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

[EddieWalker]

Venting is to create air flow through your rafters. It pulls air from the lowest spot, your eaves, and exits it out the highest spot, your peak. Or as close to the peak as possible. From what I understand in your situation, there isn't any air flow through this area.

I think you have to either create a path for air to flow through there, or pack it full of foam so there isn't enough air in there to create so much water.

Condensation happens all the time, in all parts of the house. Most of it is never noticed or an issue. It gets blamed for all sorts of silly things, but all thats happening is the air cannot hold the water vapor in it any more due to the lowering of temperature. The colder that air gets, the higher the relative humidity gets, until you hit 100 percent. In most cases, this happens in a very small space from traped air, or air next to a source that is colder then the surrounding areas. Soda cans are a good example when they are really cold and the air is a lot warmer. Metal holds its temperature longer then wood or other materials on a house, so it's always the first and most obvious place to have condensation.

Remember, you have to have an eave vent and a ridge vent. Air has to go from one place to another. You don't have to have a certain number of vents, they just have to be in such a place that you get that air movement over everything.

Hope that made sense,
Eddie

 

[dave1949]

Depending on the profile of the roof tin, the air movement from eave to ridge is going to be greatly limited by the purlins. I don't have any good ideas on how that can be improved. I certainly don't think a continuous ridge vent will hurt, but if may not achieve the usual results as with a rafter roof.

I guess if you primarily see this on days following use of the ventless heater, and only in that area, you could put a lot of blame on that. If so, maybe switching to a vented model would get the condensation down to something you could live with.

 

[MarkV]

I would not be surprised if the ventless fireplace isn't a big part of the issue. Keep an eye on the interior ceilings even with a good felt overlay.

MarkV

 

[pacerron]

Ok sports fans, I have a problem.

Yesterday I walked around the deck and I see a puddle of water 2' in from the eve under the porch/roof transition line. Looking up there is a pretty steady drip and walking along the back porch(North side of house) I see 4 more smaller spots that are wet. I thought I had a leak, but no, it's apparently water vapor condensation on the underside of the metal roof. It was 28 degrees yesterday morning and there was a frost. At the time I walked around it was 11AM and the sun was full on the South side of the roof. There was no drip/leak on the South side porch which means it must have been burned off by the sun. Here are the pic's:

Puddle:

The exit point of the water:

Another spot about 8' in from the eve:

Remember...on the roof, I have 1.5" decking, then 30 weight tar paper, then 3" styrofoam, then purlins, then tin. On the porch I have 4/4 oak decking, then 2" high density foam with vapor barrier backing, then purlins, then tin.

Pondering this I think I should have also put tar paper under porch roof and tucked it up under the transition so any water would continue out over the porch and exit. But, my concern is that no matter what I do I will have this melting water vapor under the right weather conditions and it will still be running down between the tin and the purlins. Since the purlins all run horizontally water will collect/soak in/collect eventually causing an issue.

Maybe this is not a big deal, but it bothers me to have that much moisture between the foam insulation and the tin and I have to wonder if there is a definitive solution???

The others have covered normal roof venting, even the builder, Eddie Walker who said:
-------------------------------------------------------------------------------------------------------------------------------------------- " Remember, you have to have an eave vent and a ridge vent. Air has to go from one place to another. You don't have to have a certain number of vents, they just have to be in such a place that you get that air movement over everything.

Hope that made sense,
Eddie"
----------------------------------------------------------------------------------------------------------------------------------------

The problem is they are not reading or understanding your roof layers where you have the horizontal purlins blocking air flow from the eaves to the ridge every couple feet or less and creating water dams at each purlin.
In your climate I would be concerned too since the water trapped by the upper edges of the purlins may freeze then thaw, then accumulate more and repeat the freeze build up and pop the washers on your roof nails causing potential rain leaks in addition over time.
Someone mentioned metal barn roofs having the problem.
I have a little 24 x 36 open on one side horse shelter that I didn't insulate the roof because of the open side being an opening 14 feet high by 36 feet long. That was a mistake. Cold nights and warm mornings particularly in the spring and fall causes water drops to fall inside, almost like rain. After 20 years the purlins and trusses have turned black from this moisture.

In comparison, I have 2 other barns built at the same time, one 40 x 60 and the other 64 x 84. They both have metal siding all the way around and the same type metal roofs but have insulation sheets on top of the purlins. Both barns are completely dry and the purlins and trusses look as white/yellow as new, except for some swallow dung. They never drip. This is important because we have equipment in one and hay in the other.

