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I have a walk-up unfinished attic in an old ranch house in Pennsylvania, where we see summer temperatures over 100F and winter temperatures well below freezing. Because there is planking over the joists the in-joist insulation over most of the living space is realistically limited to R-25.

The attic/roof is ventilated with soffit and ridge vents.

Right now (it's still 100F outside!) I'm thinking of ways to reduce the cooling load on the central HVAC. I wouldn't mind moderating the temperature in the attic itself year-round either.

I have a roll of AtticFoil-like radiant barrier. If I staple that to the exposed roof rafters (without blocking the soffit-to-ridge vent circulation) I understand it will reduce the heat gain on sunny days.

But given that the house also goes through a cold season during which heat gain would be a bonus, should I avoid such radiant barrier insulation in this location and application? (I'm not sure how to balance the seasonality, especially given that there is less sun exposure during the winter due to shorter days and lower sun transit.)

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I have the same situation in Pa but have also tried radiant barrier with the ventilation holes. I am still in a quandry about how to use it. This is what I have experienced: I laid the foil over my insulation as often recommended. In the spring I went into the attic to check on the nature of things. I lifted a section and my fiberglass insulation was short of sopping wet! It took me short time to yank it all off of the fiberglass for it to dry out.

I have concluded that I am not able to put down radiant barrier as I most likely have airs leakage through the ceiling causing heat to meet the cold. I do have an attic fan and could put baffles in for even better ventilation.

I am left with this last attempt to achieve at least some temporary benefit before the big redo: I am tracking the sun and will staple foil in a strategic spot on the attic rafters. The spot that is potentially hot in the summer (in my case the NW corner) but receives little warmth in the winter because of daylight hours. This should help the summer AC. I am not ready to yank out all my insulation and look for leaks. One step toward betterment at a time.

I keep wanting to believe radiant barrier is wonderful but people with season changes better do their insulation/ventilation/air leakage homework before they think radiant barrier is going to be the best thing since sliced bread!

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You could install a "roof and attic ventilation fan" in the attic. They are temperature and sometimes humidity actuated. Since you have ridge vents the fan should be mounted as high on the roof and as close to the peak as possible. However, with a vent fan, some short cycling of hot air from the ridge vents is possible which will reduce the fan's effectiveness.

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I believe the atticfoil literature says to lay it across the insulation in that climate zone, though I'm sure they'd support you purchasing enough to do both the lay on top method and also rafters. They put out a second flavor of their foil with larger holes to avoid creating a vapor barrier when laying it on top.

Even with the foil, if you have poor insulation/drafts in the house it's going to get hot / cold regardless of what's happening up top.

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