Insulation calculations

[Ashful]

I was going to do as ironpony suggests, but that's an interesting idea, woodgeek. There will be large double-hung windows at each end, and it's primarily storage space, so daylight is not a huge issue. However, definitely something worth consideration. I was going to have a trap door for access anyway, so swapping from an insulated panel to something transluscent should not be a problem.

 

[semipro]

I've been thinking of stuffing a translucent bubble wrap "lasagna" into some of our skylights with a transparent separator below to support it, a mod of Woodgeek's suggestion. .
The bubble wrap should insulate well but I suspect its not designed to hold up to light long term and light transmission would suffer as more layers of bubble wrap were added.

 

[Oldman47]

I see you got advice from friends to use glass batts. You already know about Roxul batts for use in block off plates but those things insulate better than glass batts and are made for a nice snug fit in a standard wall. My new home has R-23 batts in the 2x6 wall sections and R15 batts in the 2x4 sections. (I used a double wall with 2x6 outer wall, then a vapor barrier and 2x4 inner wall.) Compare that to R-19 in glass in a 2x6 wall and R13 glass in a 2x4 wall. In addition to better insulation the stuff also sheds water fairly well, not perfectly, and does not degrade like glass does. To use it you would need to install a separate moisture barrier like 4 mil plastic but unless you consider kraft faced batts as an adequate barrier you will do that with glass too. Just some things to think about.

 

[Ashful]

Interesting question for the foam board in the rafter bays. If you recall, the plan is to fasten foam board to underside of purlins, and then spray foam to the foam board, 3.5" thick in each rafter bay. The roof deck (purlin space) will be vented at soffit and ridge.

Do I want to go with foil face on this foam board? My thinking is that, given solar loading on a dark'ish metal roof, this barn has always run hot. I'm in a mid-Atlantic clime, and so we normally go with heating theory of placing vapor barrier toward the inside. Here, I'm spray foaming with closed cell, which is a vapor barrier in itself, but fact remains it will be warmer under that roof deck (outside of insulation) than inside the building, most of the year. Also, people who have insulated under metal roofs, as I'm doing, have often reported condensation problems under roof deck. So, foil faced XPS (foil to outside), might be a good idea, in my mind. If nothing else, it's a drip pan (albeit, not a very good one), taking any condensation back down to soffit.

Thoughts?

On windows, already got 6 Andersen 400-series double-hungs, a lighted entry door, and garage doors that are 1/3 glass. There will be lots of natural lighting!

 

[Ashful]

Figured it's time to post some follow-up, here. Preparing for the 3" closed-cell foam to be sprayed on Tuesday, which means I'm making all final adjustments to wiring, cribbing to which wallboard will attach, etc. Will post more photos over the next few days... in the mean time, here's a lot of wiring! This will be a workshop space, hence the mils of AWG-10/2 and AWG-12/3.





Later today, I'll get some photos of the foam board I installed in each rafter bay, and between collar ties (still finishing it, actually), as a backer for spraying the foam. That will add another 1" of insulation, so this will be one well-insulated barn... surely overkill for the 55F heat / 85F cool thermostat settings I'll have out there.

Note end wall cavities are 7" - 12" deep, completely inside original timber frame. So, there's no thermal bridging due to studs... foam will fill in completely behind the 2x4's, which are spaced off sheathing.

 

[Ashful]

It has begun:

 

[woodgeek]

Did you decide to the keep the cupola open for daylighting, or seal it off?

Or a translucent 'trapdoor'?

 

[Ashful]

There will be a trapdoor for access, and I haven't decided on foamed OSB solid panel vs. translucent, just yet. My thinking is that this area will be undoubtedly an insect graveyard, and so if I go translucent glass, I may get some light but will be looking at dead bug silhouettes. Insulative value of foamed OSB is also better, so that's the direction I was leaning.