Insulating a house in the desert (Phoenix, AZ)


Old 12-02-10, 09:51 AM
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Join Date: Jan 2010
Location: Phoenix, AZ
Posts: 106
Insulating a house in the desert (Phoenix, AZ)

Hey folks,

I bought my house a year ago and, having now experienced both of our AZ seasons (hot and not-hot) here, and seen how much it costs to keep the house warm/cool, I'm interested in trying to snug up the house a bit.

I've been researching to see if it is going to be a DIY job, or if I should bring in a pro. During my research, I've found two opinions on how to insulate a desert house:

1) Keep the house and attic(s) as separate zones, as it were. Put the insulation on the attic floor to keep hot hot hot attic air from penetrating into the living spaces during hot season, and to keep the warm air in the living spaces during not-hot season. Coming from WI and NC, this makes sense to me, but... neither WI nor NC have highs of 115 and lows of 95 on a regular basis.

2) Make the house and attic one zone and insulate to keep the heat out entirely. This would, apparently, be accomplished by spray foam on the underside of the roof and the installaiton of an attic fan or two to move hot air out of the attic space, keeping the attic from reaching 140+ in the summer and keepingthe living spaces cooler. Part of their argument is that the HVAC ducts themselves are very inefficient R-wise and, by including the attic in the insulated zone, it would decrease the loss at the ducts. My thinking, though, is that I can just wrap the ducts themselves in insulation... ? And, in my head, this makes sense, but then I wonder what keeps my cozy-warm air in my house during the 'winter'?

What are folks' opinions on these approaches? I'd hate to spend a lot of money on re-insulating only to find out that my nice, thick R-60 (hopefully) barrier is doing a really good job of keeping the heat in my house... in the summer.

Also - currently, I have a very scant layer of nasty & filthy blown-in fiberglass (fills the space between the joists in many places, but there are also some places that are simply voids). I still have a lot of work to do in my attics (house is a remodel, and I need access to the wiring & plumbing up there) and I don't want to dig through 18" of blown-in stuff everytime I work up there.

Ideally, I'd love to have this crap removed and replaced with 2 layers of R-30 batt insulation. What are folks' thoughts on that?

Finally, Lowe's has a radiant barrier product that looks like it can be affixed to underside of the roof or to the rafters and that would prevent/slow the transfer of heat from the roof to the main attic area. Thoughts on this?

Sorry for the long long long note. This is just new territory for me, so I have a lot of questions and want to make sure I get it right.

Thanks in advance, everyone. Any input is greatly appreciated.

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Old 12-02-10, 10:21 AM
Tolyn Ironhand's Avatar
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Location: Twin Cities, MN
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I would go with #1. It would be cheaper, a DIY job and I don't believe #2 will prevent the attic from getting hot.
Before adding the insulation I would add one or more powered attic fans and make sure your soffit or gable vents are in place and open. This will pull in cooler (relative) outside air and exhaust hot attic air. They even make solar ones that might be a good choice in AZ. No wiring required.

Gunguy, A Mod here, lives in AZ. I'm sure he will have some great words of wisdom.
Old 12-02-10, 01:47 PM
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Well...I may live here..but we are pretty different in our climates. Your summers are hotter and winters milder.

Personally, I'd empty out all the old stuff....seal up any penetrations between attic and living space with caulk or low expansion foam, and blow in enough cellulose insulation to get to R-50 or so. Make sure any venting is clear. My house has relatively poor airflow in the attic due to the design..and I wish I could increase it, but I really don't want to go with power vents...they require maintenance. When it's re-roof time (or after kid graduates college) I'll go with ridge vents. Can't do soffit vents w/o a lot of work, no soffits! lol My gable vents are plenty big..just need more exhaust.

Even with my 2x6 exterior walls and full insulation, the rooms in the sun get much warmer. I do the closed blinds and such in the summer to mitigate it as much as possible.

I know there are adherents to the hot roof/conditioned(?) attic....but I can't break loose from what has worked for a long time. As to the films and reflective paints...boy...I hope they do some un-biased studies and get the info out.
Old 12-03-10, 11:40 AM
Thread Starter
Join Date: Jan 2010
Location: Phoenix, AZ
Posts: 106
Thanks, guys! I shall go with the tried'n'true method of separating the attic from the living space, and shall look in to a number of solar-powered attic fans (2 per attic, perhaps?)

So, thoughts on batts vs blown in?

Personally, I prefer batts, simply because I do have to work up there still and don't want to have to wade/dig through the blown-in to get to pipes/wires/fixtures/etc. Plus then I'm not dragging the insulation into the house on shoes/clothes/hair/etc.

I know blown-in is easier to ensure you get all the corners filled and that there aren't any gaps. Batts require more work to get proper coverage. Other pros? Cons?

I figure, if I'm going to hire someone to get the old stuff out, and if I go with batts, I can simply pay them to do the first layer. The 2nd layer (and 3rd? Would it be best to lay down 2 R-30 layers, or one R-30 and 2 R-x, each at 90 degrees) probably wouldn't require as much cutting/fitting, so I could do it myself, and seal up cracks between attic & living space at that time/at my leisure.

Whereas, if I do blown-in, I'd probably have them pull out the old, then I'd need to seal everything up before I put in the blown-in.

Thanks again for folks' input! I really really appreciate it.
Old 12-03-10, 11:50 AM
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If you do 2 layers of batt at 90 degree angles it would prob be ok. Personally....I'd rather rake out some loose fill..then put it back after.

I also THINK blown cellulose is better at keeping down air flow....

The solar fans don't move much air for the cost. If you really want power vents, I'd go with hard wired. Make sure you have enough intake air.
Old 03-25-14, 02:56 PM
Join Date: Mar 2014
Location: USA
Posts: 1
Talking fyi from SRP

The purchase and installation of an electric attic fan can be expensive and not very economical. In fact, electric attic fans can actually cost more to operate than they save in air-conditioning costs. With only 8% to 10% of the heat in your home coming in through the ceiling from the attic, you don't have a large margin to work with to generate major energy cost savings. Even if you were able to reduce the heat gain from the attic by 25%, the overall effect on the air-conditioning load would only be 2% to 2.5% (25% of 8% and 10%). Also, by adding an attic fan you run the risk of creating a negative-pressure situation in the attic. This negative pressure can draw air-conditioned air out of your air handler, ductwork or interior spaces through leaks and cracks.

The best thing to do is make sure that your attic is well-insulated and that you have ample exhaust and supply air vents to provide for natural ventilation. If you have adequate natural ventilation, there is no need to pay to run an attic fan.

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