Condensation barrier for metal roof

Cheap5.0 · Nov 1, 2011

I have two options, i can purchase (from the post frame kit supplier) 1/8" single bubble R-3 condensation barrier for $260 and it will be delivered with the kit.

Or i can go to Mendards/Home Depot and have a bunk of 7/16" OSB delivered for around $400 and use that as the condensation barrier.

Whats the best route here?

kbs2244 · Nov 1, 2011

I wouldn’t consider OSB a condensation barrier.

Cheap5.0 · Nov 1, 2011

Thanks, that would have been an expensive mis-step.

I spoke with Mike @ DIYPoleBarns and he said the 1/8" foil backed insulation is the most popular condensation barrier (@ $400 for my 30x40 4/12 roof), but he also said tar paper or house wrap would work just as well and be cheaper.

richtersrodz · Nov 1, 2011

Is your roof already on? And now that you say that, can you use house wrap on a roof?
I've seen it on the walls, but never a roof.. I'm just curious.. and looking to learn any new
info.. I have a pole barn, built from years previous to my ownership, with no insulation..

John in OH · Nov 1, 2011

I had a similar discussion with my builder prior to installing the steel roof on my new garage/workshop ( see build thread: http://www.garagejournal.com/forum/showthread.php?t=98038&highlight=retirement , posts #51 & 58)

I had originally spec'd out the bubble-type condensation barrier over the trusses, but the builder wasn't comfortable with doing that installation so he proposed ... for the same price ... to install OSB and felt paper. I couldn't really see any down side to his proposal so that's how we went forward. I guess time will tell if it was correct or not. Sure made for a pretty installation job!

Cheap5.0 · Nov 1, 2011

richtersrodz said:
Is your roof already on? And now that you say that, can you use house wrap on a roof?
I've seen it on the walls, but never a roof.. I'm just curious.. and looking to learn any new
info.. I have a pole barn, built from years previous to my ownership, with no insulation..

The structure has not been erected yet, im still working out the details but it is ordered already.

The Mike i mentioned above, owns/runs DIYPoleBarns AFAIK. He made that house wrap suggestion to me, so i would say its a valid option. He said its basically anything that will create a small air barrier between the metal roof and the inside air of the building. Thats all it takes to keep condensation out.

Thats when he also suggested that I could use felt paper as the cheapest option.

John in OH said:
I had a similar discussion with my builder prior to installing the steel roof on my new garage/workshop ( see build thread: http://www.garagejournal.com/forum/showthread.php?t=98038&highlight=retirement , posts #51 & 58)

I had originally spec'd out the bubble-type condensation barrier over the trusses, but the builder wasn't comfortable with doing that installation so he proposed ... for the same price ... to install OSB and felt paper. I couldn't really see any down side to his proposal so that's how we went forward. I guess time will tell if it was correct or not. Sure made for a pretty installation job!

How long have you had the OSB/felt combo up there? It is about the same price as the 1/8" single bubble foam, but would take a lot more work to put it.

As i was told, the 1/8" foam comes in a 120'x6' roll, that is simply run across the purlins end to end. Then you cut it, and do the next section. There is no need to secure it (unless it is windy on that day you install it), as the metal roofing will be screwed through it into the purlins.

richtersrodz · Nov 1, 2011

Cool... I know if I had my choice, I'd probably rather work with the house wrap.. more
coverage, and no tar residue.. Only down fall, is carrying that big roll up on he roof.
I didn't know if the tar paper would help seal nail holes or something.. (Not questioning
your source..

)

Cool looking build thread John... Now I have another to read through and slobber over.

larry_g · Nov 1, 2011

In my build I used what is here called an 'insulated vapor barrier'. It is 2" of fiberglass on a plastic barrier. My building came with precut rolls that were just rolled out on either the roof or walls and sandwiched between the purlins/girts. I had it in my old shop built in the '90s and never had a moisture problem here in wet Oregon. I also have it in the new shop seen in the build below. I'm happy with it.

On edit here is a thread about it http://www.garagejournal.com/forum/showthread.php?t=119142&highlight=insulation

lg
no neat sig line

John in OH · Nov 1, 2011

Cheap5.0 said:
How long have you had the OSB/felt combo up there? It is about the same price as the 1/8" single bubble foam, but would take a lot more work to put it.

