How to get R30 in a 6" space?

[Willy]

If you do not need a air flow space what size/type foam board would fill a 2X6 space and give you a R30 insulation. This would be for a space above a porch ceiling sofet that becomes a floor above and needs to be insulated for R30 to be to code. Thanks, Mark

[rakuz66]

Isn't a 2x6 only 5 1/2 inches deep???  If you have an actual 2x6 then I guess you could buy that 1.5 inch rigid insulation board , it's R-7, so that would give you R-28. 

[Willy]

Isn't a 2x6 only 5 1/2 inches deep???  If you have an actual 2x6 then I guess you could buy that 1.5 inch rigid insulation board , it's R-7, so that would give you R-28. 

I read some where about a  R Factor of 8-9 per inch for different types of foam? Just not sure what type it was. Yes it is a 5 1/2" space a standard 2x6 size. It is only a 6 ft wide by 10 ft long area to insulate and not have to deal with making the floor thicker in that spot. Mark

[tc-vt]

Professionally applied sprayed closed-cell polyurethane foams advertise r-values of around 7 per inch at installation.  As the foam ages (ie. the blowing gas is gradually replaced by air) in the first 5-10 years after installation, the r-value drops to around 6 per inch.  Blue board (Dow) and pink board (Owens Corning)  have r-values of 5 per inch.

If you need more than the r-value of 27.5 the board insulation gives you in the 2x6 space here are two ways to do it.  Either fur out the bottom of the rafters/joists to give you the needed additional 1/2 inch thickness or put 1/2 inch board insulation across the bottom of the rafters/joists.  The latter method gives the added benefit of breaking the thermal bridge across through the 2x6.

Tom

[Willy]

Professionally applied sprayed closed-cell polyurethane foams advertise r-values of around 7 per inch at installation.  As the foam ages (ie. the blowing gas is gradually replaced by air) in the first 5-10 years after installation, the r-value drops to around 6 per inch.  Blue board (Dow) and pink board (Owens Corning)  have r-values of 5 per inch.

If you need more than the r-value of 27.5 the board insulation gives you in the 2x6 space here are two ways to do it.  Either fur out the bottom of the rafters/joists to give you the needed additional 1/2 inch thickness or put 1/2 inch board insulation across the bottom of the rafters/joists.  The latter method gives the added benefit of breaking the thermal bridge across through the 2x6.

Tom

Thanks it is looking like I may end up just using 2X8s in this area and lose 2 inches of head room. I only have 6 ft at this point so 2 inches makes a difference. Either that or lower the porch head room 2 inches which has allmost 8 ft of room.  The porch roof is 24 ft long but only 10 ft of it will be a floor for the loft area. I might have to drop the whole porch ceiling down 2" to keep it flat but the other 14 ft is 2X6s off hangers so it would not be hard to just drop the hangers instead of going with a 2X8 on them. Mark

[firefox]

I would think that you would be much better off not dropping the 2x6s and putting the
sheet foam insulation over the ends of the 2x6s. The wood really conducts the heat away
so any thing you can put in its path is a plus.
Just my 2 cents.
Bruce

[Willy]

I would think that you would be much better off not dropping the 2x6s and putting the
sheet foam insulation over the ends of the 2x6s. The wood really conducts the heat away
so any thing you can put in its path is a plus.
Just my 2 cents.
Bruce

The 2X6s are in a non heated space above the porch. I am at the point of now putting 2X10s on the ceiling just to solve the insulation problem there. The inspector mention I would have a problem with the 2X8s insulation wise not strength but the added cost of the wood is not near the price of the insulation. Telling him I am changing them out will make it easier to get my loft approved to. Mark