Ventilation of roof. Eaves to eaves

[människa]

Is it enough?

Planning to extend the house and open up to the ridge at the same time. It would be nice to avoid making a ventilated ridge.

Right now it's "plegel," meaning metal that is supposed to look like roof tiles. But eventually, it will become a standing seam metal roof.

There will be 300mm of fiberglass insulation in the roof, 35mm air gap under the roof sheathing, and a vapor barrier 1/3 in/out in the insulation.

The roof has a slope of 28°.

 

[Aunty]

Is it enough?
I'm planning to extend the house & simultaneously open up to the ridge. It would be nice to avoid making a ventilated ridge.
Currently, it's plegel, which means sheet metal designed to look like roof tiles. But eventually, it will become a standing seam metal roof.
There will be 300mm of glass wool in the roof, a 35mm air gap under the boarding & a vapor barrier 1/3 in/out of the insulation.
The roof has a slope of 28°

Why do you need an air gap?

You don't need an air gap in a roof; it will only cause more problems than it solves.

 

[människa]

Yes, a non-ventilated solution is not completely ruled out yet. But I must decide in a few days. More opinions on this are welcome.

 

[Aunty]

The simple answer is that an air gap in the roof only adds more moisture than it reduces. The reason for ventilating roofs initially no longer exists due to today's increased insulation amounts and improved lambda values.

The only advantage of an air gap in the roof is to dry out potential construction moisture, but this can be solved in other ways.

 

[Cancerman_777]

Okay? So if the roof gets a leak, water comes down to the råsponten, do you mean that the air gap should not be there?

 

[Rickard.]

the simple answer is that an air gap in the roof only adds more moisture than it reduces. The reason for ventilating roofs initially is no longer relevant due to today's increased insulation quantities and improved lambda values.

The only advantage of an air gap in the roof is to dry out any construction moisture, but that can be solved in other ways.

Strong assertions for relatively unproven methods that I know many smarter and more knowledgeable than I do not agree with. An important function that ventilation also has is to provide some space over the entire lifespan of the house for penetrating moisture both from outside and inside, which I would argue is unavoidable in houses meant to last generations.

In my geography, an air gap is proven effective, and in my years, I've never encountered any problem caused by air gaps or attic ventilation, and even air gaps with no further air exchange (kind of like the OP's idea but also even "worse") seem to work.

 

[Aunty]

Ok? So if the roof gets a leak, water comes down to the råspont, are you saying that the air gap shouldn't be there?

First of all, I don't think you know how an air gap works and where it should be. The air gap should be under the råspont towards the inside of the house.

Secondly, the air gap is not meant to handle a water leak; there is no way for an air gap to manage that amount of moisture. It is meant to handle humid air, not free water.

Thirdly, houses weren't built with an air gap in the roof to handle moisture but for completely different reasons.

Fourthly, an air gap will increase the moisture load in the roof because humid air will move into the air gap and condense against the cold underside of the råspont. To solve this problem that arises with ventilated roofs, additional hard insulation has been placed on top of the råspont before laying tiles or whatever is to be used.

 

[Aunty]

Strong statements for relatively untested methods that I know many smarter and more knowledgeable people than I disagree with. An important function that ventilation also has is to give some space for penetrating moisture both from outside and inside over the entire lifespan of the house, which I would claim is unavoidable in houses that are meant to last generations.

In my geography, air gaps have proven effective, and during my years, I have never encountered any problems caused by air gaps or attic ventilation, and even air gaps without further air exchange (like the OP's idea but also even "worse") seem to work.

but that's not true, just ask any moisture expert, or read research on the subject. just google it

 

[Rickard.]

but that's not true, just ask any moisture expert, or read research on the subject.
just google it

Strange that 99% of all new houses are built with ventilation then if it's worse in every way

 

[Cancerman_777]

first and foremost, I don't think you know how an air gap works and where it should be.
The air gap should be under the sheathing towards the inside of the house.

Second, the air gap is not meant to handle a water leak?, there are no possibilities for an air gap to manage that amount of moisture. It's meant to handle humid air, not free water.

Third, houses were not built with an air gap in the roof to handle moisture but for completely different reasons.

