Sunday, November 29, 2009

Winter: downside of a radiant barrier?

One thought that has been on my mind lately, with our cold weather fast approaching, is whether the radiant barrier will make it harder to heat my house throughout the winter. One nice thing about living in central Texas has been that, even on the coldest winter days, the sun has enough power to warm the house pretty significantly by afternoon if the day is sunny.
But the radiant barrier rather effectively rejects that radiant energy. This, of course, is a bad thing during the winter, but a really great thing during the summer. The balancing factor during the winter is the fact that a lot of heat that would typically radiate out of the warm attic during the night will be reflected by the barrier back down into the house, keeping the house from cooling as quickly during the night as it otherwise would.
So some questions arise:
  1. Does the radiant barrier help me or hurt me overall during the winter months?
  2. If it hurts me, does it cost me more energy during the winter months than it saves me during the summer months?
  3. How can I find out the answers to questions 1 and 2?
As it turns out, my home is heated by natural gas, and the company was willing to provide my natural gas usage since 2005. So I will be able to perform some analysis.
Just as there is Cooling Degree Day data out there on the web, there is Heating Degree Day data as well.
When this winter is over, I plan to perform a natural gas usage vs. Heating Degree Day analysis, similar to the kWh usage vs. Cooling Degree Day analysis I performed for the cooling system of the house. The graphs generated from that data should help answer questions 1 and 2.
As usual, there will be complicating factors. First, when I finally completed the radiant barrier this spring, I blew in a bunch of extra insulation. That will certainly affect my results, but I don't have a very good way to account for it. Second, there are really only about 3 months of significant natural gas usage in the house: December, January, and February. That is not very many monthly data points when compared to the 5-6 months of significant cooling usage in the summer, meaning that it may be harder to identify trends. Finally, my cooling energy is measured in kWh, but my heating energy (burning natural gas) is measured in "ccf" which is 100 cubic feet, a volume of natural gas (presumably at atmospheric pressure), so we'll need a conversion factor between the two which may carry its own complications.
Despite these difficulties, we should be able to determine in general whether my cooling-focused improvements have helped or hurt my house's heating-related performance.
Stay tuned until after the gas meter is read in February to find out!

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