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Old 01-12-2012, 08:41 PM   #1
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Smashing down blow in insullation


I have blow in insullation in my attic and I want to build a small room up there for storage. I am concerned though if I lay down floor boards over top of the blow in and nail it down that it will severly decrease the insulating capability of the blow in. Does anyone know if this is true?

Should I just remove the blow in from where I want the room to be and lay down fiberglass batts under the new floor? Or will it make no difference?

Currently I have 21" of blow in up there.

Here are some pictures with illustrations of what i am tryign to do.




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Old 01-12-2012, 08:43 PM   #2
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Smashing down blow in insullation


Have a yard sale instead.

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Old 01-12-2012, 09:09 PM   #3
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Tarheel are you a quitter?
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Old 01-12-2012, 09:15 PM   #4
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Smashing down blow in insullation


I would suggest that you also look for some outdoor storage space (i.e. shed, etc) or try additional shelving. That looks like a bit of a pain in the rear of a project and for minimum yield on storage space.
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Old 01-12-2012, 09:20 PM   #5
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Smashing down blow in insullation


Unless you have at a bare minimum of 2 x 8's for ceiling rafters with no more then 12' spans that ceiling was not built for any storage.
No the insulation can not be compressed, if you did it no longer would do anything. It needs the air spaces between the fibers.
I agree add a garage, Buy a shed.
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Old 01-13-2012, 09:36 AM   #6
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Smashing down blow in insullation


The rafters are 2X8's and nothing that heavy is going up there. It is just christmas light boxes, a fake Christmass tree, some ornaments and baby stuff. We have one kid and more coming so we need a place to store this stuff while its not being used.

Let me rephrase my "Small Room" comment to long hallway. This room would extent over 35' in length. Yes the room ceiling would be low but it would be very deep to allow storage for a ton of light weight stuff spreadout over a very long span. This is a very small house with no yard, no basement and no room for a garage. I have no choice. I cannot afford to rent a storage locker either. They are $100 a month around here minimum.

So since I cannot compress the blow in like I assumed, can I achieve a descent insullating results by removing the blow in where I am going to put down the floor boards and put in fiberglass batts?
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Old 01-13-2012, 10:19 AM   #7
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Smashing down blow in insullation


Quote:
Originally Posted by BackYardBuilt
The rafters are 2X8's and nothing that heavy is going up there. It is just christmas light boxes, a fake Christmass tree, some ornaments and baby stuff. We have one kid and more coming so we need a place to store this stuff while its not being used.

Let me rephrase my "Small Room" comment to long hallway. This room would extent over 35' in length. Yes the room ceiling would be low but it would be very deep to allow storage for a ton of light weight stuff spreadout over a very long span. This is a very small house with no yard, no basement and no room for a garage. I have no choice. I cannot afford to rent a storage locker either. They are $100 a month around here minimum.

So since I cannot compress the blow in like I assumed, can I achieve a descent insullating results by removing the blow in where I am going to put down the floor boards and put in fiberglass batts?
How far are the joists spanning? Makes a big difference if you're okay or not.
And don't compress the insulation.

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Old 01-13-2012, 10:39 AM   #8
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Tarheel are you a quitter?
Space for a few boxes of Christmas ornaments is one thing...
this ain't that.
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Old 01-13-2012, 05:13 PM   #9
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Smashing down blow in insullation


The rafters are fine to hold this light weight stuff. I am a mechanical enigneer and trust me, I could put 300 LBS supported over just two of these rafters and it would be fine. The rafters in the attic are exactly the same dimensions that support the second floor. The vertical walls under these rafter run perpendicular under this proposed room which would also provide even more rigid support. I'm not worried about the structural integrity of this house not being able to support some toddler equipment and some x-mas stuff. All I am wondering about is the difference between the fiberglass batts and the undisturbed blow in.

Thats all I am asking. I think I'll just call the manufacturer and ask them for thier R value of the blow in and compare it to the R value of fiberglass batts under the new floor I will nail down.

Thanks for the help fellas.
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Old 01-13-2012, 05:21 PM   #10
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Smashing down blow in insullation


Quote:
Originally Posted by BackYardBuilt
The rafters are fine to hold this light weight stuff. I am a mechanical enigneer and trust me, I could put 300 LBS supported over just two of these rafters and it would be fine. The rafters in the attic are exactly the same dimensions that support the second floor. The vertical walls under these rafter run perpendicular under this proposed room which would also provide even more rigid support. I'm not worried about the structural integrity of this house not being able to support some toddler equipment and some x-mas stuff. All I am wondering about is the difference between the fiberglass batts and the undisturbed blow in.

