I used aluminum kitchen foil for a radiant barrier. The payback for radiant barrier installed is pretty far out, but doing it your self with $30 in foil pays for itself quicker. I did this in the 850 sqft attic of the original part of my house. My friend sprayed lo-mit2 radiant barrier on it 10 yrs ago. The spray ons tend to to reflect only about 80%. During some down time I decided I wanted the air gap/channeling effect you can get with a foil barrier. Already having the spray on I was reluctant to spend a more on store bought barrier so I opted for Reynolds wrap. The trick is getting the heavy duty version and doubling the ends to support staples. I have to temp probes in my attic, one just above my insulation which now hovers about 1 degree above the outside high. The other probe is 2 feet higher, the height of my hvac. It tends to run 5-7 degrees warmer on really hot days. Highest I've seen it was 107f on a 100f day. I have a ridge vent, one gable vent and no soffit or eave vents. It works better than I expected. I feel a lot of it is from the channeling effect of keeping the heat in the rafters going up to the ridge vent.
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