Cool Roofs?
I am in the process of selecting the new roof for my house. I was going to choose one of the roofs that are rated by the coolroofs.org. It states cool roofs has the following benefits:
Energy savings and global warming mitigation
Reduction in urban heat island effect and smog
Improved occupant comfort
Comply with codes and green building programs
After talking to the roofer and looking at my budget, the roofer says that I should just choose a light color roof. cool roof such as Owen Corning Duration¯ Premium and CertainTeed LANDMARK Solaris Max costs a lot more the other roofs. He said that I am basically paying for reducing global warming and reducing urban heat island effect. I will not enjoy the benefit of cooling my own house (or very marginally) assuming the house is insulated. In California, we are required to have R-38. I am on a budget and would not want to spend the extra money on cool roofs if it does not help cooling my house.
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Replies
Amy,
Roofing color is the most important determinant of radiant solar gain. A light-colored roofing is all that you need to keep down summer heat. White, of course, is the coolest. Massive roofing materials that are self-venting, like Spanish tiles, also tend to reduce attic temperatures.
I should add, though, that after roofing color, a radiant barrier under the rafters (assuming it's an attic and not a cathedral ceiling) and continuous roof venting are the next most important elements of a cool roof assembly.
Tile roof is in my budget and I am going with composite asphalt shingle roof. I have cathedral ceiling, open loft that is connected to the cathedral ceiling, and attic. I can use radiant barrier for the attic. What should I do for the cathedral ceiling/open loft portion?
What's "continuous roof venting"? Is it the solar power fan that draws air out of the attic? Or passive vent holes around the edge of the roofs? Is the purpose of continuous roof venting for reducing heat trapped in the attic in the summer? Since I am in heating climate, I would think it's beneficial to have heat stay in the attic rather than venting it out in the long heating season?
BTW - I am planning on putting the furnace in the attic. Do you have suggestions on how to insulate the area around the furnace without having to insulate the whole attic? I assume it's more efficient to put the furnace in an insulated area.
Thanks!
Amy,
If you are in a heating climate -- that is, if you spend more for heating than for cooling -- then you don't want a cool roof. The sun is your friend. When the sun heats your roof, you spend less money on heating fuel.
"Continuous roof venting" means providing an air channel between the top of the insulation and the roof sheathing. This channel has inlets at the soffit and outlets at the ridge.
Several reasons for such venting have been proposed, including cooling the roof sheathing and removing moisture that passes through the insulation. The validity of the logic behind these reasons varies; suffice it to say that it is possible to design a roof with or without ventilation. Either method can work if the details are right.
If you have R-38 ceiling insulation and don't have air conditioner ducts in your attic then roofing color (and radiant barrier) will have a tiny impact on your cooling loads. You say that you plan to put your furnace in the attic -- that's a bad idea. If you have no other viable choice, then you need to make sure you have very tight and well insulated ducts. The impact of the cool roof on cooling loads will be a little larger under this scenario but still almost certainly less than 0.1 kWh/sqft. It would probably be more cost-effective from an energy use perspective to invest the extra $$ in a more efficient flat screen TV.
Also, sloped residential cool roofs are a fairly expensive and not very effective approach approach for addressing urban heat island / earth albedo effects. Those impacts are largest for flat roofs that can be coated white,
The Solaris shingles look like a good investment. You have the upfront Energy Star rebate, the ongoing reduction in cooling costs, and the longer expected lifetime due to lower shingle temperatures. Reducing the neighborhood's temperature is just a bonus you don't need to quantify.
I have to disagree with Martin about dark shingles reducing costs in a heating climates, because the low angle of the winter sun causes much less heating effect in the winter vs. the heating problem in summer.
"Shasta White" shingles are the lightest of the regular-priced shingles, but unfortunately, they make the house look cheap. That's not a problem if you can't see much of the roof from the curb.
