GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Guest Blogs

Basement Insulation — Part 1

Getting ready to insulate the basement walls of our Massachusetts home

When we bought our house on Martha's Vineyard, the basement walls were uninsulated.
Image Credit: Marc Rosenbaum

A common truism (that isn’t) is “heat rises.” Actually, what rises is air that is warmer than the surrounding air. Anyone who has lived with a wood stove knows this — it’s a lot hotter at the ceiling in the room with the stove than it is at the floor. But heat flows from hot to cold, so it readily goes from our houses down into whatever connection they have with the ground, because the ground is cooler than the temperature most of us like our homes to be at.

A surprising number of buildings have no insulation between the finished, occupied portion and the ground. In a concrete slab-on-grade building, such as many schools, having no perimeter or sub-slab insulation leads to cold floor edges, and (especially if the floor is carpeted) mold.

Most houses in New England have basements (or crawl spaces). There are two locations for insulation between the finished above-grade space and the ground: The first option is to insulate the first-floor joist cavities, above the basement; and the second option is to insulate the basement walls and floor slab. Either option can work well to slow down the flow of heat between the house and the ground.

If I insulate between the floor joists, how cold will my basement get?

Insulating the frame floor is usually done with fiberglass batts, those pink or yellow items that facilitate coughing and itching. This method is cheaper than bringing the whole basement into the thermal enclosure. Putting the thermal boundary at the first floor, if done well, works fine at reducing heat loss to the ground. The weak point of this strategy is that the basement gets colder. How cold depends on a number of things:

1 – Is the basement leaky to the outdoors? If so, it will get colder than if it’s airtight. Even in cold climates such as northern New England, uninsulated basements won’t freeze if they aren’t leaky. And frozen pipes, the bane of the northerner’s winter, are most commonly caused by air leaks at the sill area, with the pipe located in front of the leak.

2 – How much of the basement is above grade? The more foundation that is exposed to the ambient air, the colder the basement gets.

3 – What is the ground temperature? In much of New England, the temperature at the basement footing level is somewhere near 50°F. If the basement is mostly below grade, the (uninsulated) basement temperature won’t drop much below the ground temperature. The entire area of the floor slab is in contact with this stable temperature, and during the summer, when the basement warms, the sub-slab areas warms also, storing some heat that comes back in the winter as the basement cools.

4 – Are there heat sources within the basement? The most common, of course, is a forced air furnace and uninsulated, leaky ducts, or a boiler and uninsulated pipes. As the outside temperature drops, the heating system fires more often to keep the house warm, and losses from the system keep the basement warmer than it would be without the heating system. In the most extreme cases, the basement is actually warmer than the house!

Insulating the joist bays and under the slab

So one good way to get a cold basement is to do a good job insulating the frame floor above, and also insulate below the floor slab. Here on Martha’s Vineyard, where I live, South Mountain Company did this in a lot of houses, figuring that an owner could always add wall insulation to the basement but it would be hard to add insulation to the floor slab (partially because of headroom issues). My house has one inch of extruded foam beneath the slab, for example.

But insulating beneath the slab cuts the heat flow from the earth in the winter. In one case, some houses were built with spray foam in the frame floor, and two inches of rigid foam beneath the slab, and no basement wall insulation — basically the culmination of the typical Martha’s Vineyard strategy. These houses, because they had very good thermal envelopes above grade, were heated with a point-source propane heater in the main living space, and therefore had no basement heating system.

The first winter people were surprised to see the basement temperature drop below 45°F! This shouldn’t have been a surprise, though — very little heat came from above or from the deep ground, and so the basement temperature headed to a point somewhere between the ground surrounding the basement (which is coldest at grade) and outdoors.

Watch out for condensation

In humid climates, the drawback to insulating the frame floor goes beyond energy, to an air quality and aesthetic concern. After winter, the ground has cooled down. The ground at grade warms in the spring, but the basement walls, particularly down near the footing, are cool. When summer humid air gets into the basement, condensation occurs as the air is cooled down below its dew point. This is typically noticeable at the bottom of the walls where they meet the floor slab. The moist surfaces, especially if they are in contact with, and therefore cause to be wet, organic materials like wood, cardboard, paper or fabrics, support biological growth and you get the classic moldy dank basement odor.

My house was insulated with fiberglass batts in the frame floor, and as noted, has one inch of foam beneath the basement slab. As we got into mid-late December, the temperature in the basement dropped to 58°-59°F and seemed to level off. Why? Because the boiler was running, heating the house and our hot water, and all the piping for both heat and hot water were left uninsulated, which makes them very effective heat emitters.

