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Pole barn ceiling

Climate Zone 6B

I just bought a property with a large (60'x80') old wood pole barn and am going to be working on closing it in for an office and storage. I'm closing in 16'x80' right now. The only portion that I'll be heating (for now) is a 16' square at one corner of the barn. The poles for the barn are spaced every 16' and I've drawn up a "building inside a building" for this portion since I can't put any additional load on the existing structure and want to get a highly insulated office. I'm planning on a double stud wall for the office with air tight drywall details. The ceiling is where I'm having some trouble figuring out the best thing to do. I've attached a drawing showing a section cut through the space.

I'm planning on using 11-7/8" TJIs to frame the ceiling supported by new stud walls. I'd like to get some more insulation for the ceiling than 11-7/8" can provide though. Anyone have an idea or have done something similar in the past?

It seems to me I have 2 options:
-Furr down the TJI ceiling to get more cavity to fill with insulation
or
-Add rigid on top of the plywood (Wouldn't I need 5" though? - if that's the case that makes rigid unfeasible to me)

If I furr down the ceiling is it reasonable to assume I could get rid of the plywood layer on top or could condensation on the metal be a future problem that would just drip down into the insulation/ceiling? If so, I assume I need to ventilate that cavity or since I'm "a building inside a building" would that not really be necessary since the plywood could just dry to the open space above?

Thanks in advance for any advice/replies!

barn section.pdf53.72 KB
Proposed ceiling assembly.jpg15.28 KB
Asked by JMS Build
Posted Wed, 07/30/2014 - 17:55
Edited Thu, 07/31/2014 - 05:20

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2 Answers

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1.
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JMS,
You are in Climate Zone 6. According to the 2009 IRC, the minimum R-value for ceiling insulation in Zone 6 is R-49. If you are installing cellulose or blown-in fiberglass, you'll need about 14 inches of insulation above your ceiling.

Your drawing shows new ceiling joists. The usual approach to installing ceiling insulation is to blow the 14 inches of insulation above the ceiling. You don't need to install any plywood above the top of the insulation.

If you are worried about wind-washing, or any small losses in thermal performance due to convection currents, the cheapest way to address those concerns is simply to install a thicker layer of insulation. That will be a lot cheaper than any attempt to cap the insulation with plywood.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/31/2014 - 05:25

2.
Helpful? 0

Thanks Martin. There was just something about having the underside of the metal roof above exposed that made me think that there will be condensation there dripping down occasionally. If I have the heated portion well air sealed that shouldn't be a problem. I'll just pile up more blow-in on top of those joists and keep an eye on that metal surface occasionally.

Answered by JMS Build
Posted Thu, 07/31/2014 - 09:42

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