Thats all I am asking. I think I'll just call the manufacturer and ask them for thier R value of the blow in and compare it to the R value of fiberglass batts under the new floor I will nail down.

Thanks for the help fellas.
You're an engineer but you keep calling the joists rafters?
What if they span 30 feet?

Edit: Oh. I see they're trusses.

Last edited by titanoman; 01-13-2012 at 05:23 PM.
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Old 01-13-2012, 10:08 PM   #11
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Smashing down blow in insullation


You will probably find the blow-in is the same R-value per inch as batts, or pretty close. That is only part of the problem. Adding a floor surface will stop any moisture drive (stack effect) to condense water on the bottom of the decking. This is the purpose of attic ventilation. I doubt the moisture will work horizontally to get to the ridge vents, to exhaust outside as planned. The amount of mold would depend on if a vapor barrier was present or not. Here are a few links on that; http://www.buildingscience.com/docum...on?full_view=1

http://www.google.com/url?sa=t&sourc...wWATQw&cad=rja

http://www.wag-aic.org/1999/WAG_99_baker.pdf

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Old 01-13-2012, 10:37 PM   #12
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Smashing down blow in insullation


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You're an engineer but you keep calling the joists rafters?
Quote:
Originally Posted by titanoman View Post
What if they span 30 feet?
Edit: Oh. I see they're trusses.

Titan, I apologize for not using the correct terminology for you to understand my fundamental question which I was seeking advice on. (Which was about insulation and not rafters or joists) I do get some of these defining terms mixed up because I don't use them on a regular basis like I am sure many of you all do. Usually in my field of work we use calculus, physics and statics engineering to determine stress loads, material dimensions, sheer stress minimums, compare graphs, calculate bending moments, moments of inertia and lots of other analysis to make sure that things are safe and run efficiently. Please accept my sincerest apologies for not communicating these terms more clearly.

I find it funny how you seem to question the legitimacy of my being an engineer due to the mis-identification of some simple wooden parts in my attic. I wasn't saying I am an engineer and you should STFU, I was simply saying I am a man who has education in this area to know the limitations of what I am attempting. I really just wanted to end the discussion about, "Is what I am doing safe" and get the real question answered about my insulation dilemma. The pictures I posted clearly illustrate my goal.

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Old 01-13-2012, 10:59 PM   #13
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Adding a floor surface will stop any moisture drive (stack effect) to condense water on the bottom of the decking.

Gary, when you say decking, do you mean the bottom of the roof surface or the bottom of the actual floor boards I will be putting down?

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This is the purpose of attic ventilation. I doubt the moisture will work horizontally to get to the ridge vents, to exhaust outside as planned. The amount of mold would depend on if a vapor barrier was present or not. Here are a few links on that;

Gary, those were some very technical and informative links. Thank you for posting that information.
This room is only going to end up being about maybe 5 ft wide by about 3.5 ft tall. There is at least 20 ft of open insulated attic on each side of the spot I will put this storage area. So what you are saying is that any moisture trying to come up through my second floor ceiling under this storage area could cause mold? Under the new floor I am putting down or on the ceiling itself?
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Old 01-14-2012, 02:00 PM   #14
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Sorry, decking (a framers term) is the sub-flooring. You may get mold on the bottom surface because of the moisture diffusing through the insulation from the warm conditioned room below. This would be the first surface it contacts if colder than the rising air; fig.2; http://www.buildingscience.com/docum...bout-diffusion

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Old 01-14-2012, 02:12 PM   #15
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Sorry, decking (a framers term) is the sub-flooring. You may get mold on the bottom surface because of the moisture diffusing through the insulation from the warm conditioned room below. This would be the first surface it contacts if colder than the rising air; fig.2; http://www.buildingscience.com/docum...bout-diffusion

Gary
So is there any way to combat this?

What if I didn't put any insulation under this new decking in the attic but I fully insulated the outside of this storage room. Walls and ceiling. The soffit air would be able to carry away the moisture then because it would diffuse into the open attic air.

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