Different roofing professionals take different approaches to replacing residential roofs based on a variety of factors. While some will place the plywood sheathing directly over the trusses and then affix the shingles onto the sheathing, others will use a layer of foam board beneath the plywood. If you're getting your roof replaced, this is a worthwhile topic to discuss with your roofing contractor. Here are some pros and cons of using foam board under your plywood sheathing.
Pro: Better Insulation
The more that your home is insulated, the more comfortable that it will be inside — and this has a direct correlation to your heating and cooling expenses throughout the year. A big reason to use foam board under your roof sheathing is for insulation. If your roofing professional is concerned about heat loss through your attic, for example, he or she may be in favor of using foam board.
Con: Higher Cost
Incorporating foam board into your roof replacement project will increase the cost of the work. While it's possible that you'll make up this cost in utilities savings over the coming years, you'll need to pay this extra money up front. Using foam board is costlier not only because of the materials, but also because the installation of the foam board adds to the length of the project — thus adding labor costs.
Pro: More Water Protection
If some of your shingles fail, rainwater can immediately begin to soak into your plywood sheathing. Unless you catch this problem promptly, it can eventually make its way through the plywood and begin to drip into your attic. When you have a layer of foam board beneath your plywood sheathing, this material serves as an extra barrier against water. This may make a difference in keeping water out of your house
Con: Requires Sheathing Removal
A roof replacement job often just consists of replacing the shingles and not touching the plywood sheathing. If you're interested in using foam board, however, this will mean that the roofer will need to remove all of the sheathing. This obviously becomes much more of a project in terms of time and effort. It's best to only think about this job if your roofing contract needs to perform a complete roof replacement, which includes the removal of the sheathing rather than just the shingles. Generally, a roofer can discuss these points with you and point you in the best direction for your home and your budget.
For more information about residential roofing materials or construction, contact a local resource.