On December 26th, we started using the heat pump instead of the oil boiler for heat. Within two weeks, the temperature had dropped into the upper 40s. This situation promised to make the walls even colder as we entered the summer season, potentially making the smelly basement problem worse. Considering that we were heading to removing the oil system completely, we decided that we would add insulation to the basement walls.

Exterior insulation or interior insulation?

Once we decided that we were going to insulate the basement walls, we had a number of choices about how to do this. In the early days of my practice, we usually put insulation on the exterior of the foundation walls. This keeps the walls nice and warm and dry and is a very good mold prevention strategy.

There were two drawbacks to this. The first is that the insulation on the exterior of the foundation walls is discontinuous at the footing with the sub-slab insulation, so there is a thermal bridge here. As the entire building thermal enclosure gets better and better, this matters more. It really shows up if you are designing a Passive House, for example. This thermal bridge is also potentially a moisture source issue, as water in the footing, which sits on earth that may be wet, can be pulled up into the foundation wall by capillarity and on into the basement. This can be prevented by installing a capillary break atop the footing, usually a coating that fills the concrete pores and stops the transfer of water at that boundary.

The second issue is that carpenter ants and mice like foam — it’s easy to tunnel into and makes a cozy home for unwanted tenants. So over time many practitioners moved the insulation to the inside of the foundation wall. Now the concrete is cold, in fact, colder than it was when it was uninsulated. So basement air (or outdoor air in the summer) that reaches the concrete will likely deposit condensation there, and we’re back to our smelly basement. So interior wall insulation needs to be air tight.

Interior basement insulation also doesn’t want to have a vapor retarder on the interior face, because vapor diffuses from the ground through the concrete and into the basement, and if there is an interior vapor retarder the entire insulation cavity gets wets and, you guessed it, begins to decay and smell bad.

Foam insulation needs a thermal barrier

Considering the above, my preference for interior basement insulation is some form of foam insulation. There’s a lot to say about types of foam, and when one might be preferred over another, and which might be avoided as much as possible, yet for now let’s focus on just one aspect. When using foam in a habitable space, the building code requires that the foam be protected by a thermal barrier, which it defines as 1/2 inch gypsum wallboard or its equal. I don’t want to cover basement foam with gyp board, because for one thing paper-faced gypsum is an Olympic quality mold growth medium. And I don’t want the expense of building something to hang the gypsum board on, like a stud wall.

One alternative is to use a foam product like Thermax that has been tested and has an approval to be used without a separate thermal barrier. This is what I chose for my basement insulation. This foam has a foil facing on both sides, which puts a vapor retarder at the concrete wall, restricting vapor flow into the basement, and, once the joints are taped on the inside face, produces an airtight seal at the interior face.

When the frame floor at my house was insulated, the insulator made the curious choice to insulate 2×10 joists with fiberglass batts that fit 2x6s. This by itself is not unreasonable in the context that the above-grade walls were 2×6, and the insulation level to a basement that might be in the range of 45°F to 60°F during the heating season doesn’t need to be as high as the insulation level to outdoor temperatures such as the above grade walls experience.

But the insulator did a curious, and not overly clever installation, to make things easy for himself: he installed the insulation flush with the bottom face of the joists, allowing a three- to four-inch empty space between the underside of the subfloor and the top of the batts.

Where this becomes a thermal fox pass is at the edge of the floor, where a couple of inches of wood separate the house from outdoors. All around the one hundred sixty four feet of perimeter, there’s a strip with no insulation.

You can see it here in this infrared photo, taken on a very cold morning, where the warmest surface is the uninsulated exposed basement wall, and the green strip just above it is the rim joist cavity.

Dow Thermax and closed-cell spray foam

So the basement insulation job really has two parts: the concrete walls, and the wood sills and rim joist. I chose Dow Thermax polyisocyanurate foam for the concrete walls, because it is rated to be left exposed, and has a 4/1000 inch white aluminum facer that is moderately durable (there are three facer thicknesses to choose from; I chose the middle, because the thickest is really costly) and is easy to seal with foil tape at the panel edges (more on that later).

For the sill/rim area, I’m using closed-cell spray polyurethane which will be applied by our ace insulating subcontractor, Matt Viaggio. The spray foam will provide an air seal as well as insulating value, and is well suited to insulating these perimeter spaces that have wires and pipes and anchor bolts in the way of installing board stock foam.

I wrung my hands a bit about how much foam to use on the concrete. I decided to leave the batts mostly in place, so some of my thermal enclosure is still at the frame floor, and some is at the basement walls. In the end, I installed 2 inches of polyisocyanurate foam, in two 1-inch-thick layers with offset joints to enhance air tightness, for an R value of 13. I stopped the foam level with the top of the concrete walls, to allow the spray foam to cover the concrete and lap onto the top of the rigid foam.

