About them cold roofs

Hi guys.

I'm thinking about cold roofs a lot lately since i will build one soonish.
I have come to realize there are 2 types of cold roofs.

One i have observed to work (if you don't have a large vapor source underneath) is what i call fully vented.
It's layers are from top to bottom : roof covering (shingles, metal, etc), purlins, rafters, suport framing.

The other one i've seen advertised a lot and which does not look very right to me is what i call somewhat ventilated.
It's layers are from top to bottom : roof covering (shingles, metal, etc), purlins, counter battens (rafters), membrane (some form of plasic), sheeting (OSB or 1x4), rafters, supporting framing.

Now, the first type works great if there is continuous soffit and ridge venting as long as you're not having large amounts of warm vapour underneath which can condense on the inside of the covering.
It can also be very easy to inspect as everything is in plain sight.
One drawback is that if water manages to get thru the covering, it will be on your attic floor.

The second type, i don't know ...
On the plus side, if water gets thru the covering, it will drip on top of the membrane and head to the gutters.
But if there's some condensation on the inside, either underneath the sheeting (not horrible) or between it and the membrane (very bad), you're in trouble.
Also, inspecting the thing is not straightforward.

Now, i can control thru proper execution how much vapor gets from the living spaces to the attic.
Is there other possible sources of moisture that can condensate on the inside faces ?
I know dew condensates (in the morning) but it only does on top of surfaces, not underneath them.

As i plan on using metal roofing, I'm favoring the first type since metal is pretty leak proof but it won't stay like that forever.
Do you have any thoughts on the subject ?

PS
Some issues that can come up:
- piling snow blocking the ridge vent
- wind driven snow or rain getting in the ridge / soffit zone

Hi, I believe membranes used in roof construction (Tyvek, etc) are semi-permeable, ie. moisture can pass through them in one direction only. So when installed properly the moisture from inside the house is able to get out whereas moisture from the outside cannot pass inside.

As far as I know some wood-based sheeting (Agepan, etc) is made in this way also - that's why it has "outside" and "inside" markings on it. (Funny story when our house was being built and the installers ignored the German writing... So they were already half-done when i noticed "INNENSEITE" all over the roof when looking at it from the outside .

Thanks Crt.

Those membranes are not quite what they seem, not quite proven (and how can they be?).
They are somewhat permeable with a declared permeance.
But what that actually amounts to you and me and anyone installing them is a good question.

Remember, most traditional roofs were like this:


No air / vapor barrier = 100% permeable.
Obviously, they did not have so much vapor in the living spaces below (no showers / bathrooms, no gas cooking, not so much of the time inside, ...)
That's why it worked big time.

But now we have those vapor sources and our best bet is to do the air / vapor barrier together with the insulation at the ceiling in the living spaces.
Bonus is everything gets in plain sight for easy inspection.
And that is a BIG deal.

My only concern is not understanding all possible moisture sources that can humidify the attic leading to condensate on the back of the metallic roof which leads to dripping on the attic floor insulation which leads to insulation problems or worse, water penetration down below for whatever damage your imagination fathoms.
Also, water on the wooden structure causes damage if it can't evaporate soon enough.

One source of such humidity could be morning dew but i could be wrong and it's no issue here.

Anyone having any experience / stories regarding this ?

My experience is very limited since I am definitely not a builder by trade and our house is inhabited only part-time in the cold part of the year (fully in the warm part).

What we have is both Agepan boards and a Tyvek-like membrane. Ground floor + unheated attic. There are windows on the long ends of it - the house is long and narrow but they are closed most of the time. The house has external insulation sealed up to the roof.

In my understanding this is all pretty much standard - how things should be done, in our part of the world anyway. NE Slovenia, climate zone seems to be roughly similar to yours. Up to -30 C in the winter, up to 35 C in the summer.

There have been no problems with moisture at all neither during spring-fall when we're in the house full-time nor during the winter when we're there some 3-4 days every 10-14 days using the wood burning stove, using the propane heater for warm water, cooking, showering, etc. Drying a crop of industrial hemp in the attic also brought no problems except visitors giving us knowing smiles Actually ever since the first winter when floor heating drove out the moisture from the freshly built walls the house has been super dry.

This can maybe give you some idea of how things got done - http://www.dergan.net/gallery/album15?page=3

Sorry if I haven't been of much assistance, this is all I can offer from personal experience.