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Measuring home heat loss

  1. Jan 23, 2007 #1
    What is the most practical method of measuring where a home is losing heat?

    The idea is that these measurements would be the first step in calculating the most cost-effective improvement, deciding whether to increase insulation in ceiling, walls or floor, to upgrade to double glazed windows or thicker curtains or just to seal minor drafts.. and on what timescale it might be cheaper to just install more powerful gas heating instead.
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  3. Jan 24, 2007 #2


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    You won’t need to actually measure loss of each component because calculations will be accurate enough. You should measure overall heat loss, however, so you can plot your progress. Invest in an electronic thermostat that also displays the number of hours that your furnace is on per day and cumulatively from the time you reset the counter. Establish a baseline measurement, such as the number of hours of heat used during the month of February, and record the average daily temperature (it’s available on your energy bill) as well as the average daily temp inside your house. This will allow you to calculate the savings in years to come.

    To figure out where to put your money and effort, calculate heat loss through each part of your home. Charts like
    give you the insulating efficiency (R-value) of different building materials, from which you calculate heat flux as
    where A is the area.

    Example; I bought an old house in Colorado that had a 7’x9’ single pane sliding glass door with a highly conductive aluminum frame. The window was iced over most of the winter and the family room was uninhabitable. If the average R-value was about 0.6 and the daily average difference between indoor and outdoor was 30F (mild winter), then
    [tex]\dot{Q}[/tex]=3150 Btu/hr. Sorry about the British units but that’s what’s used in the US HVAC and energy industries...

    I installed a double-pane Low-E French door assembly with insulating shades, R-value about 3.3, so the savings is about 2.5kBtu/hr or 1.8e6Btu/month. My gas furnace is about 82% efficient (value available from the manufacturer) for a savings of 22 therms or $15-25 per month (natural gas prices vary from year to year) from just this one item. We would have done this upgrade for aesthetic reasons anyway, but with this kind of calculation I could determine the benefit (and also balance what sort of window with how much insulation versus cost).

    Here are the rules of thumb:
    1. Seal all drafts first. They are the biggest winners, while conversely if you don’t take care of them they’ll undo the effectiveness of any other improvements. Many of the biggest ones are hidden; for instance, the vertical chase of each “wet wall” (space for pipes behind your tubs and showers) is usually open to the attic.
    2. Next take care of the most egregious offenders, like the window discussed above.
    3. Next go from top to bottom.
    a. Attic is a huge area and is easy to upgrade. Roll out new unfaced (so they breathe) R-25 fiberglass batts to bring the thickness to around 12 inches (R-38). Fiberglass is so cheap that this is a big win.
    b. Next do windows, and garage walls (insulate and drywall) and door (insulated unit) if you have rooms above.
    c. Can’t do much with walls unless you want to demolish the house. R-15 is typical for wall with 2x4 studs, R-11 insulation, drywall inside and wood sheathing outside.
    d. Last in effectiveness is under-floor insulation.
    4. You mention a more powerful furnace; if you insulate your house, you may get by with a smaller one instead, as I did. Upgrading the furnace will save money if existing furnace is old and inefficient. New furnaces are 80+ to 90+% efficient compared to 60-70% for 30 years ago.

    The cold rooms in my house are now comfortable and the furnace rarely turns on unless in gets really cold outside. The calculations predicted pretty well the energy savings I actually observed (this isn’t rocket science after all…)

    You can get more sophisticated later if you want. I've measured the thermal time constant of my home (it's now about 40 hours), the effective heat capacity, etc. These are amusing for a physicist, but the simple equation and chart above are all you really need.

    Have fun!
    Last edited: Jan 24, 2007
  4. Jan 24, 2007 #3


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    This one can be huge.
    I cut my oil usage about 50% for a sheet of foam board, a bit of that foaming sealant stuff and about a half hour of time.
    Sealed off the opening into the attic.
    Total cost < $10.

    I found it because I was in the attic on a -20F day.
    All of a sudden I realized, I'm warm :uhh:
  5. Jan 24, 2007 #4


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    Infiltration is also the most difficult/inaccurate to calculate. And it really is a huge problem even if it is only a little bit of air because of the heat content difference.