Everything needs to be moved back from the walls

I used an innovative system developed by Hilti to attach the Thermax to the concrete walls, which I’ll describe in the next post. But first, I had to go around the basement perimeter and note each item that was closer than two and a half inches from the concrete walls, and would therefore be in the way of the foam. I ended up with this list:

– The stairs to the basement from the first floor were framed hard against the concrete

– The electric panel was mounted on a piece of plywood over flat-wise 2x4s – I was not going to mess with this one!

– The PVC waste plumbing from the kitchen sink and washing machine

– The potable water pressure tank

– The washer hoses and electric receptacle

– The light switch at the door to the bulkhead

I could leave these as they were, or move them out from the wall to let the foam run past them uninterrupted. I decided that it wouldn’t be that much more work to move them all then it would be to fit the foam around them, so I moved them.

[The continuation of this blog can be found here: Basement Insulation — Part 2]

Marc Rosenbaum is director of engineering at South Mountain Company on the island of Martha’s Vineyard in Massachusetts. He writes a blog called Thriving on Low Carbon.


  1. MJDesigns | | #1

    Nice article
    Thanks for the summary. We are in the process of planning a new build and this is a nice read. Question though ... if one were to insulate under the basement slab as well as around its perimeter with 1" or 2" of type IX (1.8 lb density) EPS rigid foam, how would the slab perimeter portion be finished if the balance of the foundation walls were insulated at the exterior? With your choice of Thermax, it just works with maybe just spray foam to bridge the gap between the polyiso Thermax and the EPS slab perimeter insulation. But with no interior rigid foam applied to the walls, what's the best thing to do with the portion of EPS foam that's visible around the perimeter of the basement slab?

  2. Marc Rosenbaum | | #2

    That's a good question and no
    That's a good question and no easy answer. With an inch, you might caulk it similar to an expansion joint in a large slab. More than that, your imagination is as good as mine! It's been a long time since I insulated the exterior of a basement so this issue hasn't come up.

  3. wjrobinson | | #3

    Marc, great to have you
    Marc, great to have you blogging useful building assemblies.

    Dan, Marc, Martin, improving GBA. Complete detailed plans, specs, costs, hours for projects located in GBA Pro area with a link in your blogs to signing up for Pro and going to the details.

    Homeowners and builders are thirsting for complete plans to go green.

    Make it a priority that by 2013 you have this last bit implemented. Seville did it right just now with his last blog series.

  4. GBA Editor
    Martin Holladay | | #4

    Response to AJ Builder
    You ask for "a link in your blogs to signing up for Pro." I could include that, I suppose. However, your might notice that right now, there is a link in the upper right hand corner of every GBA page that reads, "Free 10-day GBA Pro Trial." That link appears on every page -- at least for visitors who aren't already GBA Pro members.

    There is also a link on every page for "going to the details." That is one of the green tabs at the top of every page -- the one titled "Strategies & Details."

    I'd love to have the editorial resources to fulfill your other request: "Complete detailed plans, specs, costs, hours for projects located in GBA Pro area." In most cases, this isn't rocket science -- it's just old-fashioned journalism and research, and it takes a lot of time. Moreover, some architects and homeowners are skittish about revealing full costs. That doesn't make your suggestion impossible -- just a little difficult. As soon as GBA can afford to hire two more editors, I hope we can implement your suggestion.

    Finally, if you are curious to know the time it took for Marc Rosenbaum to insulate his basement, and how much the project cost him, he reveals all in Part 2. Stay tuned.

  5. GBA Editor
    Martin Holladay | | #5

    Response to AJ Builder
    Thanks for editing your comment to make it more comprehensible.

    It sounds like you are describing a kit home for owner/builders -- a little bit like the old kit homes from Sears. I think that a few manufacturers are already serving that market. I imagine that the market is small.

    If you Google "kit home for owner builders" you will get lots of links. Some of these kit homes are even advertised as "green" -- like this one:

  6. [email protected] | | #6

    Basements are the best
    Thanks! This is a great article,

    In building terms, I think basements are the unsung hero. They are usually built and forgotten until there is trouble. Here in Maine, half of our work is in this very place.

    I really like all of your points. I have only one that I wanted to provide a different opinion on. I am writing about the following quote:

    "Insulating the frame floor is usually done with fiberglass batts, those pink or yellow items that facilitate coughing and itching. This method is cheaper than bringing the whole basement into the thermal enclosure. "

    I couldn't agree more on the poor performance of fiberglass- but I feel insulating the walls is a less expensive option and will provide a better return on investment.

