# Insulating attic decking



## BlueBSH (Oct 29, 2009)

I am having my attic reinsulated with cellulose, but 12ft of the attic space wide by 20 ft long is decked for storage with 1/2" OSB... The "floor joists" are 2x6 which I will have probably dense packed under the flooring... but that will probably get me about R-20? the rest of the attic will be probably R-40, is there anything I can do to improve this under the decked region? Someone said to put 2" XPS Rigid foam down, that would get me another R-10 and prevent thermal bridiging from the decking..

If you where to do this, what would you do? would you put the 1/2" OSB down first, then XPS perpendicular to that, then another layer of OSB? then screw it all down?


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## joecaption (Nov 30, 2011)

http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table


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## BlueBSH (Oct 29, 2009)

joecaption said:


> http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table


Yes, I know what the R-Value recommendations are, but that doesn't answer my question...


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## Windows on Wash (Aug 30, 2011)

BlueBSH said:


> I am having my attic reinsulated with cellulose, but 12ft of the attic space wide by 20 ft long is decked for storage with 1/2" OSB... The "floor joists" are 2x6 which I will have probably dense packed under the flooring... but that will probably get me about R-20? the rest of the attic will be probably R-40, is there anything I can do to improve this under the decked region? Someone said to put 2" XPS Rigid foam down, that would get me another R-10 and prevent thermal bridiging from the decking..
> 
> If you where to do this, what would you do? would you put the 1/2" OSB down first, then XPS perpendicular to that, then another layer of OSB? then screw it all down?


Do you really need the storage? Most folks will say yes but when confronted with the idea of removing the stuff and putting it back, they often times find it is stuff they don't really need or can find other locations for.

The best solution is to rip of the floor, air seal, and loose fill insulation.

Another option is to rip up the OSB, build another floor perpendicular out of 2x6/2x8/2x10 (whatever you think the floor can handle weight wise) and loose fill under that combined floor depth of either 11"/13"/15".

The 2x10 combo would be ideal because it will allow you to get to that targeted combined insulation depth.

Dense packing is difficult and more often than not, done improperly.

Unless you pull up that OSB deck and air seal, I would also be worried about moisture migration and condensation potential via air leaks that are not addressed because of the inaccessibility with the OSB sheets on the floor.


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## BlueBSH (Oct 29, 2009)

Windows on Wash said:


> Do you really need the storage? Most folks will say yes but when confronted with the idea of removing the stuff and putting it back, they often times find it is stuff they don't really need or can find other locations for.
> 
> The best solution is to rip of the floor, air seal, and loose fill insulation.
> 
> ...


trust me, we need the storage... we have 6 closets in the house filled to the max, and a 10x15 storage room filled...  none of the stuff is mine either (hint)...

the entire floor has already been "air sealed" had a spray foam guy come out and seal all the top plates and any openings / junction boxes / can lights with drywall boxes..

now I had the perpendicular floor idea a while ago, and someone basically yelled at me and told me never do that :huh:

This is a room that at some point will be finished, we just don't have the money to do it right now, so didn't want to go overkill on this.. right now it has a whole 5.5" of blown fiberglass in it... probably R-13 effectively at most? just estimating that number blown fiber is what? R-2.2 per inch? Cellulose is R-3.3 per inch I think?

I'm having a cellulose installer doing the work, I'm just trying to do the prepwork, they are the ones that said to do the XPS decking... I just never heard of it before... and only found one reference to it online and that was a British site that basically used SIPS for decking


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## ryanxo (Jun 7, 2011)

Closed cell foam is R-7 per inch I think. 6 inches of that would be R-42.
Expensive stuff but just an idea from another DIYer.


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## Windows on Wash (Aug 30, 2011)

ryanxo said:


> Closed cell foam is R-7 per inch I think. 6 inches of that would be R-42.
> Expensive stuff but just an idea from another DIYer.


Rigid foam will work too. Still gotta pull up that floor and re-do it though.


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