Cool Shingles in St. Louis: What the Rating Measures and What It Does Not

A shingle that reflects more sun runs cooler at the roof surface, and that is the whole promise of a cool-roof product. What it does for the house underneath depends on the attic, the insulation, the ductwork and how many summer afternoons the sun hits the roof hard enough to matter. In St. Louis the sun is strong from May into September, and the cooling load is real, so the question is worth settling before the spec is chosen. This page explains what the rating on a cool shingle measures, where to find the published numbers, and what to ask your roofer to write down.

Two numbers, and what each one describes

Cool-roof ratings for steep-slope shingles rest on two measurements. Solar reflectance is the share of incoming sunlight the surface sends back, on a scale from zero to one. Thermal emittance is how well the surface gives off heat it has absorbed. A dark shingle can reflect little sun and still release heat quickly, and a lighter shingle can reflect more while holding heat longer. The two numbers together predict how hot the surface gets at midday, which is the figure most people picture when they think about a cool roof.

Those numbers describe the surface only. They do not describe the attic, the wall behind the attic, or the room under the ceiling. The roof deck, the insulation depth and the attic vent system all sit between the shingle and the living space, and each one changes how much of the surface heat ever reaches the rooms below. A reflective shingle over a poorly vented attic can still leave an upstairs bedroom hot in August.

Where the published numbers live

The Cool Roof Rating Council keeps a public product directory that lists rated solar reflectance and thermal emittance for roofing products, and the directory can be searched by manufacturer and product line. An entry there is a tested figure with a listing date, which is more dependable than a phrase on a brochure. Check the product line name and the colour, because one product line can carry several colours with different ratings. A lighter colour in the same family usually reads higher on reflectance, but the directory entry is the figure to use, not a guess from the swatch.

The bundle wrapper is a poor place to look. Some wrappers print an energy or cool-roof claim, and some print nothing about it at all. A claim on a wrapper with no directory entry behind it is a claim you cannot check yourself. Ask the roofer for the directory listing or the product data sheet, and keep a copy with the rest of the job file so that the numbers can be compared with the bundle when it arrives.

Where cool shingles earn their place in St. Louis

A reflective roof works hardest where the attic is already warm and the house is cooled hard. Homes with ducts running through an unconditioned attic, homes with a low attic vent count, and homes with a dark roof facing the afternoon sun are the cases where the surface change can show up at the thermostat. A north-facing slope in shade gains little from a light colour, and a roof under mature trees may gain little from any cool product, because the tree canopy is already doing part of the work.

Cool shingles do not change the freeze and thaw record. A reflective granule and a standard granule sit on the same asphalt and face the same January nights, so the choice does not reduce ice damming at the eave. Ice at the eave comes from heat escaping the attic and melting snow that then refreezes at the overhang, and that is a ventilation and insulation question first. If your roof has ice at the eave every winter, a cool shingle will not fix it, and the attic should be checked before the colour is discussed.

Homeowners with a standing seam or a metal roof sometimes ask whether the same logic applies to metal. It does, in part, but the rated numbers and the product line are different, and the comparison should be made on the directory figures for the product actually quoted. Do not assume that a reflective asphalt shingle and a reflective metal panel have the same rating because both are described as cool.

What a cool shingle does not do

A reflective surface does not block hail. The impact rating on a shingle wrapper is a separate test with a separate purpose, and a cool-roof product can carry any impact class. Check the impact class and the wind rating on the wrapper as separate items, and do not let the cool-roof label stand in for either one.

A cool colour also does not stop streaking. Algae on a roof is a growth problem, and the granule treatment that resists it is a different treatment from the reflective pigment. A light shingle can streak just as a dark one does, and the streak shows up more clearly on a light roof. If streaking is the concern, ask about the algae-resistant granule separately from the cool-roof rating, and check the wrapper for that claim as well.

What to ask before the order goes in

• Which product line and colour are you quoting, and what is its directory listing for solar reflectance and thermal emittance?
• Is the rated colour the colour on the truck, and will the bundle lot match the colour you chose?
• What is the attic ventilation plan, and does the bid change it, or does it assume the attic stays as it is?
• Where are the impact class and the wind rating printed on the bundle, and do they match the spec in the quote?
• Does the quote name the cool-roof product, or does it say only cool shingle?

Keys Roofing installs Atlas and Malarkey shingles, and the product name and colour should appear in the written scope so the spec can be checked against the wrapper on delivery. If you want to talk through a cool-roof choice for your house, call the office at (314) 220-2333 and ask about the attic and ventilation before you ask about colour.

Decide the attic before the colour

A cool shingle is one part of a cooling plan. The rated number tells you what the surface does, and the attic tells you what reaches the rooms. Settle the attic ventilation and the insulation depth before you pick the colour, because those choices decide whether the surface gain shows up inside the house. Once the attic is right, the colour is a smaller decision, and the directory rating is the figure to hold the product to when the bundles are on the driveway.