    For an average home that is 32x28, there is about 896sq.ft. of floor space and 960 sq.ft. of wall space. Therefore, there is about 60 sq.ft. of added surface area that needs to be insulated if you choose to insulate the walls- however there are two reasons I think this is worth the extra material. One difference is the time it takes. A wall is flat with few obstructions and thus less waste. A floor system with wires and pipes is an insulating blitzkrieg that is very difficult to do consistently and without resorting to violence. If a owner was insistent on insulating the floor as opposed to the walls- my price would be much higher.

    On top of the extra cost, I think, the savings from choosing the floor option would be much longer to realize. When you insulate an unintentionally heated (i.e. there is no basement zone) above grade, basement wall in Maine, you save approximately .50 cents per foot per year (with oil being $3.20 per gallon and the primary heating fuel). If you insulate the ceiling of the same basement, I estimate you would save significantly less because you are now only protecting the home from a 48 degree basement and not mother nature.

    What do you think?

  7. user-1140531 | | #7

    This is my preference for
    This is my preference for basement insulation for building a new superinsulated house:

    Insulate both the inside and outside of the concrete foundation and slab with extruded polystyrene foam; 2” thick on walls and 4” thick under slab. Cover the exterior foundation wall foam with extra heavy-duty polyethylene sheet to seal all the foam board seams. Lay underslab vapor retarder polyethylene sheet under foam below slab.

    Protect the above-grade portion of exterior wall foam, by covering it with ¾” foundation grade treated plywood screwed through the XPS and into the concrete. Run the plywood 4 ft. down from the sill. Below, the plywood, temporarily hold the foam in place with dabs of adhesive, and then hold permanently by the backfill.

    For the interior, friction fit foam (4” thick on walls, and 1 ½” thick on floor) into treated battens, which are screwed and glued to the concrete. Cover interior foam with heavy-duty polyethylene sheet to seal all the foam board seams. Finish walls with sheetrock, and finish floors with plywood subfloor and wood flooring.

    Exterior foam board is carried to elevation of footing bottom. No insulation is extended under the footing. Extend underslab vapor retarder to cover area under footings.

  8. wjrobinson | | #8

    No one should follow any of
    No one should follow any of your post Ron. Join GBA and follow the advice in the Pro section. Also follow BSC.

    I mean no harm Ron but two things, are you a builder and what zone and location are you suggesting designs for?

  9. wjrobinson | | #9

    Editing cellphone shorthand jibber...
    Agreed, nice, bravo, foreword ho you go... Martin. A thought to add as to rocket science. Actually Passive House and PGH levels, of extreme R insulation and energyy reducing assemblies, are;
    1-very much in demand
    2-what GBA is all about
    3-precise in what is right and what is wrong in methodology (In other words if not done precisely correct, high R and low ACH can easily lead to horrendous unintended consequences)
    4-no where to be found in whole verses incomplete synopsis here and there. (IE, this blog tells most of the story for someone to duplicate, but, not all. A complete plan like I have used to build a dozen homebuilt airplanes needs quite a bit more. Complete plans that come with a complete A to Z step by step planbook. I also built hundreds of models and many Heathkits, those kits are what I are a minimum to be called complete enough.)

    Look forward to your future day when just like buying anything "to be assembled," Green Net Zero, has many sets of "fully packaged plans" to build all "green assemblies" correctly.

    Construction needs this at the residential level and definitely at the DIYer level.

    Find someone with plans to build a Long Eze or a Heathkit project... Not only do the best plans walk you through the build, the best explain all the terms and all the whys for what to do right and what not to do wrong. And all in one place, in order, from step 1 to final step.

  10. user-1140531 | | #10

    Aj,I was a little taken

    I was a little taken aback by your terse dismissal, but after considering your comments, I think I am kind of glad you feel that way. I consider it to be an endorsement.

  11. wjrobinson | | #11

    DIYers are a huge part of
    DIYers are a huge part of your GBA audience. Also there are many stick&batt builders wandering in here checking into what all the green hoopla is.

    So, what I propose is to do more than over the course of years chat up a million ways to do a million parts of home construction you do some complete build "fill in the blank" plan packages.

    These blogs are perfect for myself, and so are all your PDF sheets. But much more exacting guidance is needed in our residential green construction world. Not sure who today puts out the most complete plans to build something similar to a green home but I may have to look into this and refer you to some close examples. For now I just know about the planes and radios I built. I learned from the plans how to solder in a resistor, what it did, and how to read the colored stripes while building something fun to play with. That is the guidance that is completely missing in construction.

