Basement Insulation — Part 2
After moving everything away from the basement walls, I used plastic Hilti fasteners to hold the polyiso to the concrete
In Part 1 of this blog series, I mentioned that I had to move a number of infrastructure items away from the concrete walls so I could have an unbroken insulation installation. In retrospect, it was a good idea to move them, and the time it took was at least partially compensated for by not needing to take time to fit the foam around the obstacles.
- The stairs to the basement from the first floor were framed hard against the concrete. I was fortunate in that the stringer against the concrete was not fastened to the concrete, but rather rested on a pressure treated 2x4 plate fastened to the floor. In the photo below (Image #2), the lower end of the stair stringer has been cut, and then moved away from the wall and scabbed back to the original stringer with a piece of plywood cut to match the stringer profile. All the treads that butted to the wall have been cut back about two-and-a-half inches, then re-nailed to the stringer. This was perhaps a two hour task, which would have been faster if I'd known about Japanese-style pull saws. The oil tank in the background is of course history...
Once I got to the foam installing process, the foam slid right in (see Image #3, below).
The electric panel I left where it was. I did install a piece of plywood adjacent to the panel over foam for the electrician to mount the PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements. and various meters.
The PVC waste plumbing from the kitchen sink and washing machine was hard against the concrete, but moving it entailed only three cuts, and two extensions with two couplings each, and one shortening with a single new coupling (see Image #4 and Image #5, below). One advantage of messing with this was that I discovered and repaired the joint that the plumber failed to glue originally :-) Maybe another two hours here.
The potable water pressure tank was moved out with two couplings after draining it — an hour's worth of time (see Image #6, below).
The washer hoses and electric receptacle, and other switches, were the hardest to move, because they were fastened to the concrete with powder-actuated fasteners — ones that are literally shot into the concrete. That took a lot of prying and I managed to whack myself in the head with a prybar when one let loose all at once.
Two layers of polyiso, with staggered joints
Once all the clearances were established, I set to work installing the foam. It went on in two layers, with the joints staggered to help it be airtight (see Image #7, below). In tight quarters sometimes a full 4-foot-wide sheet won't fit, so one trick is to cut it from the rear and snap it carefully so it bends and can be eased around an obstacle like a water heater (see Image #8, below). When it has space to flatten out the cut won't be visible.
To see the tools of the rigid foam installer's trade — at least those used by this DIYer — see Image #9, below.
Allow me to rhapsodize about the Hilti hammer drill. It has two modes, but the cool one is that the chuck oscillates in and out as well as around, which flings the hardened drill bit against a hard, brittle substance like concrete and makes drilling a piece of cake. Awesome tool — I didn't want to return it to Pete Ives.
Each of the approximately two hundred fifty holes I drilled took five to ten seconds, and I did it all on one bit (they must be hard!) I used the Hilti IDP polypropylene fasteners (see Image #10, below), and to use this one drills a 5/16 inch pilot hole and then whacks the barbed IDP into the hole through the foam (see Image #11, below). The IDP is made in various lengths to accommodate different foam thicknesses. I like the system because it's easy, and there is no metal to corrode or act as a thermal bridge. And it has a big head that acts as an integral washer to spread the load out against the foam.
Seal the panel seams with white foil tape
I cut the foam off flush with the top of the concrete so when I sprayed closed-cell foam at the rim joist I would be able to lap the spray foam onto the rigid foam.
Once all the foam was installed, I taped the seams with a white foil tape that Dow supplies. I found it difficult to separate the tape from the release paper, so I'll look for another white tape.
Once the foam is installed and taped, the white color makes an immense difference in the brightness and sense of cleanliness in the basement. It immediately got warmer and the relative humidity dropped. As of early August, I have seen no condensation moisture on concrete in the basement, a big change from last summer where the slab/wall joint and a foot or so up the wall got damp. The basement doesn't have that signature Martha's Vineyard sine qua non, the dehumidifier, and it doesn't appear to need it.
Tips for using a two-component spray foam kit
The next step was to insulate and air seal at the rim joist, so that the thermal boundary was continuous to the bottom of the subfloor above. I chose to do this with a two-component spray polyurethane kit that I bought from my friends at the Energy Federation. This kit comes has two steel cylinders, hoses, a spray gun, and a couple of dozen spray nozzles (see Image #12, below).
First I had to pull the fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. out of the last floor joist bay where the joists were parallel to the foundation wall, and I folded the batts back on themselves where the joists were perpendicular to the wall. This allowed me space to get in and spray foam against the rim joist and down over the sills to the top of the rigid foam.
I put some polyethylene over my valued junk and got all the bikes out of the basement so I wouldn't end up with tiny (or larger) foam blobs on my stuff. I wore a protective mask (regular users of polyurethane spray foams should be using an outdoor air supply, because one of the components can cause asthma-like symptoms in some people with repeated exposures — see the Spray Polyurethane Foam Health and Safety website for good information about hazards and how to deal with them), goggles, and a Tyvek suit and rubber gloves, all to keep the stuff off of me (see Image #13, below).
Once you start the process, you keep going. Whenever you stop spraying for thirty seconds the nozzle clogs and you snap on a new one. I got the whole job done in about two hours (see Image #14 and Image #15, below).
The foam didn't expand as fast as I thought it would, so I didn't get a very even job of it — some areas got more than others. I think I got the air leakage spots because I used a fan spray nozzle and sprayed into the cracks first in each area. I was told that 70°F, the temperature of the basement when I was spraying, was at the low end of the range for this foam and that I would have gotten better results had I heated the foam to 80°F or so.
The project cost $4,000
When I was done, I re-installed the fiberglass batts. I had gone back and forth about whether to remove all the batts. What I concluded, rightly or otherwise, is that it wasn't critical to me that the basement was as warm as possible. If there is no insulation in the frame floor above the basement, there will be more heat flow down to the basement and it will be warmer. By leaving the batts in place, total heat loss to the ground from the occupied portion of the house is reduced, but the basement space is between two layers of insulation, that of the floor above and that of the walls, so it will be cooler.
I reckon I spent close to $4,000 on this project, or about $25 per lineal foot of basement wall. There's a total of perhaps seven to eight days of my time in the whole shootin' match. It would be less with a house with fewer than sixteen corners. It's not primarily for energy savings; rather it is to make the basement a more usable space without needing a dehumidifier running. We see some of our clients' electrical bill double in the summer due to dehumidification. With the foam in place, vapor transport from the soil through the concrete is significantly reduced, and the warmer temperature of the basement means relative humidity drops. The change feels totally worth it to Jill and me.
Once it was all done, I could begin to put things I want in the basement in place where they belong and cease moving them around to get them out of the way. Right now there are twenty-five chicks less than a week old down there in the OK Chicken Corral, beginning their nine-week journey to our freezer. But that's another story...
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.
- All photos: Marc Rosenbaum
- Marc Rosenbaum
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