Home>Discussions>INSULATION & HVAC>What is the best way to add additional insulation to my attic in this scenario?
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drewmey
What is the best way to add additional insulation to my attic in this scenario?
drewmey

I recently bought a home built in 1910. At some point, 5"-6" of fiberglass insulation was blown into the attic, then covered with 1/2 OSB (1,100 SF). I am looking to bring up the R-Value of this attic but struggling with my options. My wife wants to retain the ability to use it as storage.

I thought about building up additional storage with 2x10 or 2x12's, blown in insulation and additional plywood over the framing. However, I have concerns about the weight and stress on the trusses. I also thought about pulling up the existing OSB to reduce the weight before building it up higher. However the OSB is screwed to the joists and I am afraid it was done to ensure that the roof trusses don't pull away from each other.

Would it be beneficial to roll out R-30 batts over 80% of the attic and leave no additional insulation wherever we end up using storage (not optimum, but easy and safe structurally. Could roll back up if we need additional storage)? What solutions would you consider instead?

hollasboy
Re: What is the best way to add additional insulation to my attic in this scenario?
hollasboy

I have 30" thick insulation around the perimeter of my attic, where the rafter bays are open, and only 10" under the storage area's floor boards in the middle area. I think it's fine to add more where you can.

As for the structural questions, OSB does not weigh that much and is usually only used as a decking material - it does not have good tensile strength like regular plywood, so it's probably not a "structural" member - if your house was framed correctly. However, if you see nothing else holding it all together, you need to be careful before taking anything apart. No one can really answer these questions without seeing your framing situation in person. A good General Contractor would be able to tell.

keith3267
Re: What is the best way to add additional insulation to my attic in this scenario?
keith3267

You will need to take up the OSB. Then you can fill the cavities completely. If you have 2x8 joists, that should give you about an R 22. If you have 2x6 joists, then you wont get as much. You can lay (vertically) 2x6's in top of the existing joists or perpendicular to the joists on 16"centers. Then fill the now deeper cavities and replace the OSB. This should give you an R30 or more.

Mastercarpentry
Re: What is the best way to add additional insulation to my attic in this scenario?
Mastercarpentry

Perpendicular raised joisting is the usual solution here with the flooring placed on top. This gives the needed space and spreads the loading across all of the existing joisting instead of overloading any in particular. If you use "I" joists that's even less weight added and will be easier to attach to the existing joists. When I do IJT's like this I screw through the flange instead of nail to draw everything together flat and keep it there. Works great.

Phil

drewmey
Re: What is the best way to add additional insulation to my attic in this scenario?
drewmey

Thanks everyone. After thinking about, I realized that you are indeed correct that OSB makes no sense in terms of stopping the trusses from shifting. Plywood would have likely been used if that were the case.

For now I will lay R-30 over existing OSB where I am not using storage (that way I can roll it up for storage later). Where ever I need storage, I'll pull up the OSB, lay 2x10's perpendicular to my joists, and place R-30 and OSB back.

That way I get R-30 additional insulation everywhere, can create more storage later if I need it, and am adding very little to no additional weight.

drewmey
Re: What is the best way to add additional insulation to my attic in this scenario?
drewmey
keith3267 wrote:

You will need to take up the OSB. Then you can fill the cavities completely. If you have 2x8 joists, that should give you about an R 22. If you have 2x6 joists, then you wont get as much. You can lay (vertically) 2x6's in top of the existing joists or perpendicular to the joists on 16"centers. Then fill the now deeper cavities and replace the OSB. This should give you an R30 or more.

The cavity is already filled completely so I don't think I need to take up the OSB unless I specifically want to use it for more storage in that area. Can just lay R-30 directly over OSB in many places.

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