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Community and Q&A

Roof and ceiling design advice wanted for steel building

Farmin | Posted in Energy Efficiency and Durability on

Hello, I have been reading a lot of posts here on roofing and insulation but could still use some advice on my project, any advice or suggestions would be greatly appreciated. I am building a steel building in Hawaii climate zone 1, 3500Sq’. Building shape either 60×60 or 30×125 with a 12 to 14′ eave height. This is a live work / garage building for a small farm. The building will have a lot of awning windows up high on the walls. I would like some insulation to block the suns heat and to reduce the sound of rainfall.

I would prefer some type of roof deck that could be painted in the interior along with the exposed purlins that would be the visual ceiling or some type of liner panel attached below the purlins.

So my thoughts on doing this are add some more steel purlins (maybe 32″ OC. ?) so that I could use a plywood deck then add 2X4’s lying flat anchored to the purlins above the deck, infiled with 1-1/2″ ridged foam and then screw the metal to the 2×4’s. Not to crazy about the wood and all the extra purlins but I do like the the fact that if the roof leaks a little the insulation wont be damaged. I am wondering if this is how you would do the ridged foam under the steel roof in this type of building?

Or the other possibility seems to be a metal liner panel or sheet rock attached under the purlins, and if I went this route what would be the easiest/ best way to insulate, the traditional steel building insulation draped over the purlins or loose batts installed from below before the liner goes up. I probaly would go with the metal liner panel which wold allow it to be installed on fewer supports than sheetrock and be less labor. But I am wondering about putting up wide loose batts of insulation from underneath seem like that would not be fun.

In general I am wondering what is the best approach, how much do I need to worry about condensation, and what to do about it. Should I consider a ridge vent (may be problematic as the building will not be one big open space and am wondering how they would hold up in a hurricane)
Do the approaches I described make sense, what are other alternatives you might suggestc


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  1. GBA Editor
    Martin Holladay | | #1

    Most steel-framed farm buildings are poorly insulated. Considering how these buildings are usually used, that may be fine -- especially in a mild climate like Hawaii.

    Any of your suggested methods will work.

    From a building science perspective, the best approach to insulating a steel building is to put all of the insulation on the exterior side of the steel framing. In your case, you could use structural insulated panels (SIPs) or an approach that some people call "site-built SIPs" -- plywood or OSB roof sheathing, followed by rigid foam, followed by a second layer of roof sheathing or purlins, followed by roofing.

  2. rocket190 | | #2

    Pretty common construction in my cold climate is to install metal decking over your steel bar joists. Then install as much foam as you want, followed by a rubber or edpm roof covering. This way you have a fully waterproof roof that is easy to repair or modify, you can keep standard joist spacing, and you have a steel deck that can be painted on the underside. The edpm roof can be the newer white variety that should stay nice and cool in the Hawaii sun.

  3. Farmin | | #3

    Thanks for those replies, I was just looking into steel decking. If I wanted to go with a layer of foam over it and then a metal roof do you know how the metal panels ( either standing seam or exposed fastener) would be secured over the foam. I think the decking painted white would be a nice ceiling but not if it has screws poking through it all over.

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