    For the calculations, the easiest way to do it would be with software - and with the trial version of a program like this, it'll give you heat loss for a single zone, doubly easy because you can pick the type of wall/window and it'll tell you the R value - no need to calculate it yourself.
  6. Jan 25, 2007 #5
    For me, my front door doesn't close perfectly flush, so I added some caulking foam where there were small gaps and now I can't notice a draft when I run the back of my hand along the seal. Also, my sidedoor mail box was extremely leaky, so I applied the same foam to it and voila, instant savings. Finally, if you have the money, consider upgrading your windows to double windows with argon in the spacing. They are fairly cheap and the heat savings are enormous.
  7. Jan 26, 2007 #6
    None of the above actually answer the question...

    I like Mr Cesium would be interested in measuring the actual highest loss from my house rather than the theoretical or most apparent.

    For example I have replaced the 80's aluminum windows in my house with Victorian Sashes to bring the house back to how it should be. I've also taken out the nasty carpet that has been down since the 50's (I know that by the cigarette cards and newspaper I found underneath) and stripped the floor boards.
    Now is it more cost effective to seal my windows during the winter, or pack the floor void with insulation. i.e block the draft in or the draw out
    1st cheap, 2nd expensive.

    And that is just in one room. I've tried using smoke matches to track the drafts with limited success and my budget doesn't streatch to a thermal imager.
    Is it possible to bodge an old ccd to respond more in the infra red?
  8. Jan 26, 2007 #7


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    A digital camera with the first lens filter removed will be more receptive to infra red. For actual measurement, there are companies which specialise in thermal imaging (thermography) who will report for you on which areas of the property are losing how much heat. I've found these services to have been extremely informative in the past.
  9. Jan 26, 2007 #8


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    A calibrated thermal imaging camera will tell you local temperature but you'll still need to use models of conduction, convection, etc. to calculate heat loss from that. A thermal camera is mostly useful to identify areas that need attention rather than giving quantitative values.

    Don't understand your question about windows, floors and drafts. Seal drafts first, as well as you can. If you can't feel any leaks in winter and don't see smoke sucked or blown, then you're doing fine. Next increase insulation in attic, then windows. Don't overlook value of insulating shades/curtains (honeycombs are good, "Warm Window" shades which seal to the wall around the windows are best because they stop convection). Insulating floors is least effective, but if your floor is open and accessible anyway (eg., open joists reachable from crawlspace) then it's easy and cheap to add fiberglass batts. Hold them in place with metal spring-rods sold where the insulation is sold. The vapor barrier goes towards the damp side, which in the western US is usually the house side. I wouldn't insulate floors inside heated areas like between 1st and 2nd floors.
  10. Nov 13, 2008 #9
    Maybe use a digital thermometer (omega.com or from RadioShack) and take measurements of the temperature of the various surfaces of the inside of your house. Measure the outside air temperature and, as a 1st approximation, take this as the outside temperature of all the surfaces of your house. Your heat transfer will be greatest through the surfaces having the temperature closest to the outside temperature.
  11. Dec 3, 2009 #10
    I have one of the Black and Decker thermal leak detectors it works nice. Looking for cold spots from the inside or warm spots from the outside is effective. If you stick a couple fans in a door/window to lower the pressure in your house the detector will be more effective at showing infiltration spots by drawing cold air in through the holes.

    That's not measuring though, that's detecting. To measure I suppose you could turn all your thermostats to something high (75-80F) and then kill the heat source when you get there. Then for each thermostat measure the temp over time. Using the delta T over time and the specific heat for air (and that you know your houses air volume) should give you your rate of heat loss in BTU/hr per zone. Then you can prioritize reducing the rate of heat loss zone by zone. Note that this rate is linear with the outside temp, if you factor out outside temp you get the heat loss coefficient which is probably the closest thing you can get to measuring a home's heat loss.
  12. Dec 29, 2009 #11
    I came here after a google search.
    I had the idea of lowering my thermostat 5 degrees and note how long it took to turn on again.

    But my biggest problem seems to be comparing it to data that I haven't found yet.
    I wanted to see my house compared to other houses.
    Even if I calculated BTU/hr it's just a number in my head.
  13. Dec 29, 2009 #12
    You can use your existing furnace to calculate heat loss, providing you know its efficiency with a duty cycle other than 100%.

    You need your Heating Degree Days for a month, your fuel usage for that month and your Outside Design Temperature to a 95% or 99% level.

    My spreadsheet that does this is almost done.
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