As i was told, the 1/8" foam comes in a 120'x6' roll, that is simply run across the purlins end to end. Then you cut it, and do the next section. There is no need to secure it (unless it is windy on that day you install it), as the metal roofing will be screwed through it into the purlins.

Not long at all ... garage was built this past summer. The builder was uncomfortable trying to install the bubble barrier over the purlins on a 7/12 pitch roof so he campaigned for the OSB/felt paper. Note that part of the cost of adding the OSB was offset by the elimination of the purlins.

I did have the bubble-type barrier installed under the new steel replacement roofing on an existing building in Ohio. It was about a 4/12 pitch and the existing purlins were only about 12" apart so it was an easy install. The bubble-type barrier on that install had a white vinyl underside and really lightened up the interior of this old building. I was quite happy with that install.

dirttracker18 · Nov 1, 2011

House wrap is not a condensation barrier. That is a very bad idea from someone that should know better. It is not meant to shed water in a horizontal position.

Cheap5.0 · Nov 1, 2011

dirttracker18 said:
House wrap is not a condensation barrier. That is a very bad idea from someone that should know better. It is not meant to shed water in a horizontal position.

How do you think felt paper would do in this situation?

Racecarl · Nov 1, 2011

I used synthetic roofing felt on my building. It is placed over 1/2" OSB and under Metal Sales standing seam concealed fastener tin. There are 10 squares on a roll and I needed 11 so I used some 30# felt to finish up, so in the long run I should be able to see which one works better. So far I have noticed NO moisure accumulation/condensation of ANY kind.

jkeyser14 · Nov 1, 2011

Synthetic roofing underlayment is the preferred method. Felt paper tends to stick to the metal roof, and then over time when the paper gets brittle and the metal panels expand and contract it rips the paper and you lose your barrier.

Cheap5.0 · Nov 2, 2011

jkeyser14 said:
Synthetic roofing underlayment is the preferred method. Felt paper tends to stick to the metal roof, and then over time when the paper gets brittle and the metal panels expand and contract it rips the paper and you lose your barrier.

Thanks, again...saved me big time!

I see this seems to be the best inexpensive synthetic material:

http://www.amazon.com/dp/B000LF5UG8/?tag=atomicindus08-20

One roll = 4'x250'. I would need two rolls to complete my 30x40 4/12 roof correct?

Cheap5.0 · Nov 2, 2011

My other option is 7/16" OSB with std felt paper then metal roofing. I am not certain if i have calculated the materials correctly though.

30x40 4/12 pitch roof = 1475 sq ft?

Lowes has 7/16 OSB for $6, and felt paper is $19 for 4 square. Adding them up, i get $352 for a OSB/felt deck. Does that sound right?

dirttracker18 · Nov 2, 2011

Cheap5.0 said:
How do you think felt paper would do in this situation?

A better choice but not optimal IMHO

kbs2244 · Nov 2, 2011

Is he talking about the top or underside of the plywood?

Neither tar paper or Tyvek is moisture proof.
In fact it is designed not to be.
I wouldn’t use it on either side.

It sounds like what your guy is trying to do is cheaply create a small air gap on the underside to act as an insulating space.
I have never heard of it being done that way.
But construction is very local conditions dependent.

I would ask to see some previous installs and a chance to talk to the owners.

Cheap5.0 · Nov 2, 2011

kbs2244 said:
Is he talking about the top or underside of the plywood?

Neither tar paper or Tyvek is moisture proof.
In fact it is designed not to be.
I wouldn’t use it on either side.

It sounds like what your guy is trying to do is cheaply create a small air gap on the underside to act as an insulating space.
I have never heard of it being done that way.
But construction is very local conditions dependent.

I would ask to see some previous installs and a chance to talk to the owners.

I beleive Mike @ DIYPB was suggesting to use the tyvek over the purlins, below the metal roof.

They do sell a 1/8" bubble foam insulation that comes in 4'x150' rolls that is used specificly for this purpose. It costs about $400 for my setup, i was hoping to find something that works as well but is cheaper.

krapie · Nov 2, 2011

The bubble stuff is a waste. I have that put down 10 years ago, about year 2 or 3 it started to get brittle and tear as the purlins and metal shifted with wind and temperature changes. I have several drips from condensation which I can tolerte in the barn, but 1/3 of my building is living space and the drywall ceiling is now stained in places where the drips continue.