I also disagree with Martin about using the roof as a passive solar heat collector in winter. In most cold climates, the roof is covered with snow for much of the winter and it's far more important to prevent ice dams, which can occur even with R-38 insulation without venting. And a warm ceiling produces discomfort in occupants. ASHRAE has determined that people like warm floors, warm walls and windows but cooler ceilings.
I would never design or build an unvented roof in a cold climate. There are multiple advantages of a cold roof, including increased roofing longevity, evacuation of vagrant moisture, elimination of ice dams and consequent leakage and damage, and - most importantly - the ability of the sheathing to dry when it inevitably gets wet sometime during the life of the building.
In the summer, however, even in a cold climate, a dark unvented roof with no radiant barrier will become an unwanted heat sink when the sun is high in the sky. Light roofing, radiant barrier, and soffit-to-ridge venting all help to reduce this summer heat gain and consequently both increase indoor comfort and decrease AC costs.
If the furnace must be in the attic, then it should be enclosed within the thermal envelope by building a small air-tight and insulated "room" around it.
This forum is so wonderful that you are all very knowledgeable - a lot more knowledgeable than the local roofers I talked to. After reading more, it seems heat xfer (energy efficiency) and moisture xfer (prevent water in and allow things to dry) are the 2 key consideration for building envelop.
- I live in Northern California with 4-5m heating/rainy season (never snow, rarely below freezing). Furnace has to be in attic, no room in main floor. Very high humidify in the winter.
- 10-20 days above 90 degree dry summer. I hope to NOT have to install AC.
I have a huge attic - I plan to dry wall around the 500sf of attic space that is over 6' high and under a flat roof. For this bonus attic game room that may be used a few times a month, I plan to use plug in space heater since I don't want to waste the energy to heat it all the time during the 4-5 months of heating season. I do worry about it gets overly hot in the summer (only has operable sky lite to vent). Since I want to finish the attic ceiling, I don't want exposed radiant barrier. I also don't want to loose significant headroom since the ceiling height is only around 6'.
QUESTION 1 - Suggestions for how to cool this bonus game attic room cool passively or with room AC (does it exist)?
QUESTION 2 - Would it make sense to insulate the whole attic or only the part of the attic that is over 6'? This way, the attic acts as a buffer between the ground floor conditioned space and outside cold/hot air. I can also put the furnace/most of the ducts in the insulated but non-conditioned space.
QUESTION 3 - What type of insulation do you recommend for attic ceiling under flat roof? IAQ is a must for me I don't want anything that will off gas. I am also concern how to get the moisture out if the roof material somehow becomes wet?
I do plan to use white color roof for the flat roof portion. One contractor recommended IB single ply ( http://www.ibroof.com/ ) and it is listed in coolroofs.org.
QUESTION 4 - Do you have better suggestion for flat roof material?
For the slopped roof, I think I will go with regular cheaper/prettier color roof.
QUESTION 5 - Robert, would you explain why people would like cool ceiling in the winter? Wouldn't my body heat radiant to the cool ceiling and feel uncomfortable in the winter?
QUESTION 6 - Robert, you indicated "If the furnace must be in the attic, then it should be enclosed within the thermal envelope by building a small air-tight and insulated "room" around it." How about the ducts? Is it more important to put the ducts or the furnace in insulated thermal envelop?
Thank you all in advance. You can probably tell that I am new to this forum and has no experience building a house.
QUESTION 1 - Suggestions for how to cool this bonus game attic room cool passively or with room AC (does it exist)?
Any usable living space is required by code to have a minimum of 8% of floor area in glazing and a minimum of half of that (4%) operable for ventilation. What code does not require, but the laws of physics and common sense do, is cross-ventilation for natural cooling and/or high/low ventilation for stack effect air movement. So you might consider installing at least one additional skylight (not on the south side) or a couple of gable end windows.
QUESTION 2 - Would it make sense to insulate the whole attic or only the part of the attic that is over 6'? This way, the attic acts as a buffer between the ground floor conditioned space and outside cold/hot air. I can also put the furnace/most of the ducts in the insulated but non-conditioned space.
