A hot roof is a type of roof assembly that is non-vented. Insulation is installed underneath the roof decking, leaving no space for ventilation. Without ventilation, the space is warmer, which is where the terminology “hot roof” comes from.
A cold roof, on the other hand, is a roof where there is ventilation below the roof, as in a traditional attic where the insulation sits on the floor joists. This attic area is essentially outside the building envelope and is meant to be cold.
Depending on the house’s structure, the hot roof design allows the attic space to be usable and livable. This is because it brings the attic into the home’s thermal envelope. The attic space then maintains the same moisture and air control level as the rest of the house. Some advantages from a house design standpoint are that electrical, storage and HVAC equipment can remain inside the envelope and are more functional as a result.
With an unvented hot roof, conditioned air cannot escape the home. This means homeowners won’t be “heating (or cooling) the outdoors” and can save money on heating and air conditioning bills. Although there are many pros to a “hot roof” there are also some cons. In order for a hot roof to function optimally and not cause damage to the underlying structure, it is of the utmost importance to air seal properly.
The best insulation for this is spray foam insulation. Spray foam insulation, also known as sprayed polyurethane, closed-cell or 2lb foam, goes on as a liquid and quickly sets to form a monolithic barrier. A downside to spray foam is that it costs more than a traditional attic insulation such as cellulose, but unlike a loose fill insulation, it doesn’t compact over time and never needs to be topped up, removed or replaced. It is extremely durable and effective and is good for the life of the building.
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