Just to clarify, I believe you mean TPO or thermoplastic-polyolefin roofing, which is generally a commercial roofing product.Sprayed urethane foam with a good acrylic elastomeric membrane coating is a good choice.
And when you are re-doing the roof, you want to consider:
- Reflectivity and emissivity
- Thermal barrier and air barrier alignment
Reflectivity and emissivity. Emissivity is the ability of a material to dissipate heat that it has absorbed. Higher emissivity and higher reflectivity are better. One role of your roof in your climate is to act as a giant shade device for your home. It is the membrane painted over the top of the foam that you want to be as reflective and emissive as possible. Look up the product you are considering on the Cool Roof Rating Council's website for its initial and three-year ratings. The more heat you reflect away, the easier the battle for your insulation to slow the migration of the rest of it.
Durability. Generally, membranes are put on in two coats. The first one is typically tinted a light gray so that the installer can verify in the second (white) coat that they have covered everything twice. Make sure the coating is sprayed on in adequate mil. thickness so that it lasts. It helps to wash the roof every year or so to maximize its reflectance, and it will need re-coating in about five years to ensure durability. The membrane is what prevents water infiltration.
Ponding. Make sure the foam is even, without significant dips or dams. These create ponds on the roof after rainstorms and shorten the life of your membrane, also gathering dust that reduces reflectivity.
Insulation. If you don't have very much, you can add about R-6.5 per inch with the foam. (I am assuming that you do not have access to your attic space.) I chose 3-1/2 inches with even a bit more on my partially exposed rooftop ductwork. It made an unbelievable difference. Between several insulation and weatherization measures, we need no air conditioning until it is over 100 degrees outside.
Thermal barrier and air barrier alignment. Make sure your attic is unvented. Depending upon the knowledge of your original builder, you may have roof penetrations (such as vent pipes) that simply open up into the attic rather than being connected through the attic straight to the appliance that is being vented. This will defeat the insulation value of your foam. Conversely, any hole from an appliance vent pipe through the ceiling and into the open attic similarly defeats the usefulness of your attic insulation. Eliminate eave vents as well, as you are moving the top of your building envelope from the ceiling to the roof. Stop unintentional air movement that bypasses your insulation.
If you have a woodpecker issue in your area, membrane-coated foam may not be the best solution as the woodpeckers may peck right into it, exposing the foam to the elements. Consult your local roofer for alternatives, such as sheet foam with a puncture-resistant cover over it.
If you are thinking about adding solar water heating or solar PV, consider having the mounting stansions installed before the foam (protecting any threads with tape). It requires coordination of the different contractors, but foaming afterwards just provides additional security that all the roof deck penetrations are sealed tight. The solar contractors can finish their installation after the roof is finished.
Winter. Realize that this giant shade device is also going to serve the same shading function in the winter, depriving you of the significant warming effect of the winter sun in your higher-elevation area. You can make up for this by improving the overall quality of your thermal envelope with measures such as sealing cracks, having adequate wall insulation and installing dual-pane, low-E squared (or low-E cubed) windows. Passive solar design can also take advantage of the low-angle winter sun.
For more information:
Read Carl Seville's Q&A "What's the best affordable sustainable siding option? Any thoughts on roofing materials and gutters?"
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