Renovation & Design

Radiant barriers can complicate insulation job

Question: I have a closed-in deck room that had the roof extended over it, with spray foam insulation over the original metal. It has an open-joist ceiling, and I want some more insulation to cover the joists. This room is on the north side. I got a good deal on rigid foam, R-5, and Tech Shield foil-lined paper roll. I wanted to cover the joists with the foil, facing down, and then with the foam boards. After, I want to spray this with paint as a ceiling, but I noticed you said that’s a potential fire hazard and not code-compliant. I am not sure about thermal dynamics, but I would do that if I could, because it is simpler and cheaper.

If I put the foam between the joists, and the tech shield at the bottom of the joists, then use thin drywall for a ceiling, would that work? Will that insulate properly and with respect to the radiant barrier, and not create vapour issues? The tech shield specifically says that you must leave four inches top and bottom for circulation. I assume that some vapour exchange could take place through both the foam board and also the drywall. Is that correct?

Thanks ahead for any clarification you can give me.

Roger Crane

Answer: Insulating the ceiling of a converted porch or enclosed screened deck can pose significant challenges. Using rigid foam is a good idea, if installed and sealed properly, but products like your reflective foil should not be bothered with.

I am not completely sure how the original metal roof of your enclosed deck was covered, but I will assume it has spray-on foam insulation, with a painted surface, added overtop. That will provide some thermal resistance, but should also provide a good water- and air-resistant barrier. If that is not sufficient insulation for the use of this room adding more insulation beneath, before heating, is a good idea. Since this portion of the building enclosure is now well sealed, you must be careful to ensure you don’t leave any air spaces below that could lead to problems.

In your initial plan, to leave the space between the ceiling joists or rafters open above the insulation could be catastrophic. This large a gap above the insulated and partially air-sealed insulation would leave a perfect place for condensation, frost, and mould growth. Because this area would be substantially colder than the room during the heating season, any air that leaked in through the foam and tinfoil membrane would condense and freeze. The water that would result from the melted winter frost would also be restricted from drying by the insulated ceiling below, and the roof above, and would lead to rot and mould growth in the cavity.

The second method that you proposed would be much more productive, as long as it was well air-sealed. You have not stated whether your foam board was expanded foam or extruded foam. Extruded polystyrene (XPS) foam generally has good air-sealing capabilities, while expanded foam, noted by the small visible balls inside, has limited air-sealing properties. If you use XPS sheathing, you may only have to caulk or foam around the edges and any seams to achieve a proper air seal. Either way, the entire ceiling joist cavity would have to be filled with insulation to prevent moisture issues.

Adding a layer of drywall below would complete the job, as not only would it provide a fire-resistant layer, but allow for proper painting, which the foam alone would not.

The installation of the foil paper membrane may be the component that you could leave out of the equation. This type of product works by reflecting radiant energy. While this may indeed work to reflect this energy back to the living space, most of the heat energy that escapes into attics and roof assemblies is not radiant in nature. Because of the stack effect in buildings, much of the heat that escapes into this area is heated air due to convection, not radiation. The proponents of this type of heat loss reduction membrane are not well versed in building science and poor installation of that type of product can cause serious issues.

Unless you are living in a desert environment in the Southwest U.S., where attic insulation is required to keep attics and buildings cool from the summer heat and sun, this product may be useless or even harmful to your efforts.

Using rigid foam for insulating a roof cavity with limited space between the joists is a good idea, as long as it completely fills the void and is properly air-sealed. Adding products like foil radiant barriers will certainly complicate the job and are otherwise useless and should be left out of the plans.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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