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Community and Q&A

Permeable insulation options with 2022 products

sb1616ne | Posted in Green Building Techniques on

I have been in the carpentry and construction space for a while, however never really had a chance to implement many of the modern insulation practices. 

What I have is a renovation and addition project on my own home in NH(zone 6). This is a 2 part question as I have both a new construction addition as well as a retrofit to the existing 1920’s & 1970’s house. 

1. Retrofit to existing house. The existing house is a mix of 1920’s construction(2×4 with blow in cellulose added in the 70’s) and a 1970’s construction(2×4 with fiber glass batts). Both attic spaces have 24″ or more of blown in cellulose. Pine T&G sheathing, tar paper and lead painted clapboards. Roughly 2000 square feet of total wall area. The interior of this existing house is in excellent condition and all windows are 5 year old Anderson 400 series that were new construction type that I installed well. 

My plan is to keep the windows where they are, peal and stick the all the exterior walls, and then exterior insulation of some kind using a typical detail of peel and stick, two layers of foam or rockwool to get the sheathing above the dew point, and then strapping, then clapboards. 

The existing foundations are a mix of field stone and poorly sealed concrete. No vapor barriers under the basement floor, etc. So there is a fair bit of moisture that needs to move through the house. Blue Skin 160 will allow the exterior to breath as long as the exterior insulation will. As a carpenter I really hate working with Rockwool and would rather deal with foam but very few of the foams will allow enough moisture transfer. The basic unfaced EPS will allow good transfer but bugs love it? I know there are some EPS and GPS products treated with the insecticide. Foam board also seems to shrink with age and EPS seems to be the worst of them? 

So my question here is: Is Rockwool the best option we have right now?  

2. New construction addition. This is addition has roughly 900 sq ft of wall area and as of today I have not framed the walls yet. I have done all of this myself from the ground up and have tried to build as smart and green as made sense. ICF below grade walls with a proper vapor barrier and insulation under the slab. Here I would love to build double stud walls and fill them with dense pack, but with this addition being connected to the old house I will be getting much more vapor generation than a stand alone new build. I am aware of the issues that double stud walls and condensation on the sheathing surface. The entire house is never going to be that tight, just the way it is!

Any recommendations on options for a good solid wall assembly that makes sense for this addition? 


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  1. maine_tyler | | #1

    I don't have a real good answer, but I'm curious about some of your notions on vapor, such as:
    >" I will be getting much more vapor generation"

    Vapor generation? From where? Likewise with the foundation wall; you are implying your concrete wall needs to dry to the outside (through potentially wet soil?). I'm not sure that makes a lot of sense, but others can correct me. If your real concern here is vapor entering your dwelling via the basement floor (?) then perhaps you could cover said floor with a vapor barrier?

    >"The entire house is never going to be that tight, just the way it is!"

    An issue with double stud walls is moist interior air contacting the exterior, cold sheathing. In other words, you need to make the wall assembly itself tight. I'm not sure I understand what that has to do with the rest of the house (which it sounds like you are tightening up a lot anyways?). Are you concerned you will have higher interior dewpoints than such a wall can manage, even given proper air/vapor controls built into the new build?

  2. Expert Member


    You can mitigate any risks to the double walls with a good interior air-barrier, and adding a rain-screen outside the sheathing.

    If you use sufficient exterior insulation to get the existing walls above the dew point, you don't need the insulation to be vapour permeable, or the wall to dry to the outside, and you can use any type of rigid insulation you want.

    If you don't use enough exterior insulation, and want drying to the outside, then you need to use mineral wool.

    No strategy to deal with interior moisture should rely on drying through the walls. It needs to be exhausted by ventilation or removed by de-humidifying. Drying to the outside is just used to remove moisture that makes its way into the walls through imperfect air and vapour barriers that should be in place to stop it.

    1. Expert Member
      NICK KEENAN | | #5

      "No strategy to deal with interior moisture should rely on drying through the walls. "

      This bears repeating. The standard cold-climate strategy is to have a vapor barrier on the interior that keeps interior moisture out of the walls. To the extent walls dry it's to get rid of moisture that either bypasses the vapor barrier, or leaks into the wall as a liquid.

      " It needs to be exhausted by ventilation or removed by de-humidifying." I would add, "or prevented from entering the house in the first place.

  3. maine_tyler | | #3

    If you want vapor permeable insulation for whatever reason, in above-grade situations wood fiber products are gaining some traction as well (though for the time being are still hard to source. A plant in Maine is releasing product within a year).

    1. sb1616ne | | #10

      I have seen this on the news and excited to see what they can offer!

  4. Expert Member
    NICK KEENAN | | #4

    My opinion is it's a fools game to try and dry the basement through the house. You're basically trying to dry out the soil that underlies the house, and you're never going to do that.

    I don't see why you're concluding that the basement can't be insulated. My first choice would be a vapor barrier sheet on the floor and closed cell spray foam on the walls and rim joists. If you don't like that there are other ways to do vapor barrier and insulation.

    Only insulating part of the house is like only waterproofing part of the bottom of a boat. Especially with the nice things you're doing in the rest of the house -- exterior insulation, double walls, ICF -- the rest of the house is only going to be as warm as its coldest link.

    If there is liquid water entering the basement -- the basement is leaking -- that needs to be fixed before you do anything else.

