The Log house Project begins........

[Robert_in_NY]

Understood, I was thinking the purlins were 1½".

 

[EddieWalker]

I don't see any issue with air flow through the purlins and R panels. What I haven't seen is any soffit vents and ridge vents to pulle the air from the bottom to the top. Air flow is all you are after, but there has to be a start and and end to get it going. Having smaller openings just creats a venturi effect and should speed up the air going through those areas. In theory, it might even help by having bigger and smaller openings. Either way, I don't see a problem with the way the roof is built. I just don't see how air is getting in and out of the space?

Again... Be very careful of over thinking, over engineering and over doing a fix. The easiest isn't always the best, but the most complicated and expensive never is the answer either. Keep it simple and stick with what is proven to work. Either create proper air flow or seal the space.

There are lots of types of foam out there and the goal isn't to get an air tight seal, just to eliminate the vast majority of the air space so there is less volume of air in the cavity, resulting in less moisture that the air can create condensation.

Eddie

 

[Robert_in_NY]

Understood, I was thinking the purlins were 1½".

Well another option would be to use a router and cut grooves to allow air flow

 

[EddieWalker]

Just to add to my previous post. Getting the air to flow as close to the metal as you can is the ultimate goal. Having the purlins going across will actually help force the air up and across the bottom surface of the metal.

Eddie

 

[pacerron]

Ron that sketch shows a 2" air space between the upper sheathing and foam.....me thinks that is a real good place to have moisture condensate on the bottom of the sheathing......no?
PA....shingles???? {Gasp}...NEVER:laughing:

Rick,
I think the 2 " or more air flow space with sufficient eave and ridge venting is common practice for a vaulted situation like yours with open rafters.
The plywood acts somewhat as a radiation barrier. The temperature and humidity differential between cold outside air on top of the roof and the warmer more humid air under it is the root of the problem regardless of the roofing being tin, asphalt, slate, tile, or some space age material.
With your purlins being horizontal, not staggered, and only 1" thick there is a good chance that any roofing material would sweat underneath in certain weather conditions. Since your styrofoam is not like "Tuff-R" or other products with a radiant and reflective layer of foil, I would really be concerned that the dripping problem you have now will grow mold in the spring. I wouldn't count on the little bump ridges in your tin to supply enough air flow. They are there mainly to add a bit of rigidity to the panels and for looks.
Up here over the past 15 years a serious mold problem has developed on most roofs that face North as well as vinyl siding, or metal barn siding that is in the shade for that matter. Didn't used to be that way. The manufacturers are having to put x years of anti mold guarantee on their products now but I bet trying to collect would be a real hassle.
A lot of the log homes that have a protective sealer are turning black as well.

If your " me thinks" were true I should have moisture or ice on the open underside of the insulation board in my 2 big barns. That sheeting is in direct contact with the bottom of my metal roof panels, since it is on top of the purlins, and it is not much thicker than plywood sheeting. My barns are not heated and I have never seen any frost or moisture up there whether the 3 big sliding doors are open or closed.

I still think you should talk to a commercial metal roofer before putting the screw drill in reverse or cutting in more ridge vents.

Gotta go.
Ron

 

[Rock knocker]

If you take the tin off, you have the luxury of fixing everything:

1) Limit the internal humidity anyway you can. Toss the ventless
2) Replace the vapor barrier. It is clear that water vapor is getting from the house onto the under-side of the roof panels
3) Increase the amount of insulation. If you repair the vapor barrier, you don't want condensate on the car decking. Try to limit thermal bridging as best you can
4) Increase ventillation above the insulation

 

[MotorSeven]

I hear ya Eddie, this one is going to be a log study.

Ron I did talk to my metal roof supplier today. He is not a roofer, but sells a ton of tin in our county. He agreed more airflow can't hurt, but he also said that this is a common thing that may or may not develop into a bigger problem.

Rock, (1)yes, (2)I'm re-thinking this, no way is water vapor from the house getting to the underside of the tin. 30 weight tar paper & 3" of foam which was all seam taped prior to the purlins going on. (3)maybe but would that not cancel out number 4?

 

[tony Cecil]

Motor7
I have been pondering this for a few days.....I think the V where the metal overlaps, still lets the air ventilate upward, and if you put a regular old cap on the roof it will let the moisture out....I also thing the ventless stove (heater) is a problem......I surely would put the vent on the roof peak, and also not use the ventless heater.....Even if the visible moisture doesn`t happen this time without the heater working, the rifge cap should work.

The rood I mentioned a while back , has the v jointed ceiling , then three in foam sheets, vantec(sp) 5/8 , composite (rubbery) overlap, and then the shingles nailed onto it....I haven`t had any problem for the last four yrs, bt I`l check as I am going up to cook the `bird` and will be there when this next chill passes through Happy Thanksgiving Tony

 

[dave1949]

Up here over the past 15 years a serious mold problem has developed on most roofs that face North as well as vinyl siding, or metal barn siding that is in the shade for that matter. Didn't used to be that way. The manufacturers are having to put x years of anti mold guarantee on their products now but I bet trying to collect would be a real hassle.
A lot of the log homes that have a protective sealer are turning black as well.

Ron

Funny you mention that. Recently I drove by our old house that we left in 2006, and the north roof, brown shingles, were completely black-gray colored. It wasn't that way when we left. I noticed the new owner hadn't keep after the trees and thought maybe just too much shade and lack of airflow, but it sounds like there is more going on than that.

 

[Kyle_in_Tex]

Some folks put a pot of water on their wood burning stove. You just need to move the pot to where the water is dripping...just kidding.

I'm thinking the ventless is a big part of the problem. Like you said, try next cold weather without it and any other moisture sources.