# Vapor barrier for metal roof?



## alittleoff (Nov 27, 2014)

I'm trying to figure out how or what to install for a vapor barrier on the underside of the roof in my shop. I want use 1/2" styrofoam like they wrap a house with, but someone told me that it wouldn't work. My idea was to strip the underside with 1×4" lumber and glue or staple the foam to it, them tape. It would give me a 4 in. Space between the foam and the roof. Using the bubble wrap or plastic will be pretty hard to do I would think because it's so flexible, for one man to hold it up and keep it striaght. I really need to get this done so I can go ahead with insulation and a ceiling. Anyone with an idea that will work?
Gerald


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## HorizontalMike (Jun 3, 2010)

At this point it looks like expandable spray foam the way to go. Expensive from what I hear, but will save much on labor costs. FWIW, I was unable to find anyone who would even consider installing anything other than foam. Not worth any contractor's time… they would touch it.


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## EugdOT (Nov 17, 2016)

They sell home kits for spray foam that cost drastically less than having someone come and do it


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## clin (Sep 3, 2015)

I also like the foam idea. But, if more conventional and DIY, whatever you do, keep a ventilated airspace under that roof. Note: VENTILATED.

No matter how good a vapor barrier, moisture will get in. Even some from the outside. So you need to allow it to get out quickly. This will reduce condensation and when there is condensation it will evaporate more quickly.

All of this also depends on where you live. There generally is no perfect solution, so you want the one that is best for your area most of the time. This is where contacting local experts is valuable. I would think the local metal building contractors would know what is best.

But, foam looks darn good for this application.


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## alittleoff (Nov 27, 2014)

What I'm trying to do right now is not insulate, but stop mosture from formING and rusting the metal roof after I insulate and possibly install a drop in ceiling later..
Gerald


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## JBrow (Nov 18, 2015)

alittleoff,

I am by no means an expert, but my understanding is that, at least where I live, the vapor barrier is installed on the warm side and after insulation is installed. In a recent remodel, 1" foam board was installed as exterior wall sheathing and fiberglass batt insulation within the stud bays. After being fully insulated, on the inside (warm side) 6 mil polyethylene sheet was installed as the vapor barrier, with seams taped with house wrap tape.

Done in this manner keeps the cold side mostly cold and close to ambient outdoor temperature. The inside where the vapor barrier is installed remains mostly warm and close to room temperature. Condensation, either inside or outside, is minimized since the temperature at the surface of the insulation is near the adjacent ambient air temperature. But since condensation is a greater problem where warm moist air contacts a cool surface, condensation on the 6 mil vapor barrier is inside the building.

In the end, to be sure, contacting your local building department for code requirements for insulating and vapor barrier installation could provide an authoritative and probably optimum answer to your question.


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## clin (Sep 3, 2015)

I think if your plan is to add an insulated ceiling later, all you really need to do is allow for ventilation of the space and probably ignore the vapor barrier. JBrow has it right. The vapor barrier goes on the warm side of the insulation. Usually right under drywall.

If the space between the insulation and the roof is ventilated you should not have condensation problems. But, again, different climates present different issues. I.E., what works best in Florida is probably not what you want to to in Minnesota.

I've read some reports that in many cases a vapor barrier makes things worse not better. The reason, often more moisture gets in from the outside than from the inside. Then the vapor barrier blocks the movement of the moisture from inside the walls to the interior. I.E., the walls don't dry out as easily from external moisture penetration.

Roofs are of course a different animal than walls.


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## rwe2156 (May 7, 2014)

I have a commercial metal building which is insulated. Typically metal building are insulated at installation with insulation specifically designed with plastic sheeting on both sides.

I would not insulate directly against the metal. If you ever get a leak, it will be very difficult to figure out where it is. I'm dealing this issue right now and it will be quite costly to seal the entire roof (4000SF).

You could use the correct insulation, but I wouldn't go through the trouble or expense of a retrofit. I definitely would not use foam, which is quite expensive.

As long as the space under the metal is not too warm it will not form condensation.

I would consider putting bats on top of the drop ceiling.

Really the best advice I can give is have an insulation company come out the look at it. My experience has been insulation can be installed for just a little more than it costs to buy.


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## builtinbkyn (Oct 29, 2015)

You need to install battens across the rafters. 1×2 screwed to the rafters would be fine. Then I would use reflective insulation. Very easy to install with staples and easy to cut and fit around structural components , followed up by sealing the seams with foil tape. This is the product I've used before. Insulation4less Installing some roof vents is a good idea, to allow airflow between the underside of the roof and the insulation.


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## HorizontalMike (Jun 3, 2010)

> I have a commercial metal building which is insulated. Typically metal building are insulated at installation with insulation specifically designed with plastic sheeting on both sides.
> ...[snip]...
> I would consider putting bats on top of the drop ceiling.
> ...[snip]...
> ...


That is what I did, put 1/4in. SolarGuard reflective insulation on when I built the garage/shop. R-10 ceiling and ~R-7.5 walls. Not much, but in South Texas this works out well.

Calling an "insulation company" sounds great, but the reality for me is that they won't touch it. NO-ONE wants to do retro-fit insulation in my area of the country. Sounds too much like "work!?", just as Maynard G. Krebs used to belt out in surprise on the Dobie Gillis Show.

Seems like everyone is now out for the hit-n-run quick money jobs…

I think Bill has the right idea in building a drop ceiling with battens that you can attach a SolarGuard equivalent to. Probably your best bet…


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