I would say that is probably the simpler part of re-siding the house.

        1. techbiker | | #5


          So would you say there is no benefit to removing the blown-in and pushing in 2" rigid foam vs just removing the blown in insulation? Or just not worth the expense? I would have a 1.5" air gap and some R-value.

          It seems the blown in insulation is actually bug-filled fiberglass, not cellulose. It doesn't seem like this changes much though.

          Thank you again

  2. GBA Editor
    Martin Holladay | | #6

    Your walls don't have an insulation problem. They have a water management problem.

    Trying to slide 2 inches of rigid foam into each stud bay will be physically awkward or impossible, and there won't be any easy way to seal the edges of each piece of rigid foam. (Sealing the edges is a required part of this technique.)

  3. Expert Member
    Peter Yost | | #7

    Hi Biker -

    You need to figure out what is getting wet and where and get your walls dry before you do anything else; I think others have made this point but I am reinforcing. I do building assessments all the time, evaluating bulk water management first. See this blog on building assessments and see if that helps you in evaluating your own home.


  4. techbiker | | #8

    Martin and Peter,

    Thank you for the information. Understood.

    It seems the flippers blew "Atticat" insulation into the wall cavities- not even designed for walls. I will dry out the structure and focus on air sealing. There are some large gaps between the wall base plates and interior sheathing/drywall that can probably be caulked.

    P.S. The flippers also blew fiberglass insulation into the attic but apparently neglected to air seal before doing so. Should I eventually remove all of the blown-in insulation from the attic and air seal up there?

  5. GBA Editor
    Martin Holladay | | #9

    It's good to dry out the assembly, but you also need to figure out how to prevent the assembly from getting damp again in the future. If the moisture is wind-driven rain, you've got a problem.

    1. techbiker | | #10

      I've been making a list of the likely worst moisture offenders. I believe priority #1 in that department is getting a ~10mil vapor barrier on the dirt crawlspace floor. I suspect 100+l/pd of moisture removal with HVAC and dehumidifiers can handle any (non-bulk) moisture in empty wall cavities for the time being. Humidity is below 50% inside now. Thanks again.

      P.S. If it's still a problem, I can add another dehumidifier. Anyone who walks in will turn into a prune in 15 minutes. lol

      1. Expert Member
        Akos | | #11

        The crawlspace would mostly effect interior humidity issues.

        Your problem is your siding. From the stackup you showed, there is no WRB bellow the 117. Any wind driven rain will go straight into the insulation and stay there. No amount of interior de-humidificaiton will help with this.

        You need to find where the water is getting trough your siding and fix it, or remove the insulation.

        Replacing the blown in fiberglass with a layer of rigid and leaving a generous vent channel might help. I'm a much colder climate, so I can't comment on the issues this might create in a hot and humid climate though. You don't want to fix one issue (wind driven rain) and create another one with condensation behind the rigid foam.

        1. techbiker | | #12

          Thank you. Understood. I think you are correct as the old insulation is brown and full of bugs. It's getting better, however I've had to pull them out of my ears, nose, and hair. Agh!

          I will focus on air sealing rather than bother adding a layer of rigid.

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