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Why Seal and Insulate the Basement Ceiling

Paul Kando

From time to time energy audit clients call to complain that, in spite of clear instructions, their insulation contractor insists on insulating part of the basement walls rather than the basement ceiling. One brash fellow even told a lady: “We don’t do that Maam”. Huh? That lady was a customer who paid for an independent energy audit which recommended that the basement ceiling be sealed and insulated. “Do you want to heat your basement?” I asked the lady on the phone, because doing that would add almost 1/3 more to the heated volume of her house and to its heat load, not to mention the continued discomfort of a cold floor. Manners aside, this is a serious matter.

Take the first floor of my own house, 960 square feet in size. When we moved here, there was nothing between this living area and our unheated basement, but two layers of wood floor about 1.5" thick. At 0.94 per inch, that’s an R value of 1.41. In the dead of winter my unheated basement assumes more or less the temperature of the ground that surrounds it. On a recent January morning, with 3ºF outside, it was 45ºF. We like to keep the temperature of our home at 68ºF, so the temperature difference (ΔT) between living room and basement is 23ºF. The second law of thermodynamics tells me that heat travels from hot to cold, in this case downwards, from living room to basement. It was a no-brainer, therefore to seal and insulate the basement ceiling.

How much heat had I been I losing before? The heat transfer equation for a whole heating season is H = U x A x HDD, where H is the heat transferred, U is the reciprocal of the R value (1/1.41 = 0.709), A is the area through which the heat is transferred (960), and HDD stands for 7342 heating degree days, the cumulative total of daily temperature differences (ΔT) as recorded by the nearest NOAA weather station, in Wiscasset. Thus, given the fairly constant temperatures of both my living room and my basement, my seasonal conductive heat loss through the uninsulated basement ceiling worked out to be 0.709 x 960 x 7342 = 4,997,259 Btus. That’s equivalent to 45 gallons of oil burned at 80% efficiency or 61 gallons of propane burned at 90% efficiency – costing, at today’s prices, $171.45 for oil and $198.25 for propane.

I sealed and insulated my basement ceiling 5 years ago. It now has an R value of about 20 and my current heat loss, 352,416 Btus for the season, is equivalent to 3 gallons of oil ($11.23); or 4 gallons of propane ($13.00). At today’s prices, over 5 years I saved $801 in oil (or $926, had I been using propane). I sealed and insulated the basement ceiling myself, spending about $900 on materials, so I have almost recovered my investment in fuel savings by now.

However, this is not all. Our basement gets wet from time to time. During the heating season, much of that moisture used to travel through the house as water vapor borne by rising warm air filtering into the cold attic through leaks in the ceiling. To warm a pint of water (weighing about one pound) takes roughly 1 Btu per degree of warming and 970 Btus more to evaporate – a total of 993 Btus per pint of water in my case. That heat used to be delivered to the cold attic, where the vapor would condense on the underside of the roof deck, slowly rotting it. The condensing water would also release all the heat it absorbed along the way. The roof would absorb the heat and melt the snow on top. The melt water would run down to the much colder eves, where it would re-freeze, causing ice dams. Next thing I’d know a bedroom ceiling would be leaking, causing much annoyance and damage.

With the basement ceiling sealed and insulated, and leaks into the attic plugged, that natural process has been interrupted. Furthermore, since the living room floor is no longer cold, our comfort level has increased considerably. That’s got te be worth something, after all the whole point of having a heated house is indoor comfort. All things considered, I figure I already got my money back from comfort and energy savings.

For all these reasons, when Mr. Brash shows up to insulate your home, it’s OK to tell him he was not hired to argue either with his customer or with the conclusions of her independent energy auditor’s report. Mr. Brash may do as he is told, or pack up and leave.