    Maybe an idea for my website. Complete Green Building Home Plans from footer to ridge to commissioning and living in.

  12. wjrobinson | | #12

    Ron, builder? homeowner?
    Ron, builder? homeowner? zone?

  13. jhrockwell | | #13

    Skip interior polyethylene
    AJ, was the interior polyethylene the last straw?

    Ron, one poorly-sealed electric outlet on your basement interior walls may allow moisture-laden air into the wall assembly. If it condenses (less likely with your exterior insulation, but possible), it has no chance to dry to the interior, a welcome phenomenon that perplexes even those with years of experience in the field, but no awareness of current building science.

    Also, it's not clear how you'd address the possible modest thermal bridge where slab meets foundation wall. Read about the "perfect" roof/wall/slab assembly elsewhere on this site, and save yourself a lot of labor and materials.

    With the clean slate of a new house, it probably isn't optimal or even necessary to insulate both sides of a foundation wall or slab. That same amount of insulation can be installed beneath the slab, and you're done - no interior poly needed.

  14. user-1140531 | | #14


    I understand your concerns. To clarity aj’s question, I am proposing these ideas for climate zones 6 and 7.

    My first objective is to keep moisture out of the concrete, and second objective is to prevent any moisture in the concrete from drying to the interior. That requires a sealed vapor barrier on the interior. Really, all that film vapor barrier is doing is sealing the joints between the foam board. Rather than sealing those joints with tape or sealant, I feel the covering them with film is easier and more reliable.

    I will incorporate a separate electrical cavity on the warm side of the vapor barrier so that no boxes will penetrate it. The electrical cavity is a fir-out on top of the vapor barrier, so the vapor barrier will be extra heavy material will all seams thoroughly clamped off and sealed, and there will be no disruption from sheetrocking or sheetrock screws.

    So first and foremost, I want a vapor impermeable barrier on the interior. With that barrier, I might as well also incorporate insulation.

    Perhaps exterior insulation could be omitted, but I would prefer to keep ground moisture from entering the concrete as much as possible, so I would include underslab vapor barrier material under the slab and on the walls. The slab needs more R-value than what would be practical if all of it were derived from foam above the slab, so I would also add foam below it.

    Foam on the exterior of the walls is optional, but I would include it for a collection of minor reasons. If the foam were omitted, the exterior vapor barrier would still need some above grade covering, so all together, the exterior foam is not that much more to add to the exterior detailing.

    I would not worry about the minor thermal bridging through the footing into the ground below. There will be continuous foam on the interior, so the only sense of a weak insulation point would be under the footing. I have read about setting footing on top of foam. I understand the thermal advantage, and the case that it is doable from an engineering perspective. But I choose not to do it because I believe it is too risky and I don’t feel that the thermal benefit justifies the risk.

  15. wjrobinson | | #15

    Ron, if you are building your
    Ron, if you are building your first green home, go GBA Pro. Follow tried and true methods. I advise against armchair made up on the fly assemblies that sound right. Math, and science and lots of us having already built lobster traps has led to the better more forgiving builds both at GBA Pro and at BSC. BSC even rates their builds for how daring they are (forgiving).

    IE... you are scared of foam under a footer. Do the math, the loads per square inch and then even with just your logic, think of the consequences. Say the foam 2 inches thick, squashes and 1/8". It would most likely squash the same amount from the same loads. I build homes. from rim joist to ridge board homes dry out and change dimension seasonally more! (You do have to research the foam. High PSI foam exists and has been discussed at GBA. We build Long Eze's out of various psi foams all figure out with some scratching on paper and now some computer time. John Denver loved his Eze for a flight or two. They are fast planes).

    More science more math and less superstition. More experience and more copying exactly what others have done successfully will get you far.

    Read up on the thread with the failed spray foam roof. Nightmare. What happened was all involved "almost" followed proper installation. Almost ended up with water pouring from the ridge.

    "Almost" in superinsulating is a disaster too often. Much more so than air leaky homes of days gone by. And overkill can be worse than under-kill. Two, three, four water trapping layers become liabilities.

    So, are you a builder? designing and building your first superinsulated home?

  16. user-1140531 | | #16

    I don’t doubt that foam


    I don’t doubt that foam under the footing could work without presenting any problem. And I realize that the math proves that foam can carry the load. But does the math really take everything into account? If math proves no settling, where does the 1/8” come from? Are you sure that it would not be ¼” or maybe ¾”?

    We are told that there will be no settling, but even if there is; don’t worry about it because it will be even settling. How does that settling work with the mechanical lines entering the wall or slab? For that matter, how does the settling work with the slab, period?

