Need help on size of insulation for attic please

[BAZZMAZZA]

Here is my situation. I am currently preparing to put in some 3/4 inch plyboard flooring in the attic above my garage. The joist are 24" centers and are 2x4 construction. I went to the local homestore (LOWES) today and could not find 24 " insulation for 2x4 construction. The store had 24" insulation for 2x6 construction but was 12" inches in thickness. If I were to use this it would mean that I would be compressing 8+ inches of insulation down onto the garage ceiling. Needless to say I walked away and decided I would ask here first before proceeding.

My questions:
Can I use 12" insulation and compress it to fit without losing r-value?
Is this size insulation made any longer?
Where can I buy this insulation Lowes, Home Depot etc. ?

Thank you in advance!

 

[ghnl]

You can compress the insulation but its R-value will then be reduced.

Could you put some 2X4's on edge on top of the current joists - in effect raising the floor? Are these the bottom chord of trusses or actual joists? Are you sure they are strong enough to carry the extra load of flooring & whatever you might place on it?

 

[BAZZMAZZA]

You can compress the insulation but its R-value will then be reduced.

Could you put some 2X4's on edge on top of the current joists - in effect raising the floor? Are these the bottom chord of trusses or actual joists? Are you sure they are strong enough to carry the extra load of flooring & whatever you might place on it?

Thanks for the response. Area will be used for storage of small items nothing heavy. They are not joists they are trusses apologize for the mistake. Thanks,

 

[Gary S]

If you are sure you want insulation for 2x4s, go somewhere else other than Lowes. It is commonly available in R11 for 2x4 walls and ceiling in 24" width.
If you can add space for the R19 above the rafters, do it that way. You won't regret the extra insulation if you can make it fit.

 

[Possum]

First off - I think you better consult an engineer about putting any kind of load on 2x4's on 24" centers. Unless the span is very narrow it sounds sketchy at best.

That being said, you have many choices;
Blown in cellulose, or fiberglass.
I know I have seen 23" kraft faced batts for 2x6 depth, you might be able to find 2x4.
You can get XPS foam board pre-scored to snap in 24" widths.
Sprayed in foam.

 

[GSSFC]

First off - I think you better consult an engineer about putting any kind of load on 2x4's on 24" centers. Unless the span is very narrow it sounds sketchy at best.

That being said, you have many choices;
Blown in cellulose, or fiberglass.
I know I have seen 23" kraft faced batts for 2x6 depth, you might be able to find 2x4.
You can get XPS foam board pre-scored to snap in 24" widths.
Sprayed in foam.

A lot of buildings are done this way. I'd only caution this. If you decide to drywall or finish the ceiling, DO NOT go up in the attic area. We recently did a job with 2x4 bottom cords and had to walk around in the attic area to insulate. The joists held us fine, but did flex a lot. After the plaster guys finished, the HVAC crew went back up to finish up some duct work and cracked the plaster in several places!

Tim

 

[Possum]

A lot of buildings are done this way. I'd only caution this. If you decide to drywall or finish the ceiling, DO NOT go up in the attic area. We recently did a job with 2x4 bottom cords and had to walk around in the attic area to insulate. The joists held us fine, but did flex a lot. After the plaster guys finished, the HVAC crew went back up to finish up some duct work and cracked the plaster in several places!

Tim

So you are saying that as long as he can live with cracked drywall, it is safe to throw some 3/4" plywood down and start hauling boxes up there? It sounds like the sheetrock is already done. A ceiling joist is a far cry from a floor joist IMO. I would guess the person with the truss plans would think so too.

 

[GSSFC]

So you are saying that as long as he can live with cracked drywall, it is safe to throw some 3/4" plywood down and start hauling boxes up there? It sounds like the sheetrock is already done. A ceiling joist is a far cry from a floor joist IMO. I would guess the person with the truss plans would think so too.

No that isn't what I am saying.....pretty much the exact opposite.



Tim

 

[Possum]

Sorry, my misunderstanding then.

BAZZMAZZA please check this link - http://www.awc.org/calculators/span...ad=5&submit=Calculate+Maximum+Horizontal+Span

That would be 5psf for sheet rock on the bottom, 10 psf for flooring, and 10psf for load.

*edited for link

 

[Falcon67]

It's a problem for us with older houses - finding 22 1/2 x 3 1/2" fiberglass. They carry the batts here locally, but no rolls.