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I know we've got an HVAC guru or two here on the board, so I figured I'd check with the good folks here first.

I'm in a circa 1986 home which has that grey blown-in cellulose insulation. There are some spots where it is, no joke, about 3 inches thick. Rafters are exposed. I want to blow in to get R-30 or better. Ideally R-38+.

My main question is: Stick with cellulose and add enough to account for some settling (10% or so) or spray in glass? Can I spray fiberglass over cellulose? The Owens Corning Attic Cat bags are $32, while the generic fiber insulation (which covers 40 sq/ft) is $10. Is there any huge benefit to one over the other?

Lastly, I will be super careful with the spray in, but I am sure some will end up on the soffit vents. Am I safe just dropping them to clear them out? I'm tempted to replace them anyways, so I assume this shouldn't be an issue?

Assuming an average of ~R-16 throughout the house right now, and only ~1300 sq/ft of area that needs to have an application, is 20 bags enough? I can't grasp the voodoo that is R rating math. If someone knows of a calculator that can take in to account existing, what you want to move to, etc. it would be greatly appreciated.

I just moved into my brand new house. In the attic, it has 10 inches of grey cellulose. Combined with the radiant barrier, the ceiling/roof is supposed to be rated R30.

Note: The walls are filled with the same cellulose and I believe have an R15 rating. Basically, it seems I'm now living in an ice chest!

Radiant barrier is a waste of money unless your A/c unit is in the attic. I have a house that was built in 1961 and in had little or no insulation in the attic. I had an insulation co come in and add 16 inches of the white cellulose to a 1300 sq. ft. house and it cost $750.00 and cut my electric in more than half per month. I went from $350 to $130, paid for itself in less than one summer

Radiant barrier is a waste of money unless your A/c unit is in the attic. I have a house that was built in 1961 and in had little or no insulation in the attic. I had an insulation co come in and add 16 inches of the white cellulose to a 1300 sq. ft. house and it cost $750.00 and cut my electric in more than half per month. I went from $350 to $130, paid for itself in less than one summer

did you notice a big temp difference in the attic after the insulation was added?

I've read 130F in my attic in the heat of the summer. I considered radiant barrier, but now I'm leaning towards more
insulation and a reflective shingle coating:

http://rubberized.com/305specs.htm" onclick="window.open(this.href);return false;
http://www.lexiscoatings.com/modified-b ... e-coating/" onclick="window.open(this.href);return false;

R-30 is 10"-12" thick and is recommended. R-38 would be a little better but after that the cost vs. savings is very little. Radiant barrier is a help in most cases, but more so if your system and/or ductwork is in the attic. 60% of the homes in Texas have the system in the attic and over 90% have the duct in the attic (DOE figures).

ghostrider wrote: did you notice a big temp difference in the attic after the insulation was added?

I've read 130F in my attic in the heat of the summer. I considered radiant barrier, but now I'm leaning towards more
insulation and a reflective shingle coating:

http://rubberized.com/305specs.htm" onclick="window.open(this.href);return false;
http://www.lexiscoatings.com/modified-b ... e-coating/" onclick="window.open(this.href);return false;

You won't notice any change in the attic temp with insulation only. When we bought our house in May we were in the same boat as you, house built in 1986 and blown in insulation non-existent or shallow and an attic that was 130+. I had a company come in and blow in enough to bring it up to R30 but the big change in the attic temp was putting in more air flow for the attic. Do you any any venting in the attic? You might need more to bring the temp down. I noticed a HUGE HUGE HUGE difference by putting in a couple more soffit vents and adding two turbine vents to the roof. Now I have some proper airflow so the attic doesn't get at hot, which helps the insulation work better.

ghostrider wrote:

Radiant barrier is a waste of money unless your A/c unit is in the attic. I have a house that was built in 1961 and in had little or no insulation in the attic. I had an insulation co come in and add 16 inches of the white cellulose to a 1300 sq. ft. house and it cost $750.00 and cut my electric in more than half per month. I went from $350 to $130, paid for itself in less than one summer

did you notice a big temp difference in the attic after the insulation was added?

