Saturday, December 11, 2010

Air infiltration

I think air infiltration doesn't get the attention it should.  Air sealing gives the best heating savings for your dollar when compared to almost every other building envelope improvement; insulating exposed concrete basement walls is the only other thing I can think of that comes close.

I'm pleased to see marketing funds being spent on this issue, like this commercial featuring David Suzuki.

Using Hot2000 to model a house located in Halifax, NS with R26 walls, R40 ceiling insulation, and energy efficient windows, about 27% of the heat loss in January comes from air infiltration.  Hot2000 doesn't count the heat loss due to the moisture in the inside air (latent heat), and recent research indicates it underestimates the total air infiltration rate.  Considering Hot2000 errors, a more accurate estimate would be 40% of the heat loss coming from air infiltration.

Although I think the best way to find air infiltration points in a house is with a blower door test (typically $50-$100), there is a cheaper way.  I read on the idea to turn on a clothes dryer, then go around your house checking for leakage.  If you have a kitchen range hood exhaust fan and bathroom exhaust fans, turn these on too.  In addition to checking the usual places like around windows and electrical boxes, check for air infiltration along the floor on outside walls.  If you have tile or hardwood floors, a bead of translucent or clear caulking between the baseboard and floor can cut down air infiltration.

Sunday, October 17, 2010


Choosing energy-efficient windows is probably the most complex aspect of building.  In this article I'll offer some advice to simplify the seemingly innumerable choices.  The focus is on northern climates, where heating energy costs significantly exceed cooling energy costs.

Most windows now include a specifications label. All these labels include a U-factor which indicates how much heat is lost out the window, and a solar gain (SHGC) which indicates how much heat from the sun is gained through the windows.  A good window should have a U-factor less than 0.3 and SHGC more than 0.5.  It should use an Inex spacer or Super Spacer at least 5/8" wide.  A window with a cheap aluminum spacer will lead to much more condensation on the window in winter.

Use fixed(picture) windows where possible instead of operating (slider/casement).  Not only are fixed windows cheaper, they have smaller frames than operating windows which allows for more glass area and a higher SHGC rating.

According to LBNL Resfen, a window rated U 0.29 and SHGC 0.56 facing south in Portland, ME will have a net gain 76,430 BTU of energy per square foot of area.  This is the amount of heat energy from 22 kWh of electricity or about 2/3 of a gallon of heating oil.  The same window facing north will have a net loss of 17,500 BTU of energy per square foot of area, so minimizing north-facing windows in a new build reduces heating energy use.

Wednesday, April 14, 2010

Hot Tub (Spa) Power

We recently purchased a 120V hot tub. We installed it in our indoor pool area, which was unheated this winter as the insulation is not completed. The unit is insulated with what appears to be low-density spray foam and has a ~2" insulated cover.  The bottom of the tub (below the footwell) doesn't seem to have any insulation.

I recorded the power use in January using a P4400 when the air temperature averaged 32F/0C and the temperature set to 103F/39C.  The tub was using 14kWh/day which works out to $50/mth @11.8c/kWh.  When we switched to TOD rates I put a timer on it so it only comes on during off-peak rates, cutting electrical costs down to around $20/mth. It will drop from 103F at 7am when the power shuts off to the low 90's by the evening so we turn on the power manually in advance when we plan to use it during the week.

A tub 8' * 3' high has a surface area of 126sf, so with a 100F delta-T and R10 insulation the heat loss would be 1260BTU/hr or 370W.  Therefore a well-insulated outdoor tub should cost around $30/mth to operate in Jan/Feb at standard power rates or half that for off-peak.