It's often easier to insulate the roof plane to get a continuous thermal and air barrier, but it's also possible (and common) to insulate the kneewalls, sloped ceiling and flat ceiling - and this may make it easier to vent the roof above this thermal layer (air chutes will be required in the slant ceiling). The furnace needs combustion air, so if it is not direct-vented there are code requirements for the sizing of outside air ducting to the furnace room. The ducts must either be within the thermal boundary or well-sealed and insulated.
QUESTION 3 - What type of insulation do you recommend for attic ceiling under flat roof? IAQ is a must for me I don't want anything that will off gas. I am also concern how to get the moisture out if the roof material somehow becomes wet?
The healthiest insulation option is cellulose with borate fire retardant (don't use material with ammonium sulfate, since it can smell and corrode metal)
QUESTION 4 - Do you have better suggestion for flat roof material?
For the slopped roof, I think I will go with regular cheaper/prettier color roof.
A living roof is a wonderful way to keep the roof cool and reduce both global warming and rainwater runoff.
QUESTION 5 - Robert, would you explain why people would like cool ceiling in the winter? Wouldn't my body heat radiant to the cool ceiling and feel uncomfortable in the winter?
It shouldn't require explaining, since this is the subjective experience of most people. We like our feet warm in winter but don't like the ceiling to be warmer than the rest of the room. Stand under a ceiling heat vent or radiant heater or work at your desk under a 100W incandescent bulb and notice how you feel.
QUESTION 6 - Robert, you indicated "If the furnace must be in the attic, then it should be enclosed within the thermal envelope by building a small air-tight and insulated "room" around it." How about the ducts? Is it more important to put the ducts or the furnace in insulated thermal envelop?
See above. If the furnace is in unconditioned space then it's losing heat to that space. If the ducts are in unconditioned space then they're not only loosing (or gaining) heat but also potentially leaking or sucking air and causing pressure imbalances in the house that may drive moisture through the envelope.
Robert,
You suggested the following roofing material: "regular cheaper/prettier color roof." What's that?
Martin,
I think Robert just cut and paste from my initial post - "For the slopped roof, I think I will go with regular cheaper/prettier color roof.".
I decided NOT to use the more expensive "cool roof" such as Owen Corning Duration® Premium and CertainTeed LANDMARK Solaris Max that have limited colors. I am going to use their regular roof that have more color choices and also cheaper.
In a heating climate, regular and prettier might be cheaper to BUY but are more expensive to OWN, If you think of the extra material cost as a deposit in a cd or money market account, you might be able to project whether your money does more for you financially at the bank or on your roof. Don't forget to factor in less wear and tear on the AC. Is there a calculator somewhere people can use to make these sorts of return-on-investment calculations?
rats I meant to say COOLING climate
Robert,
Thank you very much for all your great points. I understand most of them but need some clarification on Question 2 and 3.
Are you recommending insulating all the attic roof (cathedral ceiling, flat roof, sloped ceiling over flat roof) OR only a subset of the attic "living space" (flat ceiling, slant ceiling, knell wall)? Would you elaborate the pros and cons for cost, energy efficiency, and moisture control (if roof gets wet or moist air from inside of the house)? I also read about SIP / EPS may be a good option for cathedral ceiling. What do you think?
I like your recommendation for using cellulose for insulation. For cathedral/slant/flat ceiling in the attic area, if I use cellulose and it somehow gets wet from roof leaks, would it settle/shrink? For cathedral ceiling, would cellulose slide down by gravity overtime and create a gap on the very top? What cellulose installation method do you recommend for the cathedral/slant ceiling and flat roof?
PS
- I live in mild heating climate (long wet winters with no snow) and dry summer that does not need AC. My roof slopes at 6-12. I will use a contractor and not DIY.
- I found an article about chemicals in cellulose - http://www.healthyhouseinstitute.com/a_688-Cellulose_Insulation . After reading that, I still think cellulose is the best option available.