I've read 130F in my attic in the heat of the summer. I considered radiant barrier, but now I'm leaning towards more
insulation and a reflective shingle coating:

http://rubberized.com/305specs.htm" onclick="window.open(this.href);return false;
http://www.lexiscoatings.com/modified-b ... e-coating/" onclick="window.open(this.href);return false;

The roof of a house has no insulation value; it is there to protect the house from the elements. I delivered roofing shingles for 5 years so I got some firsthand info from sales and installers. The average roof life expectancy in Texas is 8-12 years, this is average so don't jump me (with in that time hail or heat takes its toll). If a roof is vented properly 130 deg in a summer is normal. Too much more than that and you will cook the shingles off the roof (curling at the tips). Roof turbines and vent sockets remove heat and dissipates the moisture (#1 killer of insulation).

Insulation is a blanket that keeps that heat from entering the home via the ceiling and that is where the difference is made. I talked with the insulation co and the sales rep told me about the radiant barrier is only good if your AC/HEATING unit is in the attic but remember the price vs a return. I have seen people spend upwards of $30-40 a sheet for radiant barrier and get no return for their money because at best it might drop the attic temp 5 deg’s. two extra turbine vents on the roof will do that a lot cheaper.

One year I installed electric vents thinking they would make a difference and they did. I watched my bill go up and up; they turned on at 7am and shut down at 6am the next morning just to start at 7am again. Needless to say they were trashed at the end of the summer .

Just an FYI for those who think if they by a 30-40 year roof they are getting a better product ask the co rep how many claims they ever paid on. it is pretty well nonexistent because natural occurrences get the roofing first

Couple of thoughts from 20 years of building and remodeling:

Cellulouse - Bad. Tearably dusty, will penetrate cans/lighting. I would not add any more. I have remodeled homes that had voids in the wall cavities from settling. BIG voids. After only 5 years...areas with zero insulation. When I built my house it was the "greatest" so I had to have it. That is how I know.

R38 - I'm told there is no benefit beyond r30 other than you will probably get an honest r30 .

Radiant barrier - The tech shield decking works. the aftermarket products I have no exp with.

Tack air dams to the decking to prevent insulation from blocking air flow from the soffit vents. Use ridge vents if you have enough ridge. If not air hawks will flow with no moving parts.

What makes insulation work is the trapped air. so build cat walks so no one has to step in any of it after it is blown.

My 4 cents. (inflation)

Topbuilder wrote:Couple of thoughts from 20 years of building and remodeling:

Cellulouse - Bad. Tearably dusty, will penetrate cans/lighting. I would not add any more. I have remodeled homes that had voids in the wall cavities from settling. BIG voids. After only 5 years...areas with zero insulation. When I built my house it was the "greatest" so I had to have it. That is how I know.

R38 - I'm told there is no benefit beyond r30 other than you will probably get an honest r30 .

Radiant barrier - The tech shield decking works. the aftermarket products I have no exp with.

Tack air dams to the decking to prevent insulation from blocking air flow from the soffit vents. Use ridge vents if you have enough ridge. If not air hawks will flow with no moving parts.

What makes insulation work is the trapped air. so build cat walks so no one has to step in any of it after it is blown.

My 4 cents. (inflation)

I agree with these points. I would also add that the comments about roof vents are spot on. If your house does not have continuous soffit vents then adding additional vents to match the exisiting ones is cheap and easy. Most building centers have the vent panels that will match existing ones and a hole saw on a cordless drill makes it go quickly. The roof can't breathe out any more air than it can take in, so improving airflow helps. Also when the installer is finished take a leaf blower around the soffit vents and use it make sure the vents are clear by blowing air up through them.
The hidden benefit for us after putting in a minimum of R-38 was a dramatic decrease in noise coming into the house. We live north of Dallas and while directly in the flight path for DFW and Love we do get a lot jet traffic aound us. After the insulation went in we do not hear it inside the house. Probably the biggest energy saver that we could do in our house would be to replace the lousy windows. The builders in our area all went with cheap aluminum double pane windows, that leak air and transfer heat and cold like mad. Unfortunately the house has lots of glass and the payback period gets longer than I think we will be there.