    The biggest issue I see in placing foam under the footing is the load transfer between the foam and the ground. Perhaps the load bearing at that interface could be perfected by compacting and leveling a granular base material for the foam. But if the contact is not perfectly uniform, it will overload the foam at the high points, causing it to crush and settle until uniform loading is achieved. That could be your 1/8”. What about the foam itself? Can you really know for sure how much it might shrink due to its own chemistry as it ages in conjunction with such high compressive loading?

    You say I should do what others have done as a way of insuring success. How many others have poured footings on top of foam? Have 10,000 houses been built that way yet? Overall, I would avoid placing foam under the footings, not only because of the abovementioned uncertainties, but also because I don’t believe it is that much needed. I already have R-10 inside on the floor and R-40 inside on the walls. So the footings would not be much of a thermal bridge.

  17. [email protected] | | #17

    fox pass
    Is that New Hampshirian or Vinyardian for faux pas? Or perhaps a lack of insulation creating a furry-bypass? Or is it actually a route for fox to come and go? Either way, I like it, and will test it out on my next set of plans: 'GC to ensure fox-pass free construction throughout' ;-)

  18. jhrockwell | | #18

    Don't design in a void

    It really would be useful for you to drill down into the GBA and BSC information about sub-slab insulation, vapor retarders, etc. It seems like you are going to a whole lot of trouble and expense to solve problems that don't have to exist. In particular, see this about the "perfect" wall/roof/slab:

    Why is one of your objectives to prevent drying to the interior? If your water control layer under your footing is violated, the concrete may/will absorb water up into the wall, and you will sorely wish your wall could dry to the interior (no vapor retarder on inside).

    This building science Kool-Aid is really worth drinking! Give it a try!

  19. wjrobinson | | #19

    Ron, my post confused you. I
    Ron, my post confused you. I was trying to just use your and my knowledge of under-footer slab foam as a reference to the fact that you are a fish out of water. This stuff is simple and complicated.

    And I do not advocate any individual fooling with foam under footers unless they are well advanced in there building techniques and are very understanding of homes, buildings, materials, success and failure in trying new methods. The foam under slab has sidetracked discussion.

    Are you doctor or dentist? We just aren't on the same wave length. Easy for many with me but I just can't seem to make sense of our conversations and I see you can't either.

    Time for IPAs.

    And as to the 1/8". I made up that figure. But, an 1/8" is basically ZERO when dealing with building heights and settling, jacking, etc. I have jacked level many an old camp. 1/8" as a tolerance is space shuttle worthy. two IPAs worthy, amazing, bravo time, two pats on the back time.

  20. user-1140531 | | #20

    I am not sure what


    I am not sure what you mean by designing in a void.

    I read that link on the perfect wall, but I don’t see what part of it you think I am violating with my ideas on basement insulation. Take a look at this link:

    In that discussion, Joe Lstiburek reverses his position on claiming that basement insulation needs to dry inward. But in any case, I don’t want any walls drying inward. To me, that seems like solving a problem that should not be allowed to occur in the first place. Wall cavities that get wet are going to grow mold even if they are given a route for drying to occur. So, if the drying route is inward, the mold will come in with the vapor.

    Here is another link to a discussion about insulating basements a few months ago:

    In this article, Martin Holladay says that plastic film should be absolutely avoided in basement insulation systems. He says this:

    “Should I include a polyethylene vapor barrier?
    No. Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere.”

    So here is my question: If it is okay to use XPS or closed cell spay foam directly under the sheetrock (as Martin says it is), why is it a problem to place a polyethylene film there? Both the film and the foam are relatively impermeable. What problem can the film cause that would not be caused by the foam?

    As I mentioned, I am only using the poly to seal the joints in the foam board. Otherwise, the foam alone would keep the summertime humidity away from the concrete. Also, there is the backup of having some foam on the exterior, so the concrete will tend to warm up to the interior temperature, and therefore not condense interior humidity. So perhaps the joints in the interior foam would not need to be sealed. But I don’t see how it could possibly cause a problem. Moreover, it becomes the absolute last line of defense against any inward vapor migration entering the living space.

  21. [email protected] | | #21

    Why not just spray foam the interior basement wall?
    Hi Ron,

    Why not just use high density spray foam on the interior basement wall? This would be a lot less expensive, provide a seamless layer with better R value thus stealing less living space which I assume this basement is- otherwise why put so much insulation on it?

    We spray foam the requested amount on the walls- then build a 2x4 wall to the interior, run wires etc. and we are home free. No need to insulate the outside as well.

    The spray foam will also address the rim joist area which I imagine you are planning to duplicate the same R value you have on the wall.

