Foam Roof Insulation - Polyurethane Spray Foam Benefits

 

Insulating the underside of a roof rather than a ceiling creates many other benefits as well. Historically, we ventilated roofs in an attempt to prevent moisture problems and reduce heat build-up.

 
Current research shows that much of the moisture in attics comes from damp basements or crawl spaces, as well as from the living space. Research also shows that if we address crawl space, basement, attic and living space moisture, we do not need to ventilate an attic. In fact, by ventilating an attic, we can often make a moisture problem worse.   
 
Attic moisture problems are a result of moisture condensing on cold roof surfaces. Adding more vents causes the attic to be cooler, especially at night, which causes more condensation to form on the underside of the cold roof. Cutting a hole in the roof causes a bigger hole in the top of our "chimney", which makes the "chimney" draw better, pulling even more moisture upward. I have not seen any attic moisture problems solved by adding attic ventilation, with the exception of ice damming. (Ice damming is a “warm” attic phenomenon, and can better be addressed by reducing the amount of heat leaking into the attic.) Unfortunately, the building codes haven't kept up.
 
Ventilation to reduce summer heat build up in an attic has also been challenged recently by research done at the Florida Solar Research Center and the Building Research Council in Illinois. Much of the heat in an attic is from radiant heat transfer. The hot sheathing radiates heat to the ceiling or other objects in the attic.  To cool an attic, outside air is vented through attic or insulation is added to the ceiling to prevent the attic heat from warming the living space.  Research has shown that the ventilation rate would have to be quite large to make much difference in an attic temperature.  In the summer, the best you could possibly achieve was outside temperature.  With a very large fan using lots of energy, you might get close to outside temperatures. In winter, this would result in a colder attic as well.
 
Ceilings are usually insulated because of the ease of piling up cheap insulation. Recessed lights, outside walls, sloped or tray ceilings and knee walls all create a non-uniform thermal “cap” on the building and result in voids in the insulation.  The real-life R-value of an insulated ceiling is very often less than the claimed R-value.   
 
Ducts are often located in the attic, which exposes the coolest/warmest air in the house to the hottest/coldest environment in the house (depending upon the season).  This does not create a very energy efficient situation. As much as 10% of the heat or AC can be lost by placing ducts in an unconditioned attic.
 
From an energy standpoint, ducts and air handlers should be located within the conditioned space.  This reduces heat transfer to the outside, and reduces some concern of duct leakage.  Recently, building researchers proposed making crawl spaces into unvented, conditioned plenums, which is now accepted by code.  More recently, building researchers proposed making attics into conditioned space by eliminating ventilation and insulating the underside of the roof rather than the ceiling.  As a building researcher, I fully support both concepts.   
 
A roof system insulated with spray foam reduces energy several ways.  Energy loss from ducts located in the attic is essentially eliminated. The top of the building is much tighter resulting in less infiltration and exfiltration, so excess moisture isn’t pulled into the attic. Infiltration through the ceiling is also reduced.   In addition, the attic temperature is lower, which further reduces energy loads.
 
In a standard insulation system, ceiling insulation reduces the transfer of heat from the attic to the living space (in the summer).  Attic temperatures can often approach 140F during the day.  Most of this heat enters the attic space through a multi-step process. First, solar energy warms the shingles and sheathing.  The hot sheathing then transfers heat to the rest of the attic through conduction, convection and radiant heat transfer. 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 by as much as 40F.  Both conduction and convection heat transfer are proportional to a temperature difference, so that heat transfer will be reduced proportional to a drop in surface temperature.  Radiant heat transfer, though, is proportional the 4th  power of the temperature difference.  The reduction in radiant heat transfer resulting from an insulated roof can easily exceed conduction and convection reductions.

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 that 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”.