Insulating Garage Ceiling

[ducatithunder]

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.

 

[ducatithunder]

Screwed the pooch on this post. Can the mods move this to the General Forum. Thanks!

 

[The Cobbler]

done!

 

[ducatithunder]

done!

Thank you!

 

[Leaflessshadetree]

IMO insulation is the opposite of a thankless task. The thankyou comes with every hour spent in a heated area on a cold day and with every utility bill.
I do questing using XPS vs thicker batts.
I haven't insulated that type of joist but typically I've stapled at the bottom of the joists so there was no air space between the insulation and the ceiling. Will that "sag" over time reducing the air space?

 

[ducatithunder]

IMO insulation is the opposite of a thankless task. The thankyou comes with every hour spent in a heated area on a cold day and with every utility bill.
I do questing using XPS vs thicker batts.
I haven't insulated that type of joist but typically I've stapled at the bottom of the joists so there was no air space between the insulation and the ceiling. Will that "sag" over time reducing the air space?

Lol, it’s not thankless, I was being melodramatic from the groundhogs day ritual I’ve been doing for days.

It’s tough to get it to conform to the ijoist unless you cut out the bottom to key in. That is not worth the time and energy to do to every piece. If it was a standard joist then it would have been face stapled.

If the XPS is installed then the batt above can’t sag over time. To me it seems to be added value with the given challenge and joist. The paper is tight and supports the batt. There is a slight sag but I don’t see it sagging and filling into the gap. I fluffed the batts after stapling to allow any compression to release during install.


Sent from my iPhone using Garage Journal

 

[ducatithunder]

slow day on the journal ...

I have read that XPS is more stable for moisture then Polyiso panels. That is the XPS is less prone to absorption of moisture. I could get my hands on foil faced iso panels cheaper then the XPS panels.

 

[Jeepster04]

I do not believe you would have moisture forming between the plywood and insulation kraft paper. Moisture condenses when warm air hits cold surfaces. Since everything will be on the warm side of the insulation it should, in theory, be fine.

 

[Tim Kennedy]

ducatithunder:
How big is this building? Just curious as to why 12" oc was used instead of 16" or 24" oc.

 

[ducatithunder]

ducatithunder:
How big is this building? Just curious as to why 12" oc was used instead of 16" or 24" oc.

The garage is 28x46 with 12ft rear and 14ft front walls and a monopitch roof. Just under 1300sqft. Its a 1/12 pitch that is roughly 50x36 to the edges of the facsia. Its how the roof was engineered. The added cost to 12"oc vs 16" and the deflections for load made the case for 12"OC.

The walls are R23 Rockwool.