What is a Hot Roof System?
By David Braddy LEED GA
Building science is changing rapidly and some of those changes are challenging traditional building methods. One of the most beneficial changes is also one of the most controversial, which is the hot roof system or the unventilated sealed attic.
A hot roof is a properly sealed unvented attic, instead of a traditional vented roof which consist of an insulated attic floor and open soffits where air enters and then leaves through the top of the roof.
A few years ago, the scientists at the Oakridge National Laboratory in Tennessee tested properly vented and totally sealed attics. They found that proper venting only reduced the roof deck temperature by 3 to 5 degrees Fahrenheit.
But, when an attic was insulated using the Hot Roof Theory, the attic temperature adjusted to within 10 degrees of the ambient temperature of the house. Because the attic is heated or cooled by air that would normally escape from the house, it does not raise the load on the heating and cooling system.
However a vented attic typically reaches 130 -140 degrees, when the attic temperature exceeds outside temperature shingle failure can occur and if you have duct work or HVAC equipment in an attic it has to operate in extreme conditions causing excess energy to be used. This also creates the perfect opportunity for moisture damage due to condensation forming on equipment and duct work.
Spray foamed attics have foam applied directly to the roof decking, and the attic space isn’t ventilated. The lack of ventilation is why they are called hot roofs.
In a standard insulation system, ceiling insulation reduces the transfer of heat from the attic to the living space (in the summer). Attic temperatures often approach 140F during the day. Solar energy warms the shingles and sheathing and then transfers heat to the attic. The 140F temperature of the underside roof surface drives the heat transfer process.
By insulating the roof surface with spray foam, the surface temperature exposed to the attic (the temperature driving the heat transfer) is reduced dramatically.
The benefits of including the attic in the insulated space are:
* Duct leakage and heat loss/gain from ducts is much less of an issue.
* Air sealing is easier in the roof than in the ceiling.
* Dust and loose insulation are less likely to migrate down to the living space.
* Tests show energy costs are lower when the attic is sealed.
Further information is available from ASHRAE (8700-527-4723) in a publication titled ‘Vented and Sealed Attics in Hot Climates’.
For more information check out this video
The next question is what type of spray foam is best for a hot roof system; there are two types open and closed cell.
Both of these can be used if properly applied, but there are big differences between the two. Open cell is moisture permeable and should not be used in a hot roof system without a vapor retarder. It has a R value of approximately 3.5 per inch; on the other hand closed cell is moisture impermeable has an R value of 6.8 per inch and is a vapor retarder. Closed cell has excellent adhesion qualities as does open cell, but closed cell actually becomes part of the roof structure making it much stronger.
We will have more on the differences between open and closed cell foam next time.