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i am considering putting up a radiant barrier in my attic, with a product like AtticFoil, to 1) reduce the attic temperatures as my A/C ducting is in the attic and 2) to reduce heat convection into my home as well.

Are there any drawbacks or concerns to installing a radiant barrier in the attic? I will staple them to the bottom of the rafters inside the attic, leaving gaps at the top and bottom for proper attic ventilation & airflow.

Home is located in a relatively temperate climate (San Francisco Bay Area) with summer temps occasionally reaching over 100 but generally has highs in the low 80s during the summer months.

*on a related note, attic currently has ~R19 cellulose insulation, which I plan on increasing to R50. I figure doing the radiant barrier first would be better since I would otherwise end up stomping on the insulation and compressing it.

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Radiant barrier in the attic is an excellent idea. At the typical roof temperatures you see in the summer, even in a temperate climate, radiant transport of heat is significant and a properly installed radiant barrier can be very effective in blocking or reducing radiant transport of heat.

California Title 24 now requires some kind of radiant barrier. My house was built just before this requirement took effect but I elected to pay the small premium to use TechShield radiant barrier anyway and it has been worth it. I reviewed the AtticFoil site and they seem to have a good product for retrofit applications. Your plan sounds like a good idea and I don't see any major issues aside from the usual ones of "work carefully, try to avoid falling through your ceiling, prepare for the high temperatures up there, watch out for dust etc".

Do try to stay away from the "over-insulation" method as that loses effectiveness as dust builds up on the top (working side) of the barrier. Your plan to use the "open-ridge" method is the best as dust buildup on the bottom (working side) of the barrier is not as much of a problem.

Insulation of the ducts is not a binary yes/no question any more than attic insulation is. funkadelic understands this and that is why he plans to increase his attic insulation from R19 to R50.

Yet duct insulation is typically no more than R4 to R8 while carrying the coldest air in the house. It is the part of the HVAC system that has the least insulation while needing it the most.

No doubt there are some installers who think that as long as "cold" air comes out the register there is no problem but this ignores the question of how cold is the air at the end of the duct vs the air that went into the duct and how much cooling power has been lost in the duct.

Who would accept a water pipe that leaked half of the water it carried?

The Federal government has done many studies of ways to reduce duct loss because it does make a difference in your energy efficiency. There are even suggestions to move the HVAC ducts into conditioned space as this would cut duct heat/cooling losses to zero. I did this in my house.

For retrofit, moving ducts is impractical so the next best is to bury the ducts. You have an excellent opportunity to do this when you upgrade to R-50 of cellulose.

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/ns/eemtg082011_a8_duct_retrofit.pdf

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I've got nothing against people adding more insulation, but the benefit of more insulation is much lower in a temperate climate than it would be in southern Arizona or Maine. You should also consider other parts of the home, such as older windows, before assuming that more attic insulation is the best way to spend funds towards energy conservation. –  BMitch Jun 27 '12 at 17:43
    
funkadelic says outside summer temps are typically in the 80s. He and I live in the same area and that has been my experience also. So the delta-T across the windows will only be 5 to 10 degrees while the delta-T to the attic will be much higher, probably 40 to 60, as attic temps of 120-140 are likely. Given this situation, heat transfer from the attic is much greater than from windows and if air conditioning is the concern, funkadelic is right to work on his attic. –  Philip Ngai Jun 28 '12 at 3:53
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The AC ducting should be insulated if it's in the attic (it would be inside the duct so you wouldn't see it). And with a temperate climate, your benefit from more insulation may not offset the cost. But otherwise, I see no reason not to.

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The benefit of the radiant barrier will drastically decrease when you add R-50 insulation and bury/seal/insulate the ducts. I would skip it in such a moderate cooling climate such as the SF bay area. Central valley might be a different story.

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