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Community and Q&A

Attic insulation, radiant barrier, and ductwork

HhsHwZFSCs | Posted in Energy Efficiency and Durability on

Background: The house is located in Atlanta Georgia and is a 3000 square feet two story with a high roof attic and the second floor HVAC and ductwork present in the attic. The house was built in 1984. Cooling bills avg about $55-60 per month this Summer. I am after better comfort. Attic Temperature highs around 103.

Question 1 – I have an area with a house fan on the second floor hallway ceiling that has wood decking over the old insulation in the attic. I want to keep the decking to get to the furnace. The area in the hallway has an HVAC return and the thermostat is located there. I want to increase the insulation without raising the floor. I would like to remove the old settled blown in insulation about 6″ or so and replace in the cavity with several cut pieces of XPS foam board 3 to 4 pieces stacked in the cavity and then reinstall the plywood foor. I will also remove the house fan. My concern is moisture build up between the ceiling dry wall and the foam board. This would not be the whole attic floor by about 10% of it max.

Question 2 – In the same attic I have installed radiant barrier under the roof rafters. I did not want to use spray foam due to outgassing and cost. My thought was to attach 2 inch XPS or polyiso to the rafters to improve the attic insulation. However, I am not sure if this can cover the underside of the radiant barrier directly. In addition, I have R 3.8 duct insulation on the ductwork which is raised above the floor. If I add additional insulation will this help much. I have already sealed the duct leaks with mastic.


Scott Razzino

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  1. GBA Editor
    Martin Holladay | | #1

    1. Your plan to install rigid foam insulation between your attic joists (under the plywood walkway) will not cause any moisture problems. Each layer of foam should be carefully air-sealed with caulk or canned spray foam.

    2. It makes little sense to install insulation along your rafters unless you do a thorough job to create a properly insulated, sealed, conditioned attic. In your climate, that means installing R-30 insulation under, over, or between your rafters -- not just 2 inches of rigid foam.

    3. Radiant barriers only work if they face an air space. If you install rigid foam tight to the shiny radiant barrier, it becomes worthless.

    4. Your biggest problem is probably your R-3.8 duct insulation, which is woefully inadequate. You need to either re-insulate your existing ductwork or replace all of the ductwork in your attic with new insulated ductwork with at least R-8 insulation. More insulation is better!

  2. 5C8rvfuWev | | #2

    Scott, you might also want to re-calculate your A/C expenses and attic temp. I live just east of you and, while I'm waiting to build my house, live in an 800 s.f. apt on the first floor of a 2-story building built in 1992. But my A/C costs this summer have averaged $80/mo and up, or about 25% more than yours with about 25% of your area to cool! In addition, the temperature in the parking lot outside, let alone in an attic, has often gone well over 103 degrees. I hope your numbers are a typo cause otherwise I'm getting screwed!

    Joe W

  3. Scott | | #3


    My temperature is calculated from a remote thermometer in the attic. The height of the thermometer is mid level. I am not seeing temps any higher that 103 since I installed the radiant barrier. My avg electric bill is somewhere between 55-60 without cooling costs. With cooling costs I have been at $108 for June, $112 for July, and $118 for August. I have two 17 Seer Two Stage Carrier HVAC system one for upstairs and one for downstairs. The downstairs does not run much. Highest electric bill was $145 two summers ago in July - August timeframe. I have done alot of air sealing and insulating.


  4. HhsHwZFSCs | | #4


    To clarify your answer #2.. Can 6 inches of polyiso rigid board with foil on both sides be attached to the rafters to meet the R30 number? While I know this would be expensive it would avoid sprayfoaming. Why are sprayfoam contractors only adding 4 inches of foam in roof decking channels? If I calculate this that would be only R19 on the roof decking.

    Also, answer #3... a friend and I keep disagreeing about installing rigid foam or polyiso board over my current radiant barrier. He says the foil faces both sides so covering the bottom side facing the attic floor should not reduce the effects of the foil since the underside facing the roof deck is still foil reflective surface. Do you agree?

  5. GBA Editor
    Martin Holladay | | #5

    Your plan should work. Six inches of polyiso should give you about R-39. Remember that you also have to insulate the gable walls, if there are any, and you have to create a careful air barrier at the perimeter of the attic, where the new roof air barrier meets your existing wall air barrier. All seams between the polyiso panels should be sealed with housewrap tape or canned foam.

    Spray-foam contractors are suggesting only 4 inches of spray foam for a few reasons:
    - They are afraid of driving away customers with their high prices;
    - There are limits to how much foam can be sprayed in one pass without causing your house to catch fire, because the installation of spray foam causes an exothermic reaction;
    - Many of them are ignorant about building science basics, especially when it comes to R-value.

    If a radiant barrier (shiny foil) faces an air space, it will raise the R-value of the air space. If another building material is installed tight to the radiant barrier, the radiant barrier becomes worthless.

  6. Riversong | | #6


    A radiant barrier does not reflect heat, it diminishes the amount of infra-red radiation that is absorbed and emitted.

    A shiny metal (or white colored) roof is a reflective barrier, since it reflects the visible spectrum (about half) of the sun's radiation. Under a roof, where there is no visible light, it is the IR absorptivity and emissivity of a surface which are important.

    Attic radiant barriers perform best when the shiny (lowE) side facing downward, because this not only reduces dust accumulation which reduces its effectiveness but also cuts down on IR emissivity to the attic floor and house ceiling.

    So, attaching XPS to the existing radiant barrier will render it null and void. But attaching foil-faced polyiso insulation will simply move the radiant barrier down to the lowest foil surface facing the attic floor, and it will dramatically reduce the heat conduction to that surface, further diminishing the IR emissions.

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