I don't know of an easy solution to your problem. You might call a couple commercial roofing contractors and see what they say before jumping into something. I would have probably added 1.5 in foam sheet insulation between the purlins and then put a membrane over the surface before putting down the tin. Then any condensate would have to flow down the bump channels if you have barn type roofing. If your tin is similar to copper roofing with soldered seams I would have used layers similar to the way that is done.
Have you ever posted pictures of your roof layer build up as you did it?
Ron

 

[EddieWalker]

I was thinking that the purlins and rafters overlapped and created openings. If they are solid, and there isn't any way to move air through them, then filling up the void with foam is gonna be your best, long term solution.

Eddie

 

[pacerron]

You might be able to get foam pumped in to the voids between the purlins by removing the end flashing trim on each end of your cabin like is done in wall cavities up here if the insulation contractor
can get a hose into the 1.5 inch space toward the center of the house from each end and can develop enough pressure to fill the cavity as he pulls the hose back out.
How long is your roof?
Ron

 

[bindian]

A vented ridge cap is a good idea, it might reduce the condensation by air flow.

The way you describe your roof under-layment, it is possible the ventless is involved, but I would think condensation from that would form on the underside of the tar paper, or maybe the foam, since they are the vapor barrier.

Moisture will move through materials unbelievably. I once had a shower/tub with a sheetrock ceiling above, then a layer of plastic for vapor barrier stapled to the underside of the trusses. Condensation would form on the underside of the plastic, on the attic side of the sheetrock, and that was happening under fiberglass insulation to boot -- which theoretically, should have kept that area above the dew point.

Rick,
How about a solar powered ventalator fan? I had one before that cut on and off at certain temperatures.
hugs, Brandi

 

[ENG18LT]

Be careful that the foam does not expand so much it distorts the metal roofing or worse tears the metal roofing from the fasteners. I know you probably don't want to hear this but I would pull metal roof off, put down foil faced bubble insulation then reattach metal. The insulation is the same stuff that is used in pole barn construction to prevent the condensation from "raining" inside. Maybe even try a synthetic roofing underlayment like Grace between the metal and purlins. This is one of the main drawbacks of using metal roofing. I see a lot of homes in our area using metal to replace their shingles and the installers are simply attaching 2x4 purlins to existing roofing then the metal with no barrier to direct the condensation away. The selling points are the perceived longevity and lack of snow build up, but everyone I talk to about the condensation issue does not have an answer. I have asked several installers and supply houses about it as well as home owners. The supply houses just hand you a brochure, the installers say "never heard that", and home owners just give a blank look or say the installer/salesman didn't mention that. As someone that has 16 years in residential construction I am not sold on its use in the residential application. That being said builders have been using standing seem roofing for years with good results. Difference is standing seem is not secured to purlins, it is generally attached directly to roof sheathing with a good underlayment, no air gap. I wish there was a simple, tried-and-true answer for this problem, but I have not come across anyone that has the answer, at least not yet. Maybe try contacting the manufacturer of the metal roofing and see what they have to offer as a solution.

Good luck, sorry for being long winded.

 

[MotorSeven]

Pacer has a grasp of it pretty good:
"The problem is they are not reading or understanding your roof layers where you have the horizontal purlins blocking air flow from the eaves to the ridge every couple feet or less and creating water dams at each purlin."

However there is an vertical air pathway(raised rib) built into the tin sheets every 12" and that same rib creates a 'vent" of sorts at the ridge cap. Maybe by putting in a more or proper vented cap I can get more air movement and that would be the least expensive & least labor intensive attempt to remedy this.

Eddie is spot on ref the nature of condensation and it's inherent issues. Hindsight says I should have at least put a vapor barrier on top of the purlins but I have never seen this done. Not so convinced that foam adhered to the tin will even stop condensation. The quonset has 2" of sprayed foam directly on the metal and water will even drip off the foam during certain weather conditions.

Yesterday I had more water dripping, but about half the amount of the day before. It looks like weather conditions will affect this quite a bit. I appreciate all the ideas...keep 'em coming while I monitor the "drip":thumbsup:

 

[Kyle_in_Tex]

Perplexing. Even though there are vertical pathways at the ribs, condensation can occur where the flat areas touch the warmer underlayment, . Even where there is little or no gap between metal and insulation. It would seem to me that the tarpaper would be good enough but maybe there needs to be an impermeable vapor barrier membrane. Hate to think of that moisture saturating your insulation on a regular basis. It is probably related to the amount of heat and moisture of the propane stove and the permeability of the 1.5" roof decking. Thinking that overnight the moisture freezes underneath the roof then obviously melts and comes out.

I'll defer to the construction experts as to what to do but IMO, there needs to be another impermeable layer between decking and insulation. But what do I know???