    BTW, what is your usual rim joist detail with this type of construction? Is this a Passive House or something to require so much insulation? The payback from this must be 100,000 years :) And my last two questions; have you done this type of wall before and do you have any pictures to share?

  22. user-1140531 | | #22

    Bo,This is a superinsulated

    This is a superinsulated design for a small, compact house that furthers that energy efficiency objective. The joist detailing is a complexity particularly where they run perpendicular to the insulation, and I agree that spray foam would be handy for that detail.

    I have not built this design, but I have built one with the insulation on the exterior of the basement. I am looking what is right and wrong with that one and starting with a clean sheet of paper. This new design is only a 1300 s.f. foundation. The basement is intended to be living space if needed. But with a small, tight, superinsulated house, the main issue with the basement is to not let it drag down the quality of the aboveground space. And to meet that objective, I have concluded that the basement must be extraordinarily well executed.

    I don’t have any digitized pictures of the first basement system. I ran 2” XPS all the way down and over the side of the footings, and another 2” down to four feet. I also ran a 2” XPS skirt all around, extending out about 2 ft. That is a couple feet below finish grade. I also ran a full exterior drain tile system of 4” PVC in a bed of drainfield rock. I covered the insulation above grade with ¾” CCA plywood, and stained it. I considered several different approaches to covering that area. The neighbor troweled on something on his exterior basement foam that pealed off a couple years later.

    I believe that good soil engineering and soil surfacing is critical to a properly performing basement. All of the backfill was high quality fill sand compacted in 6” loose lifts. The surface of the backfill is sloped for drainage, capped with a membrane consisting of a layer of heavy duty polyethylene sandwiched between layers of geotextile fabric, and covered with 12” of crushed ¾” trap rock, tamped to settle it. The fabric/poly layer is brought up to the ¾” plywood, turned upward, and stapled to the plywood.

  23. user-1135248 | | #23

    adapting to *existing* foam
    Not to try and hijack the thread, but I've got a somewhat
    interesting one ... there's already an inch of old white *EPS*
    on my basement walls that was already here 20+ years ago when
    I moved in. [Evidently the previous owner read in Pop Mech
    or something back in the seventies that it would save energy
    and glued 2x8s of this stuff all over the walls.] While nobody
    has mentioned it yet the time may come when I need to apply
    some sort of ignition barrier to it, likely an intumescent
    coating of some sort. However, I don't want that to become a
    vapor barrier on the wrong side of this stuff! Any recommendations
    on permeable coatings that are fire-marshal acceptable? While
    the ultimate right answer is to rip it all and replace with
    Thermax, I'm not quite up for doing that yet.


  24. GBA Editor
    Martin Holladay | | #24

    Response to Hobbit
    Q. "Any recommendations on permeable coatings that are fire-marshal acceptable?"

    A. Gypsum drywall.

  25. Expert Member
    Dana Dorsett | | #25

    That much XPS... (response to Ron Keagle)
    That much XPS would have a net 100+ year lifecycle greenhouse effect greater than the energy use it offsets, if it's blown with HFCs (like almost all N. American XPS is.)

    Where you can, R-for-R substituting iso or EPS would yield a roughly 200:1 reduction in greenhouse potential related to the blowing agents. EPS works fine under slabs, and on the exterior of foundations too (with the possible exception of high termite-risk zones, and there XPS would have similar issues.) Iso (foil or fiber-faced) is suitable for insulating the interior side of foundation walls, but could slowly saturate with water if used on the exterior or under slabs.

  26. [email protected] | | #26

    Drywall is the best idea
    Hello Hobbit,

    I agree with Martin- drywall is your best bet- cheap, easy and safe!

    We use DC-315 as a thermal barrier paint. However the only foam that it is rated to cover is polyurethane, (also plywood and several other substrates)- not polystyrene like you have.

    As I understand it, and that is very little, EPS and XPS have very low melting temps and turn to liquid so soon that the paint does not have time to protect the foam for the required amount of time to be considered a thermal barrier- 15 minutes. At this point, I know of no paint that will protect any foam other than polyurethane.

    Drywall, on the other hand, is rated to cover them all!

  27. wjrobinson | | #27

    Drywall can be purchased that
    Drywall can be purchased that uses no paper on the faces, instead fiberglass. Check your humidity.

  28. Tallie | | #28

    Basement Refinishing
    I live in a 1914 home in Zone 5. The basement walls are block with a smooth surface. They were once painted. The paint is peeling and there is some effervescing. I have not had any water issues in the 7 years I've been here. Though it feels somewhat damp during the hot summers.

