Heat loss through walls by BTU rate

EDIT - on the subject of BTU

Sorry admins can you get in touch with me, I’m unsure if I am allowed to ask this question. I’ve redacted it.


Looking for an explaination on,

  1. How best to calculate degree days, and
  2. How this relates to rate of heat-loss regarding BTU’s, and
  3. How you could use this information to compare heat loss at one time of the year versus another time of the year.


Hi Mark / Adam,

I think you’re the two running most of the wall type sessions on Sundays, figured its best to try to draw your attention to this question.

BTU’s and energy loss generally seem to be a fairly confusing topic. I’m wondering how I should be thinking about a rate of loss for a given wall type, and how that fluctuates through the year / how to compare.

IE, for a given BTU energy loss during the summer, I’m assuming that needs to be somehow based on degree-days or at least related to it. And then how would you extrapolate that to winter, would it need to be based on R-values, opposite-degree-days, something else?

Its easy to find explainers on these topics in isolation, but of course the exam combineis them in unexpected ways that are often confusing …

Many thanks

Ben -

If you recall, the formula for calculating heat loss is:

Heat Loss (btu/hr) = ((Area(sf) * Temperature Difference(°F))/R-Value (Btu °F h/sf)) * 24 (24hrs a day)

Using this formula, the Temperature difference is the value that would fluctuate from day to day as the outside temperature varies from day to day (i.e. The thermostat is set at 70, and mondays temperature was 25°F, and Tuesday was -15°F). To get other points in the year, you would simply need to input the outside temperature for the various days and calculate accordingly.

If you were wanting to calculate the heat loss for an entire year or a heating season, you could use the following formula :

Heat Loss (btu/hr) = ((Area(sf) * Temperature Difference(°F))/R-Value (Btu °F h/sf)) * 24 (24hrs a day) * (HDD)

The total Heating Degree days (HDD) can be found in both the NOAA data and ASHRAE 90.1 based off of the nearest weather location to the building site. (typically an airport or weather station)

Similarly, to figure cooling loss (Heat Gain) you would use the same formula but Replace the HDD with the CDD (cooling degree days). Since cooling cannot be created, the cooling loss is actually a heat gain through the wall system.


Thanks for the explanation Adam, that’s really helpful. Still trying to remember if this is given on the exam resources?

I sometimes struggle with the HDD / CDD, would it just be easiest to mark the delta as a positive value for the purposes of the equation, and just keep in mind if you are heating or cooling based on the question scenario … ?

I’m not certain if it is or not, so it might be beneficial to remember this formula for the exam.

you could keep this positive if that works for you. however I tend to keep it as is, personally. By doing this I can see the overall heat loss( btu/hr). If I get a negative number (as you would with cooling days), then I know that it was actually a heat gain. But again, either one works as long as you are mindful that you are doing a heat gain vs a heat loss.

Hopefully that makes sense.

Thanks for the help!

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