Part VI: Staying CoolChapter 10. Passive and Low-Energy Cooling
Reflective and Cool Roofs
What a lighter roof can and cannot do for summer heat, including attic ventilation.
- Difficulty
- Intermediate
- Cost
- Varies
A dark roof absorbs the sun and runs hot. Some of that heat conducts into the attic and then into the rooms. A cool roof reflects more of the sun and, if it also has a high infrared emittance, gives heat back to the sky instead of holding it. The indoor effect is largest when the attic is poorly insulated. A thick attic already blocks most of that path. The roof deck still runs cooler, which can help the roofing material, and the house may barely notice.
A cool roof does not shade a west window, does not flush the house at night, and does not fix a leaky duct in a hot attic. Do those first if they are the actual problem. The shade guide and the attic insulation page are often the higher return.
Difficulty
- Level
- Intermediate
- Time
- A reroof or a coating project, which is not a casual afternoon
- Skills
- Reading a reflectance number and staying off a steep roof
Cost estimate
Varies
Choosing a light rated shingle at the next reroof can be a small premium. A coating on a low-slope roof is a maintenance product with its own price and a recoat date. House paint on shingles is not the cheap version of either. It is a way to void a roof warranty and trap water.
| Choice | What it is |
|---|---|
| Light or cool-rated shingles or metal | A roofing product with a published reflectance |
| A coating made for that roof | A maintenance layer with a service life |
| Random white paint | Not a cool roof |
Materials
| Item | Qty | Notes |
|---|---|---|
| The roof you actually have | 1 | Slope, material, and whether the warranty allows a coating. Low-slope and steep roofs are different products. |
| A product with published reflectance and emittance | 1 | Aged numbers matter more than the day it was applied. Some energy codes require a listed cool roof. Check. |
| Attic insulation and vents | Already, or first | A cool roof on top of an empty attic is the wrong order. Vents still have a job. |
What the roof can change
Sun that never enters the roofing does not become heat in the deck. A surface that also emits infrared well will not sit as hot for the heat it does absorb. Shiny metal sometimes reflects the sun and emits infrared poorly, so it can still run hotter than a white surface that looks less like a mirror. The pair of numbers, reflectance and emittance, is the specification. A single word, white, is not.
In a hot climate with a thin attic, people feel this. In a cold climate with a good attic, the summer gain was already small, and a darker roof was contributing a little heat to the attic on sunny winter days. Switching to a cool roof can add a small heating load and remove a small cooling load. If you almost never cool and you heat a lot, do the arithmetic with your bills before you chase a white roof for comfort. The heat-loss page is the winter side.
Attic ventilation still matters. A cool roof is not a reason to block soffit or ridge vents. Those vents remove moisture. A coating that seals a roof which needed to dry, or that closes a vent, creates a wet attic. Follow the roof maker and the coating maker together. If they disagree, stop.
What it will not do
West and east glass admits sun straight into the room. No roof color touches that. Night flushing removes heat the roof already delivered. A cool roof lowers the delivery. It does not erase the afternoon you already stored in the mass.
Coatings wear. The aged reflectance is the honest number, and a dirty roof reflects less than the label. Recoat on the schedule. Do not walk a steep coated roof as if it were a floor. Many coatings are slick, especially with dew.
Metal roofs need the fastening and the underlayment the maker specifies. A light color screwed down with the wrong fastener is a leak that happens to be reflective. Snow slides off slick metal. The landing zone cannot be a door. The overhang page makes the same point about dump zones.
Insulate and vent the attic before you shop for a color
If the attic floor is thin, fix that. Confirm soffit and ridge paths are open and baffled.
Find the glass that is actually cooking the rooms
West windows may matter more than the shingles. Shade those whether or not you coat the roof.
Buy a rated product for that roof
Reflectance and emittance, aged if the sheet offers it. Code first if your climate zone requires a cool roof. No house paint.
Hire the roof work if the slope or the height is past you
Coatings and reroofs are fall risks. Warranties are paperwork you read before the first gallon.
Look at a hot afternoon and a cold sunny day
Attic temperature is the summer test. A higher winter bill with no summer complaint means the trade was wrong for your climate.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/Projects/Cooling/passive_cooling.htm
- https://www.builditsolar.com/Projects/Cooling/Shading/Shading.htm
- https://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
- https://www.builditsolar.com/Projects/Cooling/NoAC/ComfortWithNoAC.htm
- https://www.builditsolar.com/Experimental/RoofCooling.htm
- https://www.builditsolar.com/Experimental/RadiationCool/EvaproRad.htm
- https://www.builditsolar.com/Experimental/VentSize/
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.