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Your Utility Bills: Blocking the flow of radiant heat

Dear Jim: We have plenty of attic insulation, but it seems that I can still feel heat from the ceiling. It is actually warm when I touch it, and the rooms are uncomfortable. What can I do to fix this?

– Ronnie M.

Dear Ronnie: What you are experiencing is radiant heat from a hot roof. On a sunny summer day, a dark shingle roof can reach 150 degrees. This heat radiates downward to the room ceiling below, making it much warmer indoors.

Standard thermal attic floor insulation, such as fiberglass, rock wool, cellulose, etc., is not very effective for blocking radiant heat flow from a very hot roof. Its primary benefit is blocking conductive heat loss, particularly during winter.

During summer, when the sun first warms the attic in the morning, the thermal insulation blocks the conductive heat transfer downward from the warm attic area. As the roof gets much hotter in the sun though, radiant heat transfer becomes the predominant mode of heat transfer to the rooms below.

The best method to block this radiant heat transfer is a combination of a radiant heat barrier and adequate attic ventilation. The barrier blocks the direct heat flow from the roof to the room ceiling, and the ventilation carries away the hot attic air. The ventilation also helps during winter to avoid moisture condensation in the attic.

Reflective attic foil is effective for blocking radiant heat flow. Aluminum foil, with reinforcing mesh or kraft paper, is commonly used in attics. Aluminum has natural low emissivity properties, so when it gets hot from the roof above, it does not radiate the heat on down to the rooms.

Reinforced attic foil is available in rolls of various widths. I use 4-foot-wide rolls. It should be stapled up under the roof rafter leaving a gap between it and the roof sheathing. The air gap is important for both the effectiveness of the foil and to allow for ventilation above it.

The neatness of the installation is not important for its effectiveness, so it is a simple do-it-yourself installation project. Although it seems counterintuitive, if you use the least expensive single-sided kraft paper-backed foil, face the shiny aluminum side downward toward the ceiling.

The best type of attic ventilation is a combination of inlet soffit vents and an exhaust ridge vent. This creates an excellent year-round air flow pattern to carry away heat during summer and moisture during winter. Flexible roll-type or rigid ridge vent is easy to install and equally effective.

When installing the foil, don’t staple it all the way down to the attic floor. This would block the air flow from the soffit vents. Also, stop just before the ridge vent opening so air is drawn from the attic area, as well as from between the foil and roof.

The following companies offer attic foil: AtticFoil, atticfoil.com, 800-595-8772; Fi-Foil, 888-926-4364, fifoil.com; TVM Building Products, 888-699-1645, tvmindustries.com; and ridge vents: Cor-A-Vent, 800-837-8368, cor-a-vent.com; and Lomanco, 800-643-5596, lomanco.com.

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Dear Jim: I have old heat-control permanent window film on my windows and want to remove it.

What is the easiest way?

– Jen L.

Dear Jen: Try using a single-edge razor blade or window scraper to loosen the film from the top edge.

Spray the exposed area with a solution of half ammonia and half water, and slowly work your way diagonally across, peeling it off from top to bottom.

If this doesn’t work, spray the film with the ammonia solution. Cover the window with plastic wrap, and let it stay wet for 10 to 12 hours.

After this sweating of the film, the previous removal method should be effective.