Updated: Mar 2, 2022
What is a "Hot Roof"? This style of roofing involves any roof where insulation (Typically closed cell Spray Foam) is applied directly to the underside of roof framing and sheathing - completely Insulating the underside of the roof. The roof surface (when hit by sunlight) is much hotter - thus the name "Hot Roof".
Below: Closed Cell Spray Foam being applied to OSB sheathed Roof.
What are the Benefits of a "Hot Roof"?
Very Efficient and complete home insulation.
Quick / Easy Installation.
Reduction of Ice Dams.
Condensation cannot get to the home framing - thus there are fewer (if any) moisture/condensation/mold issues.
What are the Problems with "Hot Roofs"?
Most Asphalt shingles cannot handle the heat of a "Hot Roof". There are special roof shingles (Designed to handle the increased heat generated by "Hot Roofs") We have seen 85-90% of Hot Roof installations had the wrong type of asphalt shingles installed on them, with serious overheating, shingle damage, and premature failures of the roof shingles. Typically, we have seen hot roofs with the wrong asphalt shingle roofs loose 25% to 50% of the expected shingle life! (We have inspected well over 100 "Hot Roofs" and have seen them at many stages of their lives - and their failures)
Leaks. When flashings, penetrations or damage causes a leak in a Hot Roof, the water that penetrates is often "stuck" above the foam insulation -and the framing can rot, attract insects and be damaged - often with no external signs or symptoms until a complete failure (rotted out framing failing) happens. THESE ISSUES ARE ESPECIALLY IMPORTANT TO HOMES WITH OSB (Structural Waferboard or "Oriented Strand Board") ROOF SHEATHING - SINCE OSB WEAKENS AND FAILS MUCH MORE QUICKLY IF WATER IS TRAPPED AGAINST IT. While Ice Shield waterproof membranes (High Temperature Membranes are typically needed for hot roofs)
Lack of framing access. When you completely cover the roof rafters, trusses, and roof sheathing, you loose access to the framing for future inspections. Cracks, bows, sheathing warps and pops, truss issues and other problems are all now hidden and cannot be inspected for. I always suggest to clients that every framing member be completely inspected, and any defects, underframing, or other issues be corrected before any insulation is applied to it. Even these precautions cannot prevent future framing issues - they only lessen the preventable ones.
ANTS! Carpenter ants spend their lives looking for nice soft rotted moist wood to hollow out and nest in. Ants will nest in anything that is soft, and when water does leak into the framing system - and gets trapped against the wood framing - a perfect storm -of moist rotting wood (Which ants can smell at a distance) and easily carved out soft foam next to it- is created. When Carpenter Ants find this rot they will often expand their nest into the foam - sometimes creating huge nests!
It's somewhat COMBUSTIBLE. The foam itself can burn, and it needs to be completely coated in a special intumescent coating - a coating that "puffs up" when exposed to heat and forms a heat barrier - to help prevent ignition of the foam. We often find this coating missing, poorly applied or incomplete. Most foam products need to be protected from heat and flame (see the warnings on foam boards) and are a potential fire hazard without this protection. Many people say "Well... it's protected in the attic / crawlspace and because of that it does not need protection." Well, Guess What? FIres DO happen in Attics and Crawlspaces - so NEARLY ALL spray or rigid foam NEEDS PROTECTION!
This is a good example of inaccessible attic framing. Note the roof framing added horizontal support framing in the middle of the rafter spans
Below: Truss framed Attic with Spray Foam Hot Roof:
Here are some links to more resources on Hot Roofs:
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