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Adding Insulation to floored attic

While updating some electrical, I recently removed the flooring in my 1936 2-story attic to find 2x6 joist and roughly 4" of loose wool insulation. I need to add more insulation, but still be able to put some plywood down and use the attic for storage. I live in the Midwest and the recommended R value for attics in my area is R-38. With only 5.5" of space to work with, I don't know if I can get over an R-19 (maybe 21). I have considered a couple options and I would appreciate any feedback or suggestions that would be the best bang for my buck.

1. Add more blown-in insulation to the floor joist under the plywood flooring. Also add batt insulation to the rafters of the roof for added insulation. Probably the cheapest option, but don't know if it would do any good.

2. Raise the attic floor to accommodate more insulation. I don't really want to do this being it will be difficult and expensive.

3. Remove existing insulation and have professionals spray in expanding foam into the joist. I was told by a pro that I would get around a R-35 with only 5.5" of space. I'm not leaning this way because of the huge price tag that goes w/ expanding spray.

Any suggestions or other options would be very helpful.


Re: Adding Insulation to floored attic

Isulating the roof rafters will work but you realize that you'll also be heating the attic which isn't so horrible unless you have a gigantic attic. How big are the roof rafters? If you insulate the roof rafters you will need to add styrofoam air baffles 'tween each joist first to allow the roof to stay vented and that will take up approx an inch or more of space. You'll also need to cut in a ridge and soffet vent where the styrofoam baffles will lead the air to. Its no that big of a deal if you have the space.

Another option is to add more blow in on the floor of the attic but you won't be able to store much up there without building the entire floor up. You could however simply build up the floor with 2x material 4' wide and drop
some 3/4" 4x8 ply down to walk over the length of the attic.
That wouldn't be all that involved and you could store a little bit of stuff up there.

Re: Adding Insulation to floored attic


I'm in the same situation as you are. I've just added attic stairs and plan to use the unfinished space for storage.

Before I can lay flooring down, I'll need to do something. Our options include:

1) Add insulation to the underside of the roof. This may reduce airflow or cause damming. It also leaves a semi-heated space in the attic because both the floor and ceiling of the attic have r-values of approx. R-19. Does a floor insulated to R-19 and a ceiling insulated to R-19 equal R-38? I doubt it . . .

2) Raise the floor by adding 2x4s or 2x6s. I'm leaning this way . . .2x6s would raise the floor to an 11" height, which would allow for an r-value close to R-38 (assuming blown cellulose).

3) Push away any insulation taller than the 2x6 and plywood over the floor. Does 1/2" plywood offer any r-value? I'm guessing maybe it adds R-1 to the R-19. Btw, 1/2" is a bit on the thin side, but I figure that it's an attic storage area, not a play room.

I think the best option is #2. It'll be a pain, but I think it's the best route. Does anyone out there have a better suggestion???

Re: Adding Insulation to floored attic

I'd love to hear more ideas from experienced people out there . . .

Re: Adding Insulation to floored attic

Considering the work and cost, put the insulation you need in the attic, it will pay for it's self, and get yourself a yard barn for storage.

Re: Adding Insulation to floored attic

2) Raise the floor by adding 2x4s or 2x6s. I'm leaning this way . . .2x6s would raise the floor to an 11" height, which would allow for an r-value close to R-38 (assuming blown cellulose).

Run your 2bys perpendicular to the existing rafters and if you decide to use fiberglass bats this will work out nicely laying them perpendicular to the rafters as well.

Just don't overdo it with storing too much stuff up there.

Re: Adding Insulation to floored attic

On the advice of an architect friend, I built a 12x12 space out of 2 inch thick styrofoam sheets (R-10 each) stacked crosswise on top of each other, then topped it with 1/4 inch plywood for a storage area. It's not a high-traffic area and I'm not lugging the weight set up to the attic for storage anyway, so he tells me the styrofoam should hold up.
I don't have a lot of headroom in the attic and at R-30 this saved about 4 inches compared with fiberglass batts, which (they're cheaper and easier) are going under the eaves where there's no room for storage anyway. The four inches makes the difference between usable storage I can walk into upright and a noggin-knocker, so it seemed worth a try.
The whole lot is on top of pine planks over four inches of a mixture of old, settled cellulose and fiberglass. I'm hoping the 1/2 inch to an inch of trapped air between the old insulation and the floorboards topped with new fiberglass will add to the R value as well.

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