Replies

1. 
   Airithol | Feb 20, 2015 12:09pm | #1

   Ted,

   I have not read the book, but I have heard of this method before. As you likely know, this reduction in your set point (from 65 to 50) is to account for the increasing percentage of heat load that is met by internal gains.

   Again, not having the book, I can only speculate, but this advice is likely quantified some how. It sounds more like a rule of thumb, based on experience, or measured data (or both). That said, I agree with it.

   For my own part, I took my envelope UA (BTU/hr °F) and divided it by estimated internal gains (BTU/hour). This results in a temperature offset to account for the internal gains, in °F. Deduct this from your desired indoor set point (72°F typically) and the result is an estimation of your buildings balance point temperature.

   Some degree days sites allow any temperature you want, instead of 50, 60, 65, etc.

   I cannot answer you about the CDD, except to say that in the summer things are reversed - internal gains are no longer free heat, they are a liability. In theory, you could add the above mentioned temperature offset to your desired indoor set point. Cooling in my climate is not a big concern for me, so I have not yet tackled that portion of the modelling exercise.

   cheers

2. 
   charlie_sullivan | Feb 20, 2015 04:18pm | #2

   The implicit assumption with CDD is that you have the windows open until it gets too hot. With the windows open, the insulation doesn't matter much. So there's no such benefit.

   In practice, good insulation can help you make use of strategies like opening the windows at night and closing them during the day. But how much benefit you get from that depends greatly on how actively you monitor weather forecasts and open and close windows strategically.

3. Expert Member
   Dana Dorsett | Feb 20, 2015 04:58pm | #3

   Using base 50 makes sense only if you have ~R40 or higher walls & attic, and U0.20 or lower windows, but in a tight house with those values it's about right, but not perfect, since even on sunny 30F days the heat load can go to zero if the windows can take advantage, and the overheating during the day may carry you through the night. The actual house design & orientation can make a huge difference in the energy use per HDD, no matter what base temp you use.

   Cooling loads in a high-R house are all about internal heat sources (electrical loads and mammalian occupants) and solar gain, not so much about heat conducted through the walls or ceiling. Using base 65 CDD is very imprecise for estimating cooling energy use for code-min houses due to that solar gain factor.

4. 
   user-4435615 | Feb 20, 2015 05:10pm | #4

   Mr. Sullivan, I see your point, but consider:
   CDDs must have some base temperature and I assume that unless specified differently it is CCD65. (perhaps I'm way off on that - What base has been commonly used in the past to calculate CDDs?) Whatever it is, it is presumably figured for the average house with average insulation and leakage like the base houses on which the HERS Index is based.
   Then compare that to the standards of a High Performance Home (HPH) :
   In both homes one would ventilate at night as the nightime temperature permits and then close everything up during the day at the point where the outside temperature-RH% comfort limit exceeds the comfortable conditions indoors.
   Now comparing the proverbial next door neighbor who has a HERS reference home to the HPH, both begin at the same time with the same internal comfortable conditions (both homes being equally subject to the same internal loads). As the outdoor temperature increases, one could go much later in the day before the HPH would require cooling, thus reducing the number of Cooling Degrees for that day. Or, might not need cooling at all that day before opening the house up for nighttime ventilation, thus eliminating all Cooling Degrees for that day. Many days during the shoulder seasons would be eliminated from CDD calculations and during the peak cooling season the number of Cooling Degrees on any given day would be reduced. Therefore a sizable portion of the CCDs would be eliminated for a particular site with a HPH. It seems to me, therefore, that the CDD base for a High Performance Home must be higher than for a RESNET reference home in order to reflect that reduction in total annual CDDs.
   Someone in Cooling Climates must have figured out a method for figuring the anual average cooling load and what CDD base to use to make that calculation.