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How much and what type of insulation?

I live in the Phoenix, AZ area which can get very hot in the summer.

I currently have fiberglass rolled insulation in the attic (about a 21 rating).

What would be the optimal amount of insulation and what type of insulation should I add to the attic?

Also the garage and the patio areas are not insulated at all but are covered by the same attic. Should they be insulated too?


Boris :)

Anonymous (not verified)
Re: How much and what type of insulation?

Solar panels or photo-thermal water heating panels will absorb a lot of energy before it even hits your roof.

Most solar installers will tell you to insulate well before you consider solar which it sounds like you have, so maybe you might consider solar. It'll offset those electric bills which pay to have the air conditioning running all summer, or use photo-thermal to heat the pool.

Yes, it's expensive, but most solar panels come with 20-30 year warranties, so you lock in a twenty year rate for electricity. Can't do that with the power company. I'll bet their price is going to go up over the next twenty years.

And yes, I admit it: It's good carbon-free energy.

Re: How much and what type of insulation?

For insulation, you want the most you can afford and can fit without compressing it (which reduces its' efficiency). You can investigate adding another layer perpendicular to the current layer, over the trusses, but if you have decking for storage, you'll need to look into removing it and raising it.

Another thing to consider is having spray foam insulation sprayed onto the back of the roof decking. It's expensive, but well worth it, as many of the current foams are environmentally friendly, and in the end you'll have an attic that can be entered in summer without succumbing to heatstroke within minutes. Many newer houses here in FL use the spray foam on the underside of the roof decking only - no insulation over the ceiling. The foam keeps the attic within a few degrees of house temperature, so you can access storage at any time of the year, or easily convert part of the attic to living space. It also helps strenghthen the roof structure, as it binds the decking to the trusses more tightly than the nails/screws already in use.

Re: How much and what type of insulation?

Spray Foam Insulation is the way to go. It is more expensive than other insulation's available, but it is the best available on the market today. I would suggest a 2lb closed cell foam insulation.

Re: How much and what type of insulation?

I agree w/ adding as much as you can afford in the attic in your zone. Add only unfaced batt insulation over the existing batts, at least 8 inches more would make a big difference in your utility bill. Do NOT use spray on insulation under the roof if your attic is ventilated [has soffit vents, gable vents or roof vents; this would be a total waste of money and could lead to early failure of your roof as the roof material would only get hotter with no heat conduction into the attic. The uninsulated portion of your attic over unheated/uncooled areas is correct.

Re: How much and what type of insulation?

Boris, First what you need to do is find the recommended R valve for your area, most attics in either extreme hot or cold call for R-38. Second you do not want to over insulate because then your house will not breath properly causing condensation build up and eventually mold. Also you want to make sure your attic has proper ventalation either attic fans or ridge vents work. But you do not want to block either of them.

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