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Old 06-28-2010, 02:14 PM   #1
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Attic Insulation - Can't seem to find the answer


My house is in Houston, Texas, known for hot, humid summers. The attic is not insulated at all, but has a gable fan that draws air across the attic in from the cool side and out the hot side.

I use my attic for storage, so using loose-fill insulation is not really an option. My thoughts are to insulate the underside of the roof using R-13 batts between the rafters (2X4) and facing them with 2" blue foam board, for a total R-value of R-23. Not the best R-value, but a lot better than R-0. The foam board would, of course, be covered with 1/2" sheetrock as a fire retarder.

I am also considering doing the same thing to the attic floor, only with decking on top of 3/4" high-compression foam board. Joints where the deck and roof meet would be sealed with expanding foam.

This is a good cross-section of my plan (minus the sheetrock):



Is this an acceptable solution? I've heard a lot of different things saying yes or no, but nothing really solid. Some people say I should leave the space between the rafters empty, others say I should cut the foam to fit between them, some say don't do it at all. You name it, I've heard it.

This configuration seems reasonable to me, in that it looks like some suggestions I've seen for insulating vaulted ceilings. The only difference is that it is an attic.

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Old 06-28-2010, 03:46 PM   #2
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Attic Insulation - Can't seem to find the answer


I'm no expert. But I don't see the point at putting insulation in the rafters.

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Old 06-28-2010, 03:49 PM   #3
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Attic Insulation - Can't seem to find the answer


The point would be to reduce the transmission of heat from the roof to the attic. I realize most people say the best way to do it is put in about 12+ inches of blow-in insulation, but that is not an option for me.
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Old 06-28-2010, 04:43 PM   #4
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Attic Insulation - Can't seem to find the answer


I too would not put insulation in the rafters. There have been a couple of threads on reflective blankets that can be installed to the bottom of the rafter that will reflect the heat. I don't know whether they are worth it or not nor do I know of any issues the use of these blankets can cause.

Reflectix insulation

I would definitely look at putting some insulation in the attic floor. I assume obtaining a good seal to reduce airflow from your air conditioned space to the attic is important in climates like yours. Perhaps someone more experienced will chime in shortly.
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Old 06-28-2010, 05:03 PM   #5
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Attic Insulation - Can't seem to find the answer


One thing I did manage to come up with as far as reflective blankets/radiant barriers are concerned was that I need to have ridge vents or attic fans that are close to the roof peak in order for it to be effective. I have neither of those.

My house was built in 1936, which means no soffit vents or ridge vents. All I have to work with is a gable vent.

In the summer, the ambient temperature in my attic is 135-140 degrees without the gable fan running. With the fan, we can achieve low 100s, but if there is a way to insulate the roof so the attic doesn't get that hot to begin with, it would be great.
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Old 06-29-2010, 12:03 PM   #6
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Attic Insulation - Can't seem to find the answer


Quote:
Originally Posted by Ags Win View Post
One thing I did manage to come up with as far as reflective blankets/radiant barriers are concerned was that I need to have ridge vents or attic fans that are close to the roof peak in order for it to be effective. I have neither of those.

My house was built in 1936, which means no soffit vents or ridge vents. All I have to work with is a gable vent.

In the summer, the ambient temperature in my attic is 135-140 degrees without the gable fan running. With the fan, we can achieve low 100s, but if there is a way to insulate the roof so the attic doesn't get that hot to begin with, it would be great.
I recently installed the radiant barrier from Atticfoil.com and have been monitoring the temps this summer, and the attic has never been above the mid 90s whereas it was 130+ last summer. In the spring it was usually 4-8 degrees above ambient then it's evened out in the 90s so far. I do have 2 turbines and about 15 soffits which help. However my kwh usage compared to last year is about the same, so I'm not sure it will pay for itself any time soon.
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Old 06-29-2010, 12:23 PM   #7
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Attic Insulation - Can't seem to find the answer


The way to set up the radient barrier to work with your gable vent:

1- Install 2x2 sleepers horizontally across your rafters.
2- Frame a wall with the 2x2 a few inches inside the attic from the vents on both sides.
3- cover the 2x2 with a perforated radiant covering.

The perforations will allow water vapor to escape. The 2x2 standoffs allow air channels from one side of the attic to the other.

You now have created a silver balloon that will slow heat penetration into the storage space, but still allows ventilation through one gable out the other allowing all rafters airflow.
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Old 06-29-2010, 02:46 PM   #8
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Attic Insulation - Can't seem to find the answer


Won't insulating between the rafters reduce the heat transfer from your shingles, increase their temperature, and decrease their life?
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Old 06-29-2010, 06:05 PM   #9
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Attic Insulation - Can't seem to find the answer


Ridgemaster plus is a typical functioning ridge vent I've seen used in the harshest northern climates. Please note ventilation is crucial and gable vents only work great if the wind is right. Soffit vents can also give you adequet moisture protection (air vent inc).. I would think insulating @ the roof level may be more productive because you stop the heat before it gets into the house.

Windows and doors also make a difference. Remember your roof needs to breath. You insulate between the rafters you better have an air space between your roofing and your sheathing. It will literally cook itself and cause premature failure (roof that is). I am however concerned with moisture condensing in the batts and I might not reccomend this (batts @ ceiling w/ rigid). A Steel roof or strapping and sheathing can create an airspace between the roof and the sheathing. Point is you seal this area up you need to get rid of excess moisture or face mold/mildew issues. You might be able to do some of this work but I think there might be more issues than your prepared to handle all at once. Insulating your ceiling with an insulated roof will cause moisture issues as your trapping the moisture between the spaces. I always think its better to build a box than compartmentalize mositure and try and remove it mechanically. It may be wise to build a shell as in insulate walls and rafters only. Again this is where a technical professional can assist you better. You dont want mold in your house. Especially if you use A/C.

Just a thought

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