Can this board be used as a walkway to access HVAC components in the attic (blown-in insulation is used) if plywood is laid either over or under the insulation board?
Dear Tom: Well, that depends. It would obviously not make sense to install foam board on top of your ceiling joists, only in the areas that will be used for the walkway. You should cover the entire 'floor' of your attic to improve the insulation throughout. Here are a few tips:
(1) Step 1: Seal all air leaks into your attic. Before you add any insulation, carefully find and seal all of the air leaks into your attic. Recessed ceiling lights are culprit #1: install Tenmat covers and seal the edges with DAP 230 sealant. See: Tenmat Model # FF130E Hoe Depot Internet # 204286308 Store SKU # 1000012747 and DAP 230, Home Depot Internet #100035980 Store SKU #284425 Great Stuff foam is good to fill larger gaps and holes -- but do not use it near anything that gets hot, e.g. a chimney or exhaust vent. For gaps near a hot exhaust vent, fill the space with Roxul rock wool, then cover it with fire resistant sealant, e.g. 3M Model CP-25WB+, Home Depot Internet #100166701 Store SKU #163096 For the large gap next to a chimney, cover it with aluminum flashing (available at Home Depot) nailed to the joists, and seal the edges with 3M fire resistant sealant.
(2) If you have an air conditioning system in your attic and you live in a region with hot summers, you should seriously consider whether you can seal your attic and convert it into "conditioned space". This will significantly reduce temperatures in your attic and house, improve the efficiency of your HVAC system and reduce costs. For example, instead of installing foam board on the 'floor' of the attic, you could insulate the roof deck under the rafters, and seal the soffit and ridge / gable vents. If your house has a relatively simple attic design, this would be a smart move. See Steps 1 to 5, below.
Step 1: Foam board: Install sheets of 1 7/8" thick Tuff-R polyiso foam board against your roof deck, between the rafters. This will add R-12.2. See: TUFF-R Model # 268441 Home Depot Internet # 100322376 Store SKU # 163832 Then carefully seal all of the joints and edges to block air leaks. DAP 230 is good for most joints and edges. The 'Windows and Doors' version of Great Stuff foam works well for larger gaps; the cured foam remains flexible to prevent cracks as things expand and contract. See: GREAT STUFF Model # 248312 Home Depot Internet # 100068117 Store SKU # 522661 (Foam board is flammable, however, do not install it next to anything that gets hot, e.g. a chimney or exhaust vent.)
Step 2: Fill your rafter bays with batts of Roxul rock wool, which provides R4.2 per inch. Roxul rock wool is also easy to install. See: Model # RXCB301525, Home Depot Internet # 205972559. Use Simpson insulation wires to hold the batts in place. See: SImpson Model # IS16-R100 Internet #100375163 Store SKU #594333. This is the 16" version; a 24" version is also available if your rafters are spaced 24" apart. Depending on the depth of your rafters, this will add ~R-14.7
Step 3: Cover your rafters with sheets of Tuff-R foam board, screwed to the underside of the rafters. This final layer will break all of the "thermal bridges" created by exposed wood, and add R-12.2. Again, seal all of the edges and joints to block air leaks.
Step 4: Do the same thing with your gable end walls -- the triangular walls at the ends of your roof
Step 5: Foam board is flammable and must be covered with 1/2" drywall if your attic is accessible and used for storage. Simply screw sheets of drywall to your rafters, on top of the foam board. This does not need to look pretty -- just cover the foam board.
This is all do-able for the average home owner, without any special tools. The result will be a roof with good R-39.7 insulation, and a tight air seal, year round. Plus your HVAC system will be much more efficient, running in a much cooler attic.
However, if this project would be too daunting or costly, seal all of the air leaks into your attic, then cover your sealing joists with 1 7/8" thick Tuff-R and carefully seal all of the edges and joints. Then cover the 'walkway' section with 3/4" plywood -- and cover the rest with drywall, if you attic is used for storage.
I hope this is helpful,
Date published: 2016-12-08