Monday, February 8, 2010

Stack effect

I had been frustrated recently by the fact that even after our rather assiduous insulation efforts, this old house was still not keeping heat as well as I would like it to this winter.

The evidence for this is in the firewood pile, which has been going away just as fast as it usually does. I want to slow this combustion down.

So I went on a mission, before the Superbowl, to seal up air gaps. One of the items I teach is how to calculate building heat load, and so am very familiar with the general injunction to fix the building envelope first, then do the insulation. But when we went through this house, before we moved in, we did new drywall all around, placed new foam board insulation under the new drywall, and put in new windows all around, so I had always thought that the place was about as tight as I should get it. There were a couple air leaks I knew about, but you shouldn't make a house heated by wood too tight.

Well, I was wrong. The remaining air leaks were large, and now I fixed them it's a lot more comfortable in this place.

I started with a window that had a saggy vinyl frame on one side. I put in shims and closed the gap. Then I caulked up a couple of cracks in our two old kitchen doors.

Next I switched to insulation. The cellar-head space, which is also the space behind the stairs, had never been insulated, although the cellar ceiling had R 19 fiberglass I put in three years ago to keep the downstairs floor warm. I put R 19 below the staircase and on one wall and watched the inside temperature of that wall, measured with the laser thermometer, climb from 59 F to 65 F. I need a bit more R 19 to finish the job. I think I may detail it with foam board over the glass, too, so the R 19 can't sag.

Then I put clear caulk on the gaps between the stair treads and the wall, preventing cold air coming up from the cellar this way. Standing on the second or third step of the cellar stairs, I saw light through a big gap under the cellar door threshold. Some spray foam took care of that.

Finally, standing over the floor vents for the old forced air oil furnace, I could feel cold air gently rising. I measured the temperature of this air at 47 F. Each vent is about a square foot of air gap! I knew that this mostly disused furnace was a source of cold air, but I never realized quite how much.

Aimee had some magnetic plastic stick-on sheets for keeping dog hair out of the vents in the summer. I went to get these and covered up all but one cold air return and two of the smaller vents.

The combination of these treatments, but especially the vent closing, was impressive. Almost immediately the downstairs began to climb from a stable 65 F to 70 F.

This is the 'stack effect" where hot air escaping though the chimneys and any cracks and gaps at the top of the house draws cold air in from below, in this case through the furnace. It also helps to explain why turning the furnace on for just ten minutes makes such a big difference to the comfort level in the house.

I would like to shut off all the durn furnace vents, but I'm worried that the basement will get too isolated from the rest of the house if I do that. We have a propane hot water tank and a lot of water pipes down there, and the more sealing and insulation I put between the main house and the basement, the less protected they are. I also have air ducts serving the basement itself, and I don't think I can stop using them.

The basement is cold. It's the dead of winter and old man frost has reached into the ground about as far as he will go. The basement walls, which are crude masonry and concrete, read from 23 F in the coldest spot, up high, where there's no soil protection, to 39 F in the warmest, down below. The air temperature is steady at about 35 F. Any air in those basement duct work is going to be chilled down. And if the stack effect means air is drawn through the duct work even when the furnace is not operating, that will just make things worse. The propane burner heats up the place where all the pipes are, which helps a good deal, and the pipes themselves are copper and conduct heat away from the propane tank nicely, but 35 F is still marginal.

When I reorganized the forced air vents to the kitchen, where we knocked two small rooms into one big room, I had a forced air duct left over, and I made a "Y" out of this, playing one jet on the basement water pipes, and the other into the crawl space under the kitchen floor where there are more pipes.

The result is that the forced air furnace is now a kind of fail-safe back-up to keep the pipes from freezing. So I can't close off those remaining three floor vents.

Not without putting some other source of heat in both places. Or insulating the whole thing with Core-Bond or similar. I'm not sure I want to do either because we use the basement as a root cellar.

I'll figure something out. The furnace will die of old age, probably, and I'll tear the stupid ducts out and put in some other kind of furnace system. The insurance company will require us to have a furnace, so I can't just not have one. Probably I'll run Pex hot water tubing under the floorboards and above the R 19, from a second, smaller propane tank, and so have underfloor heat. That will make Aimee, who likes to go barefoot, very happy.

In the meantime, I'm pleased with the results of my labors. I was able to shut down the big wood furnace three hours earlier than normal, while we watched the game in perfect comfort with the whole house heated to 72 F by the 20,000 BTU kitchen wood stove and a small electrical heater. And the oil furnace didn't kick in at 4 am like it normally does when it's cold out. The outside temp was 10 F, which is a bit warmer than it has been, but still pretty cold.

It just goes to show, you can never get too detailed about building envelope and penetration. Every gap counts.

1 comment:

  1. Way to go Mick. Our farm house in Missouri was the pits, but this house in Vermont is pretty darn tight.


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