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Will one layer of insulation weigh down the layer below it?


agielchinsky's Avatar
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11-15-14, 06:46 PM   #1  
Will one layer of insulation weigh down the layer below it?

Hello,

My attic floor has 2x6 joists with old beat up rockwool insulation in it. My plan was to pull out the old insulation, put in some faced rolls of R19 and then at some later point put some rolls of unfaced R30 perpendicular to the R19. The R30 would rest on the joists and only compress the R19 a little bit.

If instead of filling the joists with R19 I used R30, would the second layer of insulation weight down the first since the first layer would be several inchs above the joists?

I live in northern NJ and would prefer to get to R60, but don't want compressed insulation. In this part of the attic I've got 18 bays that are about 27ft long by about 15-16 inches wide.

Thanks

 
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Furd's Avatar
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11-15-14, 09:48 PM   #2  
It would probably be okay but is there a reason why you don't want to use blown-in insulation?

 
Bud9051's Avatar
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11-16-14, 12:29 AM   #3  
A gap above each joist or any gaps, are more of a problem than a bit of compression.

Bud

 
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11-16-14, 06:05 AM   #4  
Blown in for the whole thing is the way to go not batts.
Before doing anything make sure the attic is air sealed, nothing more then sealing up all the gaps around any ceiling fixtures and holes where wiring or plumbing was run.
If you have soffit venting your going to have to add baffles to keep the insulation from plugging up the air flow to the roof.

 
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11-18-14, 05:14 PM   #5  
I was planning on doing one layer of roll, doing some electrical work, and then putting the other layer of rolled insulation on. After thinking about it and watching some videos I think you guys are right about using blown in.

Should I remove the old rock wool first?

Should I put down a vapor barrier?

 
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11-18-14, 09:36 PM   #6  
Leave the rock wool.
To late for a vapor barrier, that would have gone on the bottom of the joist before the sheetrock went up.

 
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11-19-14, 05:14 AM   #7  
If it's not too tough to remove the rock wool (with intention of replacing it), you would likely benefit a great deal from air sealing prior to adding more insulation.

 
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11-28-14, 11:57 PM   #8  
Why go through all the trouble of removing the rockwool??? It may look dirty but it's perfectly fine insulation and will only add to whatever you blow on top.

Everyone says to seal everything in the attic but in an old house with old insulation, dirt, and old everything else it's really hard to hit everything in the attic, especially since it's hard to see. It's so much easier to seal within each room (ALL electrical boxes). The exception might be can lights and bath fans, which are easier to seal from above. ...I also don't believe that air can pass through 24" of cellulose as easily as people think it can.

I just insulated the attic and walls of an 87yr old house with cellulose and, although it's still early in the winter, it looks like my heating bills will nearly halve.

 
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11-29-14, 04:22 AM   #9  
Hi chimpy,
Air sealing is a must at every opportunity and every available location. As for cellulose reducing the air leakage, it will of course reduce it some, but picture a 1/2" hole going through a top plate. If it were looking at 24" of cellulose packed inside a 24" piece of 1/2" copper pipe, that would be a lot of resistance to air flow. But in reality, the air that passes through that hole is now looking at several hundred sq ft of 24" deep cellulose and can disperse in many directions. When installing new insulation in an attic, it is a one time opportunity to seal many locations and not a good time to be taking the easy way out. Just look at what is in your attic now. Would you ever consider digging down through that to do more air sealing? Yet, at some point in the future they will be requiring leakage testing to score every home and adjust the value of those homes accordingly.

As for removing the old, I have rarely seen any that I would want left in my attic. Between dirt, dead rodents, and their droppings, it is refreshing to have it gone. But not all are that bad so it is a case by case decision.

Bud

 
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11-30-14, 11:53 PM   #10  
Would sure like to see some figures on air moving through deep cellulose. A 15amp motor on an insulation blower sure has a lot of trouble moving air through a foot of cellulose if you just partially block the nozzle.

If the electrical boxes in the rooms are sealed then there's really no good place for that top plate hole to find exchange air.

If I had a nice clean attic I'd probably seal the holes, but I wouldn't vac up an entire attic of rockwool just to try and find some wire holes to fill. Just like you said, it's not worth digging down into it.

Another problem with sealing wire holes is that it makes it really annoying to change anything. I'm always removing and pulling new wires for this or that, and like to reuse a hole when I can

Fiberglass is a lot more air permeable, not to mention much slower to install and more expensive, which is why I wouldn't use it. Last I checked rockwool is as good or better an insulator as fiberglass and cellulose.

 
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12-01-14, 08:32 AM   #11  
Hi Chimpy,
The air paths into an attic are more than just the holes we create. Unless drywall is sealed all the way around, and the framing as well, air will flow from basement to attic very easily. Now, this does not mean it looks like an open window, but it will move enough air where it can create a moisture issue.

The concern arises when people are told or read that cellulose can be installed as a substitute for air sealing that ceiling plane, it should not, even though it is done all the time, primarily by weatherization programs (my opinion). The WAPs are extremely budget conscious and if they can meet their minimum requirements and save a nickel they will. Homeowners on the other hand will find that nickel well spent, as all air sealing is good. Note, we don't want homes wrapped in plastic, but typical improvements will never reach that level.

Dense pack cellulose in an enclosed wall cavity can make a big difference with an old leaky home, less so with a new home. But blown in cellulose in an attic is far from dense packed. Agreed, it will reduce the convective heat loss due to cold air circulating down through it to the ceiling below. But it just doesn't do the job of substituting for an air barrier.

I'm sure there are numbers out there, I just haven't found them, or looked that hard.

Bud

 
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