Radiant barrier or spray foam in attic?

[BadeMillsap]

I live near San Antonio, Texas ... summers are HOT winters are not that bad.

My home is 28 years old, two story with a fairly open attic space. Built in the early 1980's it does not have a ridge cap but rather two large vents at each end of the attic space. I have had an exhaust fan in the North end vent for may years forcing SOME of the incredibly hot air out of the attic but with rising cooling costs have decided to do 'something' to try and reduce the heat in the attic space and thus hopefully reduce the cost of cooling.

It seems to me there are two intriguing options::

1. A radiant barrier affixed to the inside of the roof on the attic side (typically a foil barrier stapled between the rafters it seems)

2. A spray foam insulation thermal barrier sprayed on the inside of the roof on the attic side (there is a "biofoam" version made of soybeans that seems to be popular in this area)

What I am hoping to get some "advice" on is which will give me the most bang for the buck? Both types seem to carry some energy efficient incentives in the form of power company rebates and perhaps tax incentives but what I really want to know is which will reduce the heat in the attic space the most?



Any thoughts?

[master53yoda]

There is also another possible issue that is worth investigating. Where is the ductwork located for the AC system. If it is exterior of the building it will make a major difference to seal the ductwork completely as any leakage is putting the structure into a vacuum which increase the infiltration gains.

Primary insulation is between the rooms and the attic this should be in the 12" level of insulation range.
The next area is sufficient ventilation. Which will remove heat from the attic. Many times the fans used do not have sufficient static pressure capability if mounted against a louver to generate the air flow needed.

The color of the roofing material will make a major difference in the amount of retained radiant heat from the sun.

As to your insulation question I would probably use foil covered 2" foam board in the framing against the roof, this gives you the best of both worlds an insinuation and radiant barrier.

my 2 cents worth.

[smorris]

I considered the radiant barrier for my attic a couple years ago. Found that if you don't have a ridge vent for the hot air to flow between the barrier on your trusses and the roof it was of limited value.

I put in powered vents with a temperature switch that kicks on at 100 degrees, those are fronted by vents that blow open when the fan comes on and it moves a lot of air. One at each end of the attic and it made an appreciable difference.

[BadeMillsap]

The roof is standing seam metal roofing. The duct work and in fact the entire HVAC system was replaced last year so I am at least FAIRLY confident that it is well sealed.

It is good to know that w/o a ridge vent the radiant barrier might be limited in efficency ... sadly that makes good sense and not really something I can rectify w/o serious cost involved due to the roof.

I do have the one powered vent on a thermostat so perhaps one at the other end would make some sense.

It's sounding like the foam against the inside of the attic ceiling might be a better choice.

[capncarl]

When we built our house in SW GA. I wanted the spray foam insulation, but could not come to terms with the installer. They were insulating houses all around me, like you said, spraying it directly to the bottom of the roof deck. This was wood decking/shingles, not metal like yours. I did not want this, I wanted the top of my ceiling and rafters sprayed, not my roof, let it get as hot as it wants, just keep it out of the living space. For the life of me I can not see how a shingle roof could stand the increased heat of a bottom insulated deck in south GA. I ended up using 14-16" of conventional blown in insulation.

It made me feel a lot better this winter when we had our 100 year snow, (almost 2 inches) and my uninsulated roof deck was nice and white and the spray foam roofs had every rafter showing through the snow. Not a good as they say I guess.

capncarl

[dbhost]

I live near San Antonio, Texas ... summers are HOT winters are not that bad.

My home is 28 years old, two story with a fairly open attic space. Built in the early 1980's it does not have a ridge cap but rather two large vents at each end of the attic space. I have had an exhaust fan in the North end vent for may years forcing SOME of the incredibly hot air out of the attic but with rising cooling costs have decided to do 'something' to try and reduce the heat in the attic space and thus hopefully reduce the cost of cooling.

It seems to me there are two intriguing options::

1. A radiant barrier affixed to the inside of the roof on the attic side (typically a foil barrier stapled between the rafters it seems)

2. A spray foam insulation thermal barrier sprayed on the inside of the roof on the attic side (there is a "biofoam" version made of soybeans that seems to be popular in this area)

What I am hoping to get some "advice" on is which will give me the most bang for the buck? Both types seem to carry some energy efficient incentives in the form of power company rebates and perhaps tax incentives but what I really want to know is which will reduce the heat in the attic space the most?



Any thoughts?

Spray foam is intriguing, but I am personally concerned about holding condensation / humidity against the roof deck and rafters. (more of a problem for me on the coast than it is for you in San Antonio for sure!). I am planning on ordering up sufficient quantities of "Attic Foil" from www.atticfoil.com, I think they are based in Dallas... Every little bit you can do to reduce heat gain in the house will certainly be worth it in the long run, ESPECIALLY if you are in an all electric home like me...

[BigguyZ]

I've hear that a hot roof isn't an issue. The shingles are more than rated for the increased heat.

Personally, I live in MN and the biggest issue is the cold. But regardless, I'm planning on getting spray foam for my attic ceiling- but that's largely because I'm interested in converting my attic to a living space.

I think that the 30% tax credit would work on either option- but that's a significant reduction in costs if you decide to go with foam.

Also, the closed cell foam seals air leaks and effectively blocks moisture in the roof.