Cross post ... Ceiling Insulation Thoughts

[ducatithunder]

I posted this over on in the General Topics and it didnt get much traction and got buried pretty quick.

I copied the text from my original post below.

So I am currently plugging away at the mind numbing and thankless task of insulating the ceiling in the shop. The joist are 12" OC. There are 43 total joist bays. No one makes 12" batts so I am cutting 24" R30 batts in half. Its actually working well. Perfect fit. It leaves me with about 3-4 inches of air space under the roof for air flow to the soffit. The roof is vented on the front and rear. The blocking between the joists has holes to allow for adequate air flow. The insulation sits on top of the lower flange of the joist. The kraft face is peeled down and stapled to the inside face of the flange. This leaves a about 1.5x10" area below the insulated area and the 1/4" birch plywood ceiling.

So Im spit balling here ... I have 2 options.

I can leave this empty airspace or I can rip down 1.5" XPS foam and fit it inside the joist flanges flush with the ceiling. What downside other then less insulation would there be to leaving it empty? I feel like the space would allow any moisture that could get into that space to condense on the backside of the plywood ceiling.

On the second option, I made a quick drawing in paint for reference.

It would add ~7.5R to the insulation bringing the total to 37.5R.

The XPS should act as another vapor barrier/retarder on top of the kraft paper. These would both be on the inside of the insulation facing the conditioned space. Im not trapping the fiberglass and its free to breath.

Does anyone see anything wrong with this approach? Thanks.

 

[Firebrick43]

It would be ideal not to have the kraft paper but the xps is installed on the right side of the envelope and will bring you at least up the the recommended r38.

 

[ducatithunder]

It would be ideal not to have the kraft paper but the xps is installed on the right side of the envelope and will bring you at least up the the recommended r38.

Thanks. Yeah, if I could get 24inch batts unfaced I most likely would have used that instead. I could only get unfaced rolls of 16". To much waste and cutting to make it work. The 24 faced worked well.

Ill have some time to think this over. I dont really see any downside for another $400-500 worth of insulation. There is not going back once the plywood is installed. That wont happen until the fall.

 

[dmcintosh]

I would think the added insulation can only help - but I am not an expert.

 

[ducatithunder]

Thanks guys. No one came out of the wood work with a WTF are you thinking, so I think Im good with it.

 

[ItBurnsWhenIPee]

Am I reading that right that you'll have two vapor barriers?

 

[ducatithunder]

Am I reading that right that you'll have two vapor barriers?

Typically yes. The Kraft face would be one and the foam would be another. The foam will be installed to fill the space below/between the joist flange.


Sent from my iPhone using Garage Journal

 

[Firebrick43]

Kraft paper is a retarder not a barrier. But they tell you not to use two vapor barriers because a large airspace between two, even with insulation installed isn’t good. Back to back without large gaps is ok.

 

[ducatithunder]

Kraft paper is a retarder not a barrier. But they tell you not to use two vapor barriers because a large airspace between two, even with insulation installed isn’t good. Back to back without large gaps is ok.

The XPS will be pressed flush with the kraft facing, so there should be no gap. Thanks!