Metal Roof Sweating - Help?

Home Innovation

I'm a builder in Maine, yea, the other end of the world. Anyways, I have a customer that has a ranch style house. Half the house is cathedral, the other has a very small attic. So heres the deal. I pulled the insulation out the attic, it was against the metal roof and was saturated. So I replaced all the insulation, and added proper vents between the insulation and 1x6 horizontal boards that hold the metal roof. I also found that there was no ridge vent or soffit vents, so I added both to get some ventilation across the whole length of the roof. My first findings included saturated insulation, 1x6 slats, and typar.
For some info: there is sheetrock on the cathedral, 2x8 rafters, R30 fiberglass insulation, 1x6 boards, typar, then metal roof....
Ok, I know there is no need for insulation against the roof for an attic and it works perfectly fine w/ no insulation against the roof, BUT I need to experiment in the attic so I can fix the cathedral area, which I will have to do taking the metal roof off..
So I waited a few days w/ no insulation to dry the roof, then I put proper vents in 2 bays w/ dry insulation and waited 5 days to see if condensation would occur and it did, exept now it condensates between the proper vent and the insulation. The top side of the proper vent is dry as well as the typar and wood slats that holds the metal roof....I really hate metal roofs and this is not helping my oppinion on them. Can someone help me out w/ this??? BTW: the proper vents was the pink foam ones.

mics_54

Facts:
#1) poor insulation in the cieling can cause air in the attic to become warmer.
#2) a lack of vapor barrier in the cieling can add to the moisture in the air in the attic.
#3) The sun can heat the roof causing attic air to become warmer.
#4) Condensation only occurs when moisture laden air comes in contact with a cooler surface relative to the temperature of the air. (see references to a dew point curve)
#5) There are several ways to manage condensation. Separation of cold surfaces from warm moist air. Elimination or reduction of temperature differentials of the air and the surfaces. Elimination or reduction of moisture in the air.

In a typical attic the most common practice is to attempt to utilize all three methods of mitigation. Insulating the cielings, venting the attic, installing vapor barriers. Condensation is most apparent on impermiable surfaces like metal. Condensation can still occur on wood but it isn't as apparent usually because wood absorbs moisture.

mics_54

Yet there are only two ways to saturate air and cause precipitation or condensation. One is to cool it to it's dew point thus reaching its 100% relative humidity. The other is to add water vapor to it. Precipitation will never occur outside of 100% relative humidity...if I understand the principles.

Renegade

Just a wild guess, it's a bare galvanized metal. There is one odd but easy solution.

Home Innovation

It is the standard residential (maybe americana) galvanized roofing (red in color). The thing that worries me is that its sweating between the proper vent and insulation now and not the metal. I do not know if there is any plastic or vapor barrier under the insulation of the cathedral. My guess would be "no". I just don't want to pull the metal off the roof, change the insulation and have it condensate again....I am trying an experiment in the attic by putting plastic sheating (6 mil) over the insulation (hot side) and see what happens. So there will be plastic, insulation, proper vent, 1x6 slats, typar and metal....any other suggestions that I could experiment on? Its around 12 degrees at night and only 20's during the day right now. Thank you.

Tom Struble

i feel on a residential job the metal roof should be on solid
sheathing with a tar paper or synthetic underlayment then
layer of rosin paper then the panel.
You have warm moist air goin thru the skip sheathing and condensing on the panel.Maybe you could install foam sheathing in the rafter bays and
seal along the joints

Home Innovation

That is what we will be doing. The slats are now rotted from the condensation. So when we pull the metal off, we will be pulling the slats and plywooding the roof. We will tar paper it, then apply the metal back on. But like I said earlier, w/ the proper vents in there, its sweating between the proper vent and insulation now and NOT the metal roof...so I can't blame it on the metal now....

dennis

In reverse order;

Metal roof
rosin paper
tarpaper #30 minimum
solid lumber preferred or 3/4 plywood
1.5" - 2" airspace
Insulation, skip the foam baffles
vapor barrier


my 2cents

AaronB.

Get rid of the moisture in the house if its not the roof causing the sweating.

Closed cell spray foam insulation will eliminate this problem.

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wmallock

I agree the problem is not ventilation now that you fixed that nor the metal you would still see condensation on a wood decking

The only thing i can see is the home has a great deal of heat loss due to lack of insulation vapour barrier lack of seam tape on vapor barrier etc.

rtomsi

If it is of any help to anybody, some producers (in EU most of them) are offering metal roof tiles and sheets with an anticondensation layer (e.g. dripstop etc). It is like some kind of thin textile glued to the metal sheet and it holds the condensate moisture inside, so that there is no dripping from the roof to the floor. This is only possible when deciding about a new roof and it does not help you to solve any problems with an existing problematic roofs.

Tom Struble

just thinking you may want to check into some of the synthetic underlayments that are rated for the high temperatures under metal roofs

red_cedar

If you do this then correct venting becomes more important. Most of the synthetic underlayments do not breathe. The synthetics are not a cure all for problems, they are an upgraded non breathable underlayment that can also act as a slip sheet for the metal.

Justusjeff

One big problem most people don't recognize is moisture from the burning of non vented gas heaters such as propane wall heaters or free standing kerosene heaters. I had a customer call me because the plywood sheathing in attic roof was literally saturated with water, leaking on insulation and also to the outside soffit. I went to the attic and I swear, it looked as though you wet the sheathing with a hose. For 17 years the customer never had a problem, but that year he switched from a coal stove to 2 gas non vented heaters. I talked to a salesman from gas company and he informed me that the end bi-product from these heaters is moisture and that a non vented unit should be run with windows cracked in the room or vent the unit to the outside. I would tell the owner to shut down these heaters for a couple of weeks and see if thats the source of problem. Hope this helps.