Replies

1. 
   4Lorn2 | Jun 12, 2003 09:30am | #1

   The most authoritative sources I have seen, independent and DOE studies done in the early 80s, showed that power venting was seldom cost effective, not saving its cost in electricity, and a balanced continuous ridge vent and soffit was the best performer in keeping the attic cool in both winter and summer.

   Within reason adding insulation can keep any heat from the conditioned space and increasing the insulation of the ducts may help. An AC man could do the calculations for return on investment plotted against and potential temperature. This wouldn't lower attic temperatures, it might increase it if you had a duct leak. But it lowers the impact of the heat on the air handler and so your electric bill.

   Perhaps lowering the attic temperature could be had by lowering the solar uptake or retransmission into the attic. Lighter shingles and metal roofs absorb much less energy than the much more popular black shingles. You might also check to make sure that the soffit and ridge vents are truly open. I have seen both clogged with dust and spider webs. A common event I see is that the soffit is ventilated with perforated soffit sheeting, vinyl or aluminum, and that the attic insulation was blown in after the soffit sheeting was installed. The overspray onto the soffit area, sometimes even when baffles were in place, effectively blocks the vent holes.

   Retransmission can be decreased by spraying one on many reflective paints to the under side of  the roof deck and rafters. This can be done in just a few hours with a spray gun. The job doesn't need to be neat so it goes on quickly. Another approach is to staple up a reflective foil to the rafters or analogous truss structures leaving a semi sealed environment below the rafters while maintaining a open air path between the ridge and soffit. Both methods, particularly when combined, can drastically lower attic temperatures.

   You say your attic temperatures are too high. With all due respect I'm not entirely sure that your attic is not doing OK. Perhaps your expectations are too high. A roof is a giant solar collector and the absorbed heat has to go somewhere. Depending on your climate and sunshine you might be surprised what is normal. I'm in Florida and while the books tell you that attic temperatures won't ever go above 140F, even in an unventilated attic, and generally stay no higher than 125F I can attest that I have recorded an attic at 157F and 140F is not uncommon. Welcome to the sunshine state where, IMHO,  too many fools use black shingles, inadequate ventilation and cut down all the shading trees. 

   I'm not sure any of this answers your question or helps solve the problem. Just a bit of brainstorming on my part. You are welcome to discount any or all of it as are any others on this site welcome to contradict, deride or modify anything in this posting with their superior wisdom, knowledge and experience.

1. 
   frankc | Jun 14, 2003 11:09pm | #4

   Thanks for your explanation and you may be right that I may be expecting too much.

   The soffits are clear and all the insulation is batt type. I have 2x10 ceiling joists with full insulation with an additional 3" insulation laid at 90 degrees. Changing the shingles is not an option (only 11 years on a 40 year roof. Nor is moving the A/C units as this unit supplies the second floor and there is a seperate unit supping the first floor. I have always wanted to see how the reflective foil would work but I am not sure if I want to go through the effort of both spraying reflective paint and putting up the foil. The attic space is not a simple none and with all the insulation doing anything up there is an effort. I think I will try both on the next house. Thank you again for the idea of reflective foil as I had forgotten about that

2. 
   BossHog | Jun 12, 2003 03:08pm | #2

   I also think that power vents are a waste of money.

   You'd be better served by getting the AC unit out of the attic.

   If you have built your castles in the air, your work should not be lost; that is where they should be. Now put the foundations under them. [Thoreau]

1. 
   BillHartmann | Jun 12, 2003 07:00pm | #3

   "You'd be better served by getting the AC unit out of the attic"

   Or air conditioning the attic.

   Frankc check out http://www.buildingscinece.com

   They like to make the attic part of the conditioned space. That elimates the problems with insulating and leaky air ducts that the others have mentioned.