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Old 11-09-2010, 05:04 PM   #1
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yet one more insulation question


I have a 1925 two story home with minimal attic insulation. What is up there is, I believe, rockwool. The attic is about 75% floored and the insulation under the boards varies 6 inches to nothing. I have some remaining work to do up there before I have additional cellulose blown in. Can I add the new insulation on top of the floorboards or should they be removed first. I was going to take out the floor and remove all the existing insulation but the more I think about doing that the less I want to. Is adding the new insulation on top of the flooring a really bad idea? There will be lots of places with a considerable air space between the attic flooring and ceiling below. Adding the new insulation on top of the existing rockwool in the unfloored areas a bad idea also?
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Old 11-10-2010, 10:21 PM   #2
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yet one more insulation question


The dead air spaces are not a great R value, and they certainly lend themselves to air currents, which hasten the heat getting from warm to cold. Wood is not a serious moisture barrier, so it won't hurt, but it will leave air pockets and it, too, has a poor R value. There is no reason that I can see to remove the old insulation, unless it has been compromised by rodents using it to go potty, etc. If it were mine, I'd bust a hump to get the wood out of the way; you can likely re-use it somewhere, right? Cellulose is a good choice; put in as much as is appropriate for your area, but make sure the sheet rock will support the extra weight.

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Old 11-11-2010, 10:45 AM   #3
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yet one more insulation question


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Originally Posted by jklingel View Post
The dead air spaces are not a great R value, and they certainly lend themselves to air currents, which hasten the heat getting from warm to cold. Wood is not a serious moisture barrier, so it won't hurt, but it will leave air pockets and it, too, has a poor R value. There is no reason that I can see to remove the old insulation, unless it has been compromised by rodents using it to go potty, etc. If it were mine, I'd bust a hump to get the wood out of the way; you can likely re-use it somewhere, right? Cellulose is a good choice; put in as much as is appropriate for your area, but make sure the sheet rock will support the extra weight.
Thanks jklingel. I'm too old to bust any more humps as I only have one left. Seriously, I appreciate your taking the time to answer. My ceilings are rock plaster on wood lathe. I assume weight would be less of an issue than if they were sheetrock. Think that's correct?
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Old 11-11-2010, 01:27 PM   #4
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yet one more insulation question


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My ceilings are rock plaster on wood lathe. I assume weight would be less of an issue than if they were sheetrock. Think that's correct?
Mike
Sorry, outta my knowledge data base. I just wanted to point out that adding insulation can impact the ceiling sheathing. Good luck, and you are welcome. Be sure to answer my questions, too! There is a pretty good pool of knowledge in the world....
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Old 11-11-2010, 02:44 PM   #5
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yet one more insulation question


Mike in Arkansas, I'm dealing with the same situation in my mom's attic. Her house was built in 1895 and has since been remodeled.

There are 4'x8' osb boards covering the floor of the attic. When I first went up there, I thought I would only have to seal everything up and add the cellulose.

But not checking what was under the osb was bugging me. So I pulled up a few boards and more than tripled the amount of work I now have to do up there. All of the boards will have to be lifted and removed. But that's alright because my goal is to bring her heat bill down from $600+ per month to as low as I can get it.

Under the osb is about a 2" to 3" gap between it and the top of the existing insulation. That's not good, as jklingel said. But the biggest problem I found was the gaps between the insulation and floor joists and the fact that none of the penetrations into the attic had been sealed under the osb. So there was (still is) air from the living space leaking up into the space under the osb and ultimately, into the attic.

Another thing I found is that heated air can come up into the attic from the bays of the exterior house walls because there is nothing blocking the tops of those bays. I hope this is making sense.

There are a lot of other things going on up there that need attention, but I guess what I'm saying is that if I were you, I'd pull up the boards in your attic and investigate. That way you can properly seal everything. Insulating your attic isn't enough if you're still losing heat or air conditioned air up into the space between the ceiling and the attic floor boards.

Do you have a hatch that leads up into the attic? If so, how is the seal on it and is it insulated?
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Old 11-11-2010, 04:20 PM   #6
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yet one more insulation question


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But that's alright because my goal is to bring her heat bill down from $600+ per month to as low as I can get it. Good God! $600/mo is unbelievable, regardless of where Mom is. We only burn 750 gal of oil/year, heating 2800 sf in Fairbanks.

But the biggest problem I found was the gaps between the insulation and floor joists... That is one of the big negatives of using batts, esp if the cavities are not exact.

and the fact that none of the penetrations into the attic had been sealed under the osb. I believe I read that a 1/2" hole loses the same heat as 10 sf of wall, and that may be a 10' SECTION of wall; I forget. Point is, AIR SEAL first.

Another thing I found is that heated air can come up into the attic from the bays of the exterior house walls because there is nothing blocking the tops of those bays. No top plate on the walls? Whatever the cause, that is a huge heat loser.

Insulating your attic isn't enough if you're still losing heat or air conditioned air up into the space between the ceiling and the attic floor boards. Roger that.
Good post, and good luck. I bet you can get that heat bill down to something like $150-200/mo.
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Old 11-11-2010, 10:47 PM   #7
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yet one more insulation question


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Originally Posted by gma2rjc View Post
Mike in Arkansas, I'm dealing with the same situation in my mom's attic. Her house was built in 1895 and has since been remodeled
Another thing I found is that heated air can come up into the attic from the bays of the exterior house walls because there is nothing blocking the tops of those bays. I hope this is making sense.
Do you have a hatch that leads up into the attic? If so, how is the seal on it and is it insulated?
Thanks for the post. My first house (40 years ago), built around early 1900's, had open bays as well. You could stand in the attic, look down and see sunlight coming in the gaps in the siding (no insulation or sheathing either). I do have attic pull down stairs that I will be insulating. Lots of openings for wires also that I've been filling as I replace wiring up there. Guess the best thing to do is just suck it up and do it right.
Thanks again

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