Several attempts to correct the tears with various materials and none of them lasting long, I bit the bullet and took the metal roof off the living space area. Installed OSB over the purlins, paper then metal.

If I was building a new barn today, I would install purlins, OSB, paper, and finally metal.

dirttracker18 · Nov 2, 2011

The only true way to stop the condensation is to insulate and stop the cold meeting warm, which causes condensation.

About the only good way to do that is clsoed cell spray foam. Effective but costly.

My cousin used the bubble wrap on his but it only last a few years before it started to drip. It was condensating between the bubble wrap and the metal. It dripped through the holes that hold the wrap up

Cheap5.0 · Nov 2, 2011

You guys need a pat on the back. Not kidding

I will be doing OSB/felt/Metal.

Cheap5.0 · Nov 11, 2011

One last question:

If i was to do an OSB ceiling inside, and put insulation on top of that...would that suffice as a barrier?

Justanoldguy · Nov 11, 2011

Cheap5.0 said:
How do you think felt paper would do in this situation?

In Nz 90% of all sheds use felt paper and it works great.
Used on many residential buildings as well.

dirttracker18 · Nov 11, 2011

Cheap5.0 said:
One last question:

If i was to do an OSB ceiling inside, and put insulation on top of that...would that suffice as a barrier?

Once again, felt is not a vapour barrier and should not be used as such. Insualtion can be a VB but not fiberglass. Foam panels or closed cell spray foam is.

Please do not put all that money into a shop and not use a proper vapour barrier. 6 mil poly is very cheap and IS a vapour barrier.

Cheap5.0 · Nov 11, 2011

dirttracker18 said:
Once again, felt is not a vapour barrier and should not be used as such. Insualtion can be a VB but not fiberglass. Foam panels or closed cell spray foam is.

Please do not put all that money into a shop and not use a proper vapour barrier. 6 mil poly is very cheap and IS a vapour barrier.

Thats a good idea, and i work for a company that sells pond liners. Most of those are 6-8 mill, but they are black...that wont be a problem right?

FITO · Nov 11, 2011

Are your trusses rated for the extra weight of the OSB?

Big Bad Dad · Nov 11, 2011

If it's vented properly, you should not need a condensation barrier. My 24x32 "pole barn" type garage has never had a problem. I installed a ridge vent all the way along the peak, and installed perforated vinyl soffit panels instead of the solid metal soffits that are commonly used on pole barns. Not a drop of condensation in 10+ years!

Gary S · Nov 11, 2011

FITO said:
Are your trusses rated for the extra weight of the OSB?

It would be a very worthless roof if it wasn't able to handle a lot of added weight. My stick built garage can handle 1.5 tons of shingles, and lots more tons of snow in the winter without failure. If you have a building that can't handle added weight, something was done wrong in the design and construction.

timewarp · Nov 11, 2011

Gary S said:
It would be a very worthless roof if it wasn't able to handle a lot of added weight. My stick built garage can handle 1.5 tons of shingles, and lots more tons of snow in the winter without failure. If you have a building that can't handle added weight, something was done wrong in the design and construction.

Not true, a lot of trusses are not engineered to carry weight on the bottom span, not anything done wrong in design or construction. This is to save money for buildings that will not need a ceiling. The trusses can handle the engineered load on the top but not hanging on the bottom span. The problem occurs when someone specs a building and they shop for the lowest price but don't spec to the manufacturer that they are going to want to hang a ceiling from the bottom of the trusses.

Cheap5.0 · Nov 11, 2011

timewarp said:
Not true, a lot of trusses are not engineered to carry weight on the bottom span, not anything done wrong in design or construction. This is to save money for buildings that will not need a ceiling. The trusses can handle the engineered load on the top but not hanging on the bottom span. The problem occurs when someone specs a building and they shop for the lowest price but don't spec to the manufacturer that they are going to want to hang a ceiling from the bottom of the trusses.

I will double check for my sake, but i believe the answer is yes.

The kit can come with an optional supply of osb for walls/ceilings/roof decking. But i found that by using the lowes 10% discount and picking it up my self instead of having it delivered i can cut the "optional" package costs in half.

I think they wanted about $560 if they supplied just osb and felt paper for roof decking, and if i get it from lowes im looking at $370 for the same stuff.

My point here, is yes i think the trusses are built for the extra load.

Condensation barrier for metal roof

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