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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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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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