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.