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01-07-13, 04:53 PM | #1 |
Lurking Renovator
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8" insulation in a 6" wall
well i've been a member of this forum for quite a while and have never posted because I quite frankly have had nothing intelligent to contribute. To continue with that theme I have decided to ask a dumb question. I am moving my mother in law out of her old house (dry rot, termites, black mold) and into her detached concrete block garage. I have studded the walls with 2 x 6's, intending to take the almost new 6" fiberglass insulation out of her old attic (2 layers) and insulating the walls of her new garage/home with it. Now; after the walls are studded I have discovered that the insulation in the old attic is actually 8".
Question: If I go ahead and stuff the 8" fiberglass into the 6" walls I know i will not get the full R value of 8" but will it be less than the R19 i would get if I bought 6"? |
01-07-13, 07:02 PM | #2 |
Journeyman EcoRenovator
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Overstuffing insulation is not generally a good idea. It is the air trapped in the batt that provides the insulation, and less air/more fiberglass is not as good.
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01-08-13, 08:31 AM | #3 |
Master EcoRenovator
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Compressing fiberglass insulation reduces it's insulation value, so you will have a lower R-value if you use 8" of fiberglass in a 6" space.
What you can do is seal the stud spaces at the top, bottom and all other seams, to prevent drafts, and I like using expanding foam behind all outlets and light switches as those tend to be drafty. |
01-08-13, 08:49 AM | #4 |
Helper EcoRenovator
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It may not insulate as well, but will there be that much of a difference to justify buying new?
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01-08-13, 10:01 AM | #5 |
Helper EcoRenovator
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Fiberglass insulation is very easy to split. it usually comes off in layers. Just seal the cavity as Ryland suggested, insert the insulation, peal off what you don't need and you should be good.
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01-09-13, 01:24 AM | #6 |
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01-08-13, 10:32 AM | #7 | |
Supreme EcoRenovator
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Quote:
> dry rot, termites, black mold... MarkM66, you have a serious moisture problem that needs to be resolved. If you don't address that, it will come back again, like Nosferatu rising from his coffin, to haunt you and your mother. I would guess that the walls have many air leaks and the humid indoor air is traveling through the walls and cooling, which causes the moisture in the air to condense in the insulation and provide a warm, dark, wet paradise for termites and black mold. Really, MarkM66, your mother deserves better than this. There are a few approaches to this and you shouldn't limit yourself to just one: VAPOR BARRIER - After you're done with the insulation, use a good vapor barrier to prevent air from moving through the wall. In the past, plastic sheeting has been used, and that helps. THERMAL GRADIENT - Either wrap the outside of the house with foam or spray the stud space with about 1" to 2" of foam. Then use a good insulation of your choice in the remaining space. The idea here is that the moist air hits the foam at a temperature above the dew point, and will not drop moisture. HEAT RECOVERY VENTILATION - This will get the moist air out of the house and save most of the out-going heat to the incoming air, which will be much lower in relative humidity when it is warmed. * * * But back to your ventilation quandary... If you should continue with your plan (attic insulation to walls). You will eliminate the expense and hassle of disposing the old insulation you want to remove. There is also an environmental benefit to re-using the insulation and not being part of the chain of events that would cause new insulation to be made. There would be some decline in insulation (compared to an 8" wall) but probably not as much as you might think. Packing in the extra insulation would increase the immobilization of the air in that cavity... that's a good thing. Everything that has been previously said about making air-tight cavities is very important, no matter what you decide. * * * But when you get done, you're still going to have to insulate the attic right? So why not leave the old attic insulation where it is and lay more over the top, and get more 6" insulation for the walls? (I may have gone beyond the two cents worth limit here...) Best, -AC
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I'm not an HVAC technician. In fact, I'm barely even a hacker... Last edited by AC_Hacker; 01-08-13 at 10:35 AM.. |
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01-08-13, 10:35 AM | #8 | |
Supreme EcoRenovator
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Quote:
> dry rot, termites, black mold... MarkM66, you have a serious moisture problem that needs to be resolved. If you don't address that, it will come back again, like Nosferatu rising from his coffin, to haunt you and your mother. I would guess that the walls have many air leaks and the humid indoor air is traveling through the walls and cooling, which causes the moisture in the air to condense in the insulation and provide a warm, dark, wet paradise for dry rot, termites and black mold. Really, MarkM66, your mother deserves better than this. There are a few approaches to this and you shouldn't limit yourself to just one: VAPOR BARRIER - After you're done with the insulation, use a good vapor barrier to prevent air from moving through the wall. In the past, plastic sheeting has been used, and that helps. THERMAL GRADIENT - Either wrap the outside of the house with foam or spray the stud space with about 1" to 2" of foam. Then use a good insulation of your choice in the remaining space. The idea here is that the moist air hits the foam at a temperature above the dew point, and will not drop moisture. HEAT RECOVERY VENTILATION - This will get the moist air out of the house and save most of the out-going heat to the incoming air, which will be much lower in relative humidity when it is warmed. * * * But back to your ventilation quandary... If you should continue with your plan (attic insulation to walls). You will eliminate the expense and hassle of disposing the old insulation you want to remove. There is also an environmental benefit to re-using the insulation and not being part of the chain of events that would cause new insulation to be made. There would be some decline in insulation (compared to an 8" wall) but probably not as much as you might think. Packing in the extra insulation would increase the immobilization of the air in that cavity... that's a good thing. Everything that has been previously said about making air-tight cavities is very important, no matter what you decide. * * * But when you get done, you're still going to have to insulate the attic right? So why not leave the old attic insulation where it is and lay more over the top, and get more 6" insulation for the walls? (I may have gone beyond the two cents worth limit here...) Best, -AC
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I'm not an HVAC technician. In fact, I'm barely even a hacker... |
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01-08-13, 02:35 PM | #9 |
Helper EcoRenovator
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I'm not the original poster.
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01-08-13, 11:13 AM | #10 | |
Helper EcoRenovator
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Quote:
Trying to answer your question: A quick google makes me believe that fiberglass is around 99% air (by volume) , so if you squeeze it into 3/4th the space (6" instead of 8") it is still 98% air. You say that new 6" fiberglass is R19, so I dare say the reused 8" stuffed into the 6" would be R18 or R19, but not 3/4th of R19 (R14). Last edited by Fornax; 01-08-13 at 11:25 AM.. |
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