Fourth, an air gap will increase the moisture load in the roof as humid air will move into the air gap and condense against the cold underside of the sheathing. To solve this problem that arises in ventilated roofs, they have started placing an additional hard insulation on top of the sheathing before laying tiles or whatever you will have

Yes, but water will move downwards in accordance with gravity, even through the sheathing as moisture, and not having an air gap under the sheathing sounds very strange. It may be that what you/they claim is correct, but to my ears, it sounds like a shit storm waiting to happen. If we then continue with my example of the water/moisture movement, it would end up in glass wool or stone wool instead of the air gap. What I've seen of glass wool and stone wool enclosed in a space without an air gap spells mold on the nearest organic material.
If the argument is that the air gap is unnecessary in roofs, then it should be exactly the same concerning the air gap behind a facade. We all saw how it went with the single-stage sealed houses...

 

[Aunty]

Yes, but water will move downward in accordance with gravity, even through the plywood as moisture, and not having an air gap under the plywood sounds very strange. It may be that what you/they claim is correct, but to my ears it sounds like a shit storm waiting to happen. If we continue my example with the movement of water/moisture, instead of the air gap it will end up in glass wool or stone wool. What I've seen of glass wool and stone wool enclosed in a space without an air gap is spelled mold on the nearest organic material. If the reasoning is that the air gap is unnecessary in the roof, then the exact same should apply to the air gap behind a facade. We all saw how it went with the one-step sealed houses...

but if you have a leak in the roof, neither the air gap will save you, an air gap can never handle those amounts. Moisture should never move past the sealing layer; you already have a problem. What could be argued is that the air gap should handle moisture from the inside, but there are much better ways to solve that. I don't think you understand the driving forces and why an air gap was used in the roof before. Additionally, I think you are mixing up air gap in walls with an air gap in the roof; they are two completely different things.

 

[Aunty]

Strange that 99% of all new houses are built with ventilation then if it's worse in every way

what?

far from all new houses are built with an air gap in the roof, in the wall is something completely different. in the wall it's sensible but not what we are talking about.

 

[Rickard.]

huh?

far from all houses are built with an air gap in the roof, in the wall is something completely different.
in the wall it's sensible but not what we're talking about.

That's true, but I have only one single project in a total of 10 years of experience with roofs in one way or another without an air gap with raw plywood, it remains to be seen how it holds up, but I know we have no problems with all the other with ventilated roofs.

 

[Cancerman_777]

but if you have a leak in the roof, neither an air gap will save you, an air gap never handles those amounts. Moisture should never travel down past the waterproof layer; if it does, you already have a problem.
One could argue that the air gap should handle moisture from the inside, but there are much better ways to solve that.
I don't think you understand the driving forces and why air gaps were used in roofs before.
Moreover, I think you're confusing air gaps in walls with air gaps in roofs; they are two completely different things.

I'm not a carpenter and have absolutely not studied the historical technical development of buildings.

A roof is not a waterproof layer. It is part of the building envelope.

You repeatedly say "you don't understand xxxxxx" and "I don't think you understand xxxxxx". I think you should explain instead.

Why would it be two different things? Explain where the difference lies.

And...sources for what you're claiming, please.

 

[Aunty]

That's true, but I have one single project with a total of 10 years of experience with roofs in one way or another without an air gap with board, remains to be seen how it manages, but I know we have no problems with all the others with ventilated roofs.

Still good when you know better than the entire research community, there's so much about this.

"Roofs are usually ventilated. Both steep roofs and roofs with less slope are usually equipped with air gaps. Especially steep roofs with large ventilated air volumes are considered to need ventilation. Ventilation often occurs through openings both at the eaves and ridge as well as in the gables. However, the need for such ventilation can be debated, and results from research at SP show that in a well-insulated roof without moisture ingress, i.e. in a roof that is both waterproof, vapor-tight, and airtight, ventilation is rather a disadvantage than an advantage, see figures 2 and 3. The ventilation air instead adds moisture that can condense inside the roof during clear, cold nights. An unventilated roof where no moisture is added is drier than a ventilated one. The studies also show that in cases where ventilation is really needed to dry out moisture damage, the flow is simply not enough. The air is too cold to carry the amount of moisture required to keep the roof dry. The reason for the low temperature of the ventilation air is the strong insulation in the roof, which means that the air gap is not heated from the inside. Similar results have been obtained at research institutes in Germany, Denmark, Finland, and the USA."