Tin Roof Sweating

I Recently built a sun room with a tin roof over 2x6 rafters on 2' centers with 2x4 purlins on 2' centers,insulated with r-19 fiberglass in between the rafters(no attic space) was going to rock the bottom of the rafters because of limited head room.Noticed today the tin was sweating badly above the insulation.Decided I better come up with a cure before I rock it.Does any one have a sure fire cure? Do you think dropping the ceiling just a couple inches to create a very small attic space would cure it?I've never really built with tin like this before and hate to just try something without really knowing if its gonna work.Any ideas?

Sounds like warm moist air condensing on a the cold tin.

You need air circulation between the tin and a vapor barrier on the warm side of the insulation.

The same thing would happen on a roof with wood sheathing, but probably not as dramatic. The same soggly insulation, damp gypsum and mold down the road.

I hope I understand your construction correctly.

We install steel roofs on new homes and existing homes, and there are a few precautions. The "trapped air" in the R19 is warmer, and will hold moisture....the metal gets cold, and condensation occurs. The fix is to remove the panels and purlin the rafters, making an air gap under the steel. In new homes, we do not deck the roof, but rather run a foil back insulation under the steel, to break heating, and ventilate the attic as in normal construction. Works great....of course, we do a minimum of 12 inches of blown insulation, so we are not "warming" the attic air.

It will take a lot of positive ventilation to prevent warm moist air from condensing on cold steel, more than is usually required by code. It is less of a problem if the metal roofing is on sheathing board. You will probably have to install a vapor barrier on the warm side of the insulation.

Thats what happens when pole barn mentality get involved with home construction.
As mentioned,
Solid seathing with an underlayment will help as well as creating a venting cavity not just an air space but one that allows outside air in and out.

Trapped humid air condenses due to the different exterior and interior temperatures. Ensure that you have a Minimum of a 1 1/2 inch air space above the insulation. You may need to install proper vents also known as insulation baffle vents on the top of the insulation. You MUST have 100 % continuous intake ventilation along the eave or soffits and also 100 % exhaust ventilation along the ridge, (If a gable style roof). If it is a lean to or shed style which ties into an existing wall, then you would need 100 % of that tie in to be vented.

Intake vent options:
1) Continuous length soffit vents, multiple manufacturers
2) Smart Vent by DCI Products
3) Vented Fascia board by Crane building products
4) 100 % perforated vented soffit panels

Exhaust Vent Options:
1) Ridge vents for metal roofs, MBCI, Metal Building Components Incorporated
2) Metal ridge vent, by Air Vent Inc.
3) Contact metal roof panel manufacturer for factory made or alternative approved products.

Definitely, use an underlayment under the panels. Upgraded option would be Grace Ice and Water shield in lieu of standard 15# or 30 # felt papers to moisture permeance.

Does anybody have any thoughts on the reflective faced deck sheathing for heat reflectivity???

It sounds as if the panels are already in place, so you have to decide if you want to deal with the inconvenience of re-doing all of the work now to correct the problems or to deal the the resulting problems yet to incur ub the future. It shouldn't take too long to dis-assemble and reassemble the panels as the hardest part is the cuts to fit the structure. Label each of the panels with a magic marker to use them in the same position later on.

Good Luck,

Ed

What I did on the last spec home we built, was place "double bubble" foil back, about an R3 value, over the trusses and purlin, no roof decking, and then 12 inch blown cellulose....the utilities on that house qualify for energy tax credits...the attic is vented on both gables, and since we use a classic panle roof, we do not run "closures" at the eave, so the roof can breath, if you will. No condensation problems, and the main benefit is the summer temp reduction.

You guys are in the same neck of the woods so to speak, perhaps joasis's ideas/ methods will work the best.
In a more Northern or colder climate, I believe things might be differnt.
IMO

Right now the roof has a 14'' vault on it(shed style) I think I will try to drop the ceiling to level on the inside giving me a 14"attic on the top side and use baffles on the bottom,insulate,rock and keep an eye on it for a while.If it still sweats I may have to remove the tin and put a foam barrior or something similiar under the tin.I'm a little concerned that such a small attic area may not provide enough temperature difference from the insulation to the tin. It seems that the tin conducts cold so well that it would be just as important to insulate the tin from the attic temperature to prevent condensation.