    I was thinking about using Roxul as an insulation material. The rock wool that does not retain moisture. Do you prefer rigid foam rather than Roxul? Would it be appropriate to place rigid foam against the wall, build the frame out over the foam and use Roxul as the filler insulation? And I've just learned from your blog that I should not use a vapor barrier in a basement. Correct? (thank you)

    For Zone 5 (Chicago) would you suggest bluwood or kiln dried treated lumber or regular pine for base plates and wall studs? Thanks.
    with gratitude,
    ~ Tallie

  29. Expert Member
    Dana Dorsett | | #29

    Roxul AND foam works!!
    First, if you have efflorescence it's usually an indication of high moisture content, and bulk drainage on the exterior may need to be improved.

    In US zone-5 you can't use a studwall & Roxul approach without running into mold issues. In winter the above-grade portion of the wall will track outdoor temps, and will be well below the dew point of the basement's interior air resulting in condensation (or even frost) conditions. But if you put up an interior side vapor barrier to protect against wintertime moisture, you end up trapping ground moisture in the studwall for the same (or worse) high-moisture & mold risk problem.

    But with as little as ~R4-5 of rigid foam (1" of EPS or XPS) against the foundation the interior air stops at the foam/Roxul boundary, which is much warmer (on average), and both the amount & persistence of condensing is quite low even at center-cavity in the studwall. Use unfaced Roxul, and no interior vapor retarder stronger than standard latex paint and you're good to go!

    It's always a good idea to put an inch of foam between the bottom plate of the studwall and the slab too, as both a thermal & capillary break. An uninsulated slab will almost always be at a temperature near or below the summertime dew points in Chicago, and wood directly in contact with that cool slab will take on moisture all summer just from the air even if the slab is quite dry and well drained. Many slabs that vintage have poor drainage (or none, poured directly on dirt), and can wick copious amount of ground moisture too. With an inch of either EPS or XPS under the bottom plate of the studwall both issues are addressed, and standard grade framing lumber can be used, anchoring the bottom plate to the slab with TapCons (or similar masonry screws), drilling through both the wood & foam into the concrete. Since the studwall isn't structural (the foundation wall is what's holding up the house) the compressive load aspects of the foam don't much matter.

    If you're not going to insulate the slab itself it's still worth sealing it to limit rates of capillary draw. Silane/Silex based sealers can be had which are also highly vapor-retardent as well, but even acrylic-based sealers are way better than nothing.

    Most summertime humidity in uninsulated basements are from outdoor air infiltration. Dew points in your area average in the mid-60s F and excursions into 70+ land aren't uncommon. When that air sits in a 65F basement your relative humidity is quite high. When insulating the foundation wall it's well worth air-sealing & insulating the band joists & foundation sills with an inch or two of closed cell polyurethane, that also seals to the top of your wall-foam layer.

  30. user-1135248 | | #30

    Drywall comes with the
    Drywall comes with the problem of attaching it over the existing
    inch of styrofoam, now we're talking at a minimum a bunch of
    furring Tapconned into the block and the problem of what to
    fill those new gaps with. I'm looking for direct surface
    treatment, if any, of the foam itself as an *ignition*
    barrier to meet code, not a thermal barrier to hold off an
    hour of raging fire on the other side. Drywall wouldn't
    do the latter either. Besides, I really hate drywall.

    The extent of fire hazard of this stuff is clearly overrated.
    It tends to shrink away from a heat source very quickly, and
    one must be quite persistent to get enough of it to melt
    together and sustain a flame and half the time it extinguishes
    anyways. There are some divots where it's clear that a plumber's
    torch played on the stuff for a short amount of time, and it
    didn't actually catch fire or even come close. I'm confident
    that I could weld or grind metal down there and none of any
    sparks hitting the foam would actually start a fire.

    It's plain ol' white styrofoam, same stuff that Joe used six
    inches of in his first barn retrofit so let's have done with
    the global-warming irrelevancies. Fortunately, mine is out
    of reach of the ants where it is. The nice thing is that
    EPS is quite permeable so everything behind it dries to the
    inside and clearly the previous owner who put it up got lucky
    in that regard, because I'm sure he wasn't thinking about
    moisture movement. That's my only drying option, thus the
    need for a permeable barrier if any barrier at all. And
    without putting a bunch of holes into my cinderblock.

    The best I've researched up so far is "Fireshell" from,
    but at only 0.9 perm it would be a little dicey for this.


  31. Tallie | | #31

    Thanks Dana
    WOW!! Thanks Dana!! It seems your right on target for what I need to have done. I can not thank you enough!! Would you happen to have a web site or a blog that I could also peruse? ~ Tallie

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |