Replies

1. GBA Editor
   Martin Holladay | Feb 03, 2010 04:58am | #1

   Karen,
   If you spend more money to heat your house during the winter than you do to cool the house during the summer — as most Americans do — then you're better off with a dark-colored roof, not a "cool roof." The only exception would be if you made the mistake of putting HVAC equipment and ductwork in your attic. (I hope you didn't make this mistake.)

   If you have ducts in your attic, and if you have an air conditioner, you want a light-colored cool roof. You should also consider a range of remedies to tackle the ducts-in-the-attic problem.

   Cool roofs are the darlings of electric utility executives, because cool roofs help utilities address the peak load problem. That's why utility executives often have campaigns to encourage cool roofs, even in heating climates. The utility executives want a smaller peak load on hot summer afternoons. Cool roofs help -- but on a yearly basis, they add to the homeowners' heating load in winter, and the homeowners are the net losers.

2. 
   Doug McEvers | Feb 03, 2010 09:06am | #2

   Excellent point Martin, I had a friend tell me he was considering a cool roof for his office building in the mountains west of Denver!!!! Some salesperson had his ear and I went out and did a quick energy use analysis for the building and found it to be lacking in a number of areas. The cool roof would have helped the cause not at all, this building in fact with a proper retrofit should require no AC system, maybe just a nightly purge of cool air.

   Karen,
   Building performance starts with very careful design, insulation well beyond code (double), very airtight (1ach50) or less with proper window choice, sizing and placement based on location. After thes critera are satisfied, you can fine tune to take advantage of passive and other methods.

3. 
   Karen Leu | Feb 03, 2010 09:35am | #3

   So do you not need to take into account that the summer sun shines much more directly and much longer? One of the building science experts I talked to said that a white roof is still recommended even for Minnesota because the benefit it provides in the summertime will still outweigh that of a dark roof in the winter.
   I would think that this decision can't just be made on whether you spend more on heating and cooling because the effects of the summer and winter sun can't be split 50/50.

4. GBA Editor
   Martin Holladay | Feb 03, 2010 10:12am | #4

   Karen,
   You're right that an accurate, detailed answer to your question depends on many variables, so there is no simple answer. You've made an important point -- the sun angle (as well as the number of hours of sun per day) changes from winter to summer. Other points affecting the calculation:
   1. For what percentage of the winter is your south roof covered with snow?
   2. Is your roof plane insulated, or is the insulation located on your attic floor?
   3. What is the R-value of your ceiling insulation?

   These calculations get complicated fast.

   Here's a takeaway point: the better your thermal envelope, the more irrelevant the cool-roof-versus-warm-room question becomes. If you have a tight envelope and thick insulation, it really doesn't matter.

5. 
   Riversong | Feb 03, 2010 07:13pm | #5

   Karen,

   As Martin suggested, there are far too many variables to answer that question in general. it can, however, be answered for a specific site and a specific house design.

   Relevant variables include: site topography, natural (or other) shading, percent cloudy days during each season, height of house, orientation of house, angle of roof, type and mass of roofing, limit of thermal boundary (ceiling or roof), whether the roof is vented or not (and how well).

   Minimizing East and West window area may be more important to prevent overheating in the winter than in the summer, depending on latitude. Remember that it is at the equinoxes (swing or shoulder seasons) that the sun rises and sets in the East and West. In summer, it can rise 30° or more to the South of East and will be about 50° high in the sky by the time it's due East. In the winter, the sun rises 30° or so North of East and remains low in the sky all day, shining more directly into windows.

   As others have stated, cool roofs are important for hot, sunny climates and ventilating those roofs also helps except in the most humid marine climates. But a cooler color of asphalt composition shingles will provide more longevity to the roof anywhere. Dark asphalt roofing shrinks and cracks from over-absorption of radiant solar heat.

6. 
   Riversong | Feb 03, 2010 07:15pm | #6

   Edit (when are we going to get an edit function?):

   My post should read: Summer sun rises North of East and winter sun rises South of East .