    1. sb1616ne | | #11

      Thanks for the reply,

      One thought I had when talking to my neighbor who is a excavator& farmer is will insulating the existing basement of the oldest part of the house cause any damage to it? You might be asking what he saying here. A lot of these old field stone foundations never were built to the frost depth we build to now. In the case of this old basement the history of the house makes me think it use to have a wood or coal stove in the basement then a hot air furnace(which I removed) after that. So say in NH where we normally build to 4ft below grade for footing if we spray foam this 2-3ft below grade foundation and remove nearly all the heat generation in the basement(wood stove or un insulated duct work) will I be opening up a can of worm for frost heaving?

      1. DC_Contrarian_ | | #14

        With brick there is a real concern. Brick absorbs water, and can crack if it freezes when wet. Insulating the interior of a brick building makes it harder for the brick to dry and leaves the brick colder in the winter, which can lead to accelerated aging of the brick. Stone is essentially impervious, especially the granite found in New England. It absorbs much less water than concrete. So cracking isn't an issue.

        In terms of frost heaving, what matters is the soil outside the building. If you have a basement that's probably amply deep.

  5. Expert Member
    NICK KEENAN | | #6

    I recommend this article:

    Note that Lstiburek has since changed his mind and no longer recommends a capillary break for stone foundations, stone doesn't wick nearly as much as concrete.

  6. walta100 | | #7

    Seems to me you need to rethink your goals and why you chouse those goals.

    What is the point of add a ton of insulation if you are unwilling to air seal the house?

    It does not matter if the R value of the wall is R60 or R8 if the wind is blowing thru the insulation. Insulation is pointless until you create an air barrier.

    I see little point in spending a ton of money to add exterior insulation to a wall around a drafty antique window with a R1 value. If 15% of the wall is R1 window it will bring down the walls averages so much moving the rest of the wall from 13 to 30 means the walls average is almost unchanged.

    Post a drawing for you 100% water tight flashing system of the existing windows with the exterior insulation and new siding and zero % of the peal and stick exposed to UV light. The cost of the details may well exceed cost of new windows.

    My point is this job is starting to sound like a “deep energy retrofit” and from a dollars and cents point of view bulldozing this building and rebuilding a new facsimile would likely cost less and be a much better building and more likely to be on time and budget unless someone has a sentiment attachment.


    1. DC_Contrarian_ | | #8

      I hear what you're saying, but I'm not ready to call for a tear-down from my laptop. I do think that this is a significant enough project that it's worth pricing out the tear-down option and keeping that in mind when looking at other options.

    2. sb1616ne | | #9

      Thanks for the input. This place is defiantly not a tear down, but I do appreciate your opinion here. I have read other posts you have written and its nice to see someone who questions the viability and payback of some of the projects proposed on here.

      All of the antique windows have been replaced with standard low E dual pane Anderson 400.

      Air sealing is my priority and much easier to accomplish than making this vapor tight, that is why I started this thread. Any advise on ways to limit vapor transmission through and existing basement floor with no vapor barrier or insulation under the slab? Part of the house with the existing poured walls I can get to the outside of the walls to seal and insulate externally without too much work. We do not have any standing or liquid water that gets into the basement, but the slab and walls do make the basement feel damp.
      How do vapor barriers work located on top of the slab? Will that create a moldy mess down the road?

      Could I put down a vapor barrier above the slab then a high density say 100 psi layer of foam?

  7. thegiz | | #12

    What is your level of dampness, have you tested it? I don’t think insulating your basement will destroy an old house, it is in my opinion that fieldstone foundations are extremely strong but maybe I’m wrong. External insulation at ground level has to be detailed right. Insect issues scared me out of it and I ended up insulating from the inside. For the floor it has to be raised away and separated from the concrete. If you can’t afford to loose ceiling height if either has to be easily removable flooring or material that doesn’t care getting wet. After insulating the walls in my century old home my humidifier is running at 55 but it hardly turns on. I originally bought a pump thinking my humidity would be insane but I manually dump the humidifier every few days. Insulating the walls makes a huge difference but I don’t know what dampness/wetness level you are dealing with

  8. DC_Contrarian_ | | #13

    There are three main ways that moisture gets into a basement:

    Liquid water seeping in, whether stormwater or plumbing leaks or even groundwater. Thankfully you don't seem to have that.

    Dampness coming in as water vapor from the surrounding soil.

    Humidity from the air. In most of New England, in the summer the outside air will have a dew point higher than the soil temperature. Air hitting an uninsulated basement wall will cause condensation.

    Sounds like you have to deal with dampness and humidity. The solution to dampness is a vapor barrier, which can be as simple as a sheet of plastic or a spray-on or brush-on coating. Closed cell foam is also a good vapor barrier. The solution to humidity is to insulate the basement walls so they are warmer than the ground temperature, and to make sure that the insulation is air-tight so that humid interior air can't reach the cool walls by bypassing the insulation.

    The problem with fieldstone foundations is that they are so irregular. You really need your vapor barrier and air sealing to be perfect, and it's hard to make a perfect seal on an irregular surface. Closed cell spray foam is popular because it is vapor barrier, air barrier and insulation all in one (it even blocks minor liquid water intrusions) and makes a perfect seal on irregular walls.

    The other approach is to build 2x4 walls inside the stone walls so that you have a flat, plumb surface to work with, and use foam board as your vapor and air seal and insulation.

  9. joycejackson | | #15

    I think that you need to evaluate the risks firstly with the permeable insulation options. I study Architecture at college and when I need an answer to such question, I usually go to resource for assistance. By the way, be careful with the construction part.

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