Roof Insulation
Insulation at the top of a house is one of the most important components contributing to a homes performance. While adding insulation to either a roof system or an attic may seem straightforward, there are a number of things that need to be taken into consideration for it to function properly. Here we will cover three of the most common types of roof insulation found in the Portland area, and some of the unique advantages and disadvantages of each type.
Vented Attic
A vented attic is the most commonly used in the PNW, and one of the least likely to go wrong if done correctly. A vented attic involves installing a layer of loose insulation covering the attic floor, and vents in the attic space above to allow air and water vapor to move in and out freely. If you have ever stuck your head up into an attic, and seen piles of pillowy insulation everywhere, you were looking at a vented attic.Houses with vented attics in cold climates can have problems with condensation in the attic or ice dams on the roof, but those are failures to air-seal and insulate properly, and rarely a problem hear in the Portland area. When you get the air leakage and conductive heat loss low enough, those problems disappear.
One of the biggest advantages of this system is its affordability, and ease of installation. While there are still important considerations that a builder needs to make when insulating a roof this way, the details are relatively easy to execute. One of the disadvantages of the vented attic is that it is by design unconditioned living space, and thus not a good space for HVAC equipment or ducting, as this can cause condensation. But by far the biggest problem that we see with vented attics is when people try to reclaim the attic space as living space without changing how they are insulating it. While this can sound like an affordable way to gain new living space without altering the exterior of the house, if not done properly, it can have disastrous consequences.
Exterior Insulation
That brings us to exterior insulation. Adding insulation to the exterior of the roofdeck can be a great way to convert and existing vented attic roof assembly into an unvented assembly when the builder does not have access to the underside of a roof system. This involves adding either rigid foam insulation, or mineral wool insulation to the existing roof sheathing, effectively moving the thermal barrier outside of the sheathing and keeping it warm. When converting an existing vented assembly, it is important to close any existing venting below the new layer of insulation, as this would drastically reduce the roofs R value. Builders can also use some kind of fibrous insulation on the underside of the sheathing, in what’s called a hybrid assembly. One of the biggest advantages of this style of insulation is that it can be used on older homes with 2x4 rafters that would otherwise not provide enough space to insulate effectively.
Closed Cell Spray Foam Insulation
The third way to insulate a roof is the used of closed cell spray polyurethane foam on the underside of the sheathing. Closed cell foam spray on foam eliminates the possibility of condensation on the underside of the sheathing by removing air all together. One of the biggest benefits to adding closed cell spray foam to a roof assembly is that it does an excellent job of air sealing a roof assembly, as well as insulating it. Closed cell foam has the highest R value per inch of any of the types of insulation discussed here. One of the disadvantages of closed cell spray foam is its high cost. It is by far the most expensive option of the three discussed.
There are many things that need to be considered when choosing how to insulate your roof, and not all solutions will be right for every house. Thanks for reading.
