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Old 01-07-13, 04:53 PM   #1
workaholic
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Default 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"?

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Old 01-07-13, 07:02 PM   #2
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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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Old 01-08-13, 08:31 AM   #3
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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.
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Old 01-08-13, 08:49 AM   #4
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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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Old 01-08-13, 10:01 AM   #5
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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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Old 01-08-13, 10:32 AM   #6
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Quote:
Originally Posted by MarkM66 View Post
It may not insulate as well, but will there be that much of a difference to justify buying new?
...my two cents worth...

> 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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Old 01-08-13, 10:35 AM   #7
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Quote:
Originally Posted by MarkM66 View Post
It may not insulate as well, but will there be that much of a difference to justify buying new?
...my two cents worth...

> 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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Old 01-08-13, 11:13 AM   #8
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Quote:
Originally Posted by workaholic View Post
I am moving my mother in law out of her old house ..... and into her detached concrete block garage.
Will the garage become a permanent livingspace for her or is it temporary wile the old house is being renovated?

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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Old 01-08-13, 11:21 AM   #9
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my 2 cents is to just fir out the existing studs another 2" and use the free 8" insulation
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Old 01-08-13, 12:04 PM   #10
workaholic
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Sorry I have been away for a while. You guys have been great to answer in so much detail, but I didn't give enough detail when I originally posted my question. It's a long story so here goes:

My father-in -law built this house in the mid 60's.
Crawl sapce with only 2 tiers of blocks
No drainage around the footers.
No ventilation in crawl
He drained his bathtub on the Ground in the crawl!!
the distance from topof dirt to bottom of floor joists is only 7-12"
water drains Towards the foundation on 3 sides
SEVERE MOISTURE PROBLEMS !!!
rotten joists, termites, black mold.

I am not going to try to fix any of this(I am going to have the house torn down in the summer) but am moving my mother in law to an existing concrete block garage located about 50' from the house .
Here is what I have done so far:
checked concrete floor of garage for moisture coming through. (taped plastic to floor, left it for a week, tore off plastic and found no moisture)

sealed all cracks in floor and mortar joints in walls,laid 2x4's on floor 16" centers leaving 3" gap between block wall and first floor joist, 4" fiberglass between joists, 3/4" subfloor on top of joists.

poured insulation in 3" gap between floor and block walls, built 2 x 4 walls inside but left 2" gap between block wall and 2x4 studs to fit 6" insulation and avoid thermal bridging.( this is where I plan to use the insulation out of her attic in the old house)

framed up the opening where the 16' garage door was with 2x8's and install insulated door and low-e window.

I plan to screw horizontal girts to the exterior walls on 36" centers. I am going to glue 1/2" foil backed foam insulation to the blocks between the girts, cover the exterior walls (girts, and foam insulation) with 6 mil plastic then cover it all wit 5 rib corrugated metal.

Then I will blow in 14" of cellulose in attic.
Kitchen cabinets, 90% gas furnace, gas water heater, various interior doors, etc. will be robbed out of her old house, cleaned and reused.

So.. starting at the exterior I have: metal sheathing--6 mil plastic--1" air gap--1/2" foil backed foam--8" concrete blocks--6" fiberglass--drywall inside

any suggestions will be appreciated.

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