QUESTION: Do darker shingles have any effect on heat retention in the winter compared to those of a lighter colour?
T. Stubbington, Oakville, MB
ANSWER: I am surprised your very interesting question is rarely ever posed to me during regular inspections. It is one that has been debated by home inspectors and others at various seminars I have attended, over the years. I will provide some insight about roofs in relation to shingle colour and hopefully clear up some misconceptions about attic function and purpose at the same time.
The first item to address is your question about energy conservation in relation to shingle colour. The simple answer is there is little relationship between the two, but that is not completely true. The interesting thing is that energy conservation relates more to summer use, rather than winter heat retention by darker shingles, as you have opined.
That is because light-coloured shingles have a tendency to reflect more sunlight and heat away from the roof than darker ones, which absorb more heat and energy from the sun. For this reason, in the summer months, darker shingles can increase the temperature of the attic and may slightly add to air conditioning costs, especially in a poorly insulated attic.
Before going further, we must clear up one common misconception many homeowners have about their attics. An ideal attic is one that is as cool as possible, not warm. This is particularly true in the winter, but also in the summer, for the reason mentioned in the previous paragraph. The reason we install more insulation in our attic floor than anywhere else in the home is to prevent the loss of heat and warm air from the living space. Normally, warm air rises in buildings and can cause an attic to become too warm if insulation is minimal and the ceilings are poorly sealed. If this happens, condensation and moisture problems will develop. Also, shingles will be subject to premature deterioration if they are substantially warmer on the underside than on the surface.
If you have a well-insulated, sealed, and vented attic, it will remain fairly cool to cold even on the days when the home heating system is used to the maximum. That is critical to prevent excessive condensation and frost build-up from any warm air that does leak though. It also will ensure the temperature differential between the top and bottom of shingles on the roof is not excessive. When that occurs, often due to low amounts of insulation combined with poor ventilation, the shingles will curl.
Excessive curling will cause the shingles to wear out very quickly, often in half the time of normal life expectancy. In relation to your question, it means you don't want shingles to increase heat retention, you want them to limit that feature. That is the true disadvantage of darker coloured shingles, because they will absorb more solar energy and ultraviolet rays than light ones, increasing the wear.
In the winter, heat retention from the sun by darker-coloured shingles will be subject to many variables. The most important of these is snow cover. In years with substantial snow cover, the colour of the shingles may have little effect because the white snow may reflect a large enough portion of the sun's radiant heat to prevent absorption by dark shingles. This will also depend on the pitch of the roof, with low-pitch roofs and those with numerous valleys able to retain more snow than steep roofs. In years with minimal snow, dark roofs may be completely exposed to the strong winter sun, with little shade from leafless trees. This may substantially reduce the life expectancy of the roofing, as well as heat up the attic. It is also possible a warmer attic in the winter may increase the chance of "stack effect," or warm air convection within the home, but that is open to debate.
The bottom line -- darker shingles should be avoided if the option for light-coloured ones is available. Many subdivisions have specific restrictions on the colour of shingles that can be used when building new homes. There may be an aesthetic advantage to darker-coloured asphalt shingles and many developers insist they be used to ensure relative conformity with surrounding homes.
Unfortunately, this is contrary to good building practice, which should promote the use of light-coloured roofing that will be more reflective, rather than absorptive of the sun's rays.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be emailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.