You are thinking about this the wrong way. The metal roofing does conducts heat extremely well to the outside in the winter but the real reason it is so cold is that the insulation prevents heat from the inside getting to the metal roofing. If there is a strong moisture source in the building the moisture will pass through the drywall, insulation and air space and condense on the metal roof. You must either ventilate the attic space VERY well, add a vapor barrier, spray foam insulation against the underside of the roof, or remove the source of moisture. Or all of the above.

Good response from Mighty Anvil. You need to address the ventilation problem!!!

Unless you do the sprayed on foam insulation method described in the previous post, you MUST-MUST, have continuous air flowage.

100 % intake venting PLUS 100 % exhaust venting.

There are vent products made for shed style roofs, which I mentioned before, whether it ties directly into a wall of the existing home oe in free standing.

I'm glad you took the time to read everybodys responses though, and to re-comment back. Not everybody does and it sort of leaves us hanging to find out if we helped out or not.

Ed

Well I dropped the ceiling creating a small attic space on the high side of the roof,put vented soffitt on the bottom side,put baffles on the bottom to hold the insulation away from the roof and was able to open the top side of the roof that ties into the old house so it opens into the old attic creating through ventilation.got it all rocked now I'll just keep an eye on it and keep my fingers crossed!Because of space limitations I was only able to get 6" of insulation in the ceiling, I just hope thats enough to buffer the temp before it gets to that cold tin.I've built lots of homes and never had a problem before this,using tin is a little different ballgame,guess I learned something!That spray foam insulation was the first thing I thought of but don't know of anyone around here that does it!

Next time you go to a home show, collect cards...you will be surprised how often you will need something....and remember you have a business card in a file...

You still have it backwards. Adding insulation does not reduce the likelyhood of condensation. The more insulation the more likely it is that condensation will occur. It is moisture that you must control not temperature.

Any time you have a cold surface(tin roof in winter) with warm air contacting it(coming through ceiling with little or no attic)and humidity present(outside air from ventilation) condensation will form on the cold surface!Take for example a glass of ice water or a cold can of pop in a warm place,the cold surface will sweat!It seems that a buffer of some sort under the tin such as foam or bubble wrap would keep the slightly warmed air from contacting the tin directly kind of like a coozy on your pop can or insulated cup.Am I wrong?

The insulation does not act as a buffer and will not reduce condensation. The only insulation that will prevent condensation is foam sprayed on the metal because there would then be no air space where condensation could occur.

Never known tin to work well with an underlay. In fact, Follansbee will void any inferred or actual warranty.

Must have positive ventilation.

As a side note. I suddenly had moisture and sweating in my utility room which has shingles. I couldn't figure it out for a while and didn't devote much time to it. Then, on a hunch, I moved the dryer and found a hole in the hose. Fixed it and condensation went away. It's all about ventilation and moisture control
Anybody have a carport? Some non-rainy days, mine will condense and drip as if like rain. Carports are well ventilated! But the grass and or even the ground will produce enough moisture to make it rain inside.
( My theory, not a fact that I've researched).

I was wondering about that too. I have some fiberglass panels for skylights in my shops metal roof. There is no insulation or sheetrock just the metal roof. Would I need to put in a roof vent for my shop to be ventilated?

Might not. Most shops have a floor keeping underground moisture out. And walls prevent too much moist air from passing through the area.

Each case will have it's own solution or even non-problem.

Follansbee’s warranty can be voided if any of their metal roofing is placed over fire-retardant or preservative treated wood or if it is not installed according to their specifications.

For TCS II, the underlayment specification lists, as a minimum, one layer of 15# asphalt saturated felt paper and a slip sheet of rosin sized paper as final underlayment layer.

For Terne II, the underlayment specification lists rosin sized paper as the only permissible underlayment and excludes roofing felts.

Adequate ventilation is required below the substrate to control moisture concentration and condensation.