Insulation calculations

I think the spray foam plan will make a robust assy.

Just tossing out another idea (which I hate to do because I irritate myself 2nd guessing plans )
This would be easy to install, would act as vapor retarder/barrier, allow plenty of roof ventilation, and address thermal bridging issues. The foam been the rafters could be omitted if a lesser R value was okay.
In general I'm a big fan of cellulose but not where water is more likely to enter (like roofs).


Edit: foam panels with OSB already attached can also be purchased.

Or the 2014 sprayfoam may have crumbled to dust in 30-40 years.

Missing info...do you intend to heat this structure 24/7, so it makes sense to give it house levels of insulation, or do you want a shop you can heat quickly from time to time, leading to a low thermal mass, lower R value design?

woodgeek said:

Sounds like a plan....for cheap foam board, you can buy 'used' from: http://www.insulationdepot.com/ and save money and be 'green'.

That outfit doesn't do small lots, usually trucks a large shipment, but it sounds like you want a lot of foam board, and don't care if its a little scuffed.

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Just a heads up I tried getting a quote from them for 2000 sq ft of board, the guy got my info and said they had a site in Western MD they were pulling board out of and could bring it to me, never heard back from him, he sounded like they only want HUGE industrial jobs. Just an FYI.

Yar. Your insulation cost goes like R-value, and your savings go like the reciprocal...1/R-value (which is U-value)

16 is a lot more than 9 that is more than 1 (R-vaules), but 1/16 is not very different than 1/9 when compared to 1 (U-value) .

Same as going to a 30 mpg car from a 20 mpg car saves a lot less gas than going from a 10 mpg car to a 20.

Ok, nothing wrong with fiberglass. The FG haters might talk about convection induced R-value drop at low temps. In our climate with FG batts (versus Wisconsin with loosefill FG) NOT a problem. Without a baffle there might be some wind-washing loss of R-value...don't know if its worth the effort/cost of doing prefab or DIY baffles.

I would insist, however, on a careful air-barrier. Stapled FG Kraft paper does NOT count. IF however, you taped all the Kraft paper seams (including the staples and any tears during install) and then OSB'ed under I suppose it would be ok.

Do they make tongue and groove edged OSB to keep the seams lined up?

How deep are your joists again?

One tip....sometimes different thickness batts are the same price, and compressing the 'thicker' batt (to a higher final FG density) does yield slightly higher R-value per inch, and is less likely to collapse (in my imagination) than a ultra-low density product. I've done that myself.

Forget the idea of a vapor barrier...you want an air barrier....little cracks and seams can move a lot of heated air, and bypass the insulation. In 70-80 years of stick building with batts, houses had 100% higher heat loss due to poor airsealing!

The seams in the OSB and the gaps under the stapled Kraft paper will (in typical construction) leak enough air to largely defeat the insulation.

You need to at least decide on what the air barrier will be, and then try to make it continuous. You are not going to tyvek the place, or put in a taped layer of poly, or tape or caulk all the OSB seams. Being careful with the Kraft paper...overlapping the paper edges and taping the seams, should provide a decent air barrier without too much added effort or cost.

Regular OSB is fine of course....I was just free-associating....

So, on the side walls, you can tape the tyvek seams with....tyvek tape, and that is your air barrier. Even taped tyvek needs to be lapped carefully for drainage, esp around windows, but I expect you are an expert on that. No problem with the air barrier being on the outside (in this case) because tyvek is vapor permeable.

Assuming you want to vent the steel roof, you will need a good airbarrier on the (sloped) ceiling plane because any leaks in that plane will have ready access to the vented cavity, wind driven pressure, etc.

I'm assuming you want to go as simple/fast as possible....sounds like thin polyiso stapled to the roof purlins. If all you want to do is define a venting space (by keeping the FG from pushing up to the roofing) I'd go with 1/2" xps or polyiso, which would also avoid windwashing the FG. I'd cut the foam all the same width, enough under the joist spacing so that each piece does not need to be custom fit (PITA). If you define your air barrier at the Kraft paper, then the (0.25"?) gaps on the sides of the polyiso don't matter. And allow the FG to dry to the vented space. But, to be a broken record, I would tape the Kraft seams if you are going to rely on it being your primary air barrier.

At 6" overall (old lumber dimensions) you'd prob get R-20 center of cavity, maybe R-15 with thermal bridging by the joists. Certainly not code, but workable if airsealed. If you wanted to go to higher R.....you could put some polyiso under the joists, and 2" would get you to R27 or so. You'ld need longer screws for the OSB, is all. Or you could spring for foam-OSB bonded panels and save a step (if you can source them). In the foam case you could not sweat the Kraft paper at all (or get unfaced) and tape the foam seams as an airbarrier.

IF R-15 is good enough for you....the FG seems like a simple and 'conventional' solution. If you want significantly more than that, you are back to where the thread started....building up sheet foam, the joists, or going spray foam.

You will have to detail the transition from vented roof assy to your wall assy carefully to connect the air barriers.

This is a pretty good page for calculate U-value.
http://www.energiberakning.se/
Go to U-Value. Press the English flag

I don't know what you mean by secondary barrier....if its sprayfoam, the assembly is airsealed and vapor impermeable. The joists can dry to both sides.

You can also go to 1/2" foam board, saves money and even easier to cut, tack, etc.

Joful said:

However, semipro's comment about roof replacement got to me, as I surely would replace with a different roof system, when the time comes

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That credit belongs to Bret Chase, post 7.
I appreciate the thought though.

Re the thermal bridging, when you replace the roof in 25 years, you can (1) see if the spray foam shrank (2) add another layer of sheet foam as underlayment to the new roof to bring it to R-50, and (3) get an energy rebate for the upinsulation from the Bieber administration (don't ask).

Joful said:

"Ronald Reagan? The actor? Then who's vice president? Jerry Lewis? I suppose Jane Wyman is the First Lady!" - Dr. Emmett Brown

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In an enlightened future, ScarJo would be veep. Might make the SOTU speech even harder to follow.

seal the cupola off from the attic with a trap door on bottom of cupola, so you still have access. then leave as part of attic space.

I like daylighting. Maintain the opening and cover it with a transparent sheetgood like plexi, suntuff or lexan twinwall. Or even coroplast from a sign store. Will diffuse the light, and provide enough insulation. Cover the inside walls of the cupola shaft with foil or glossy white paint beforehand for better throughput.