Basement insulation, part 2

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 prvention 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.

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 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 at House 5. 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 House 5 was insulated, the insulator made the curious choice to insulate 2x10 joists with fiberglass batts that fit 2x6s. This by itself is not unreasonable in the context that the above grade walls were 2x6, and the insulation level to a basement that might be in the range of 45F to 60F 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 sub-floor 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 below, 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.

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 thousandths of an inch white aluminum facer that is moderately durable (there  are three facer thicknesses to choose from, I chose the middle, 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 two inches of polyisocyanurate foam, in two one inch 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.

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. Photos to follow in the next post.

 

Comments

[dana]

With foil faced iso on the interior the foundation can no longer dry toward the interior, which raises the risk to the foundation sill, unless a very good capillary break is in place between the foundation sill & the concrete. Keep an eye on it! (Maybe even seasonal spot-checking the timber with a humidity meter.)

Reclaimed EPS or fiber-faced iso(from commercial re-roofing project) can usually be had for ~3cents/R/square foot, sometimes less, but you have to dig a bit to find a source (craigslist, or in central MA, The Insulation Depot in Framingham- pick up only for small quantities), and is perfect for basement insulation projects.

1x strapping and tapcons through-screwed to the foundation are one way to mount glass faced gypsum (or greenboard) are mold-resistant ways to achieve the code-required ignition barrier. In an air-tight house with a mini-split, controlling the indoor RH to something that results in a 60% RH or less in a conditioned basement should use very little power.

In most of MA subsoil temps are low enough that insulating the basement floor to at least R8 (if virgin-stock, more, using reclaimed foam) either over or under the structural slab. If over the slab, if the basement is at all susceptible to flooding a 2" concrete rat-slab poured over the foam works as an ignition barrier but if the basement is to be finished and it's flood-free, an OSB or plywood subfloor (anchored to the slab with tapcons) also meets code.

[Foam Roof]

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[spray foam wall insulation]

With aluminum foil confronted is on the internal the groundwork can no more dry toward the internal, which increases the danger to the groundwork sill, unless a very excellent capillary separate is in position between the groundwork sill & the tangible. Keep an eye on it! (Maybe even periodic spot-checking the wood with a moisture gauge.)