Ventilation of roof. Eaves to eaves

[Aunty]

How so?
The air moving in the air gap comes from outside, and is thus already cooled, so why would it condense?

It's called night radiation, which causes the roof boarding to be colder than the surrounding air, and when humid air from outside enters the air gap, it condenses against the cold underside of the roof boarding.

There is much to read about this phenomenon. It's also the reason why a thin insulation is often placed on the roof boarding nowadays to eliminate this phenomenon in roofs with roof boarding.

 

[Aunty]

Besides, I have also not been on a new build without an air gap..
Except for porch roofs..
But I mostly work with old houses

old houses should have an air gap but it is not due to moisture drying out but because of something completely different but you should know that as you work with old houses

 

[Cancerman_777]

old houses should have an air gap but it's not due to the drying of moisture, it's for something completely different but you should know that since you work with old houses

Print it out instead of being so secretive.

 

[Utsliten och utdömd]

old houses should have an air gap but it is not due to drying out of moisture but something entirely different but you should know that working with old houses

Considering that TS is asking about an existing house, maybe you should give tips based on that, if older houses are supposed to have an air gap?
TS mentions that they want to open up at the ridge and describes the existing roofing.

 

[plåtrickard]

old houses should have an air gap, but it's not due to moisture drying out; it’s for something entirely different, but you should know that as someone who works with old houses

No, I shouldn't know that since I work on the outside. Roofer. But I'm still interested in what insulation is used on the "outside" of the roof.

 

[plåtrickard]

But to address the original question, I would have chosen a vented ridge with seam roofing and full ceiling height.

 

[Aunty]

Print it instead of being so secretive.

In older poorly insulated houses, there was often a problem with snow melting due to the indoor heat, and when that water ran down towards the eaves, it froze again and created large ice sheets that were more or less harmful to the house.

Then the air gap was invented, allowing cold outdoor air to come in and cool down the roof from the inside, thus eliminating this phenomenon.

Now we have so much insulation and such good lambda values on insulation that this problem hardly exists anymore.

 

[Aunty]

No, I shouldn't know since I work on the outside. Sheet metal worker. But I'm still interested in what kind of insulation is used on the "outside" of the roof.

they often lay a hard insulation board between 20 and 50 mm.

 

[eleson]

Hmm. There's quite a bit to take in from this thread.
but / and I note that Isover points to a number of different solutions that lack an air gap

https://www.isover.se/applications/oventilerad-konstruktion

 

[människa]

Thank you all, especially Aunty, for all the information! But I still don't know which direction to go. The house is old, but it will be reasonably well-insulated. The planned 30cm insulation in the roof will be a 300% increase. To make it even more complicated, I can mention that we've already remodeled part of the house where we've kept the air gap, removed the attic, replaced the old 10cm insulation with 30cm, and made sloped ceilings. My question is whether we can do the same all the way up to the ridge, and then down the other side to the eaves. With or without a ventilated ridge. We are also going to extend the house (along with the ridge and eaves) this spring. So that part will be completely new, and there you could make an unventilated construction if you want I will raise the question with our timber supplier

 

[människa]

We will probably have to replace all the råspont during the roof replacement because it was really black in one spot. So it's possible to remove the air gap at that stage.
But should one use a vapor brake instead of a vapor barrier then?

 

[AnderssonH]

Hello!

Since it is an older house, I believe you should execute this with ventilated eaves from both sides and a ventilated ridge. Use a vapor retarder. A vapor barrier construction is based on a "plastic bag effect," and all walls and roofs should be airtight, which in your case will not be achieved as the old part of the house will still be very leaky.

Almost all new constructions use well-ventilated attics/roof constructions. It is well-proven.

However, Aunty has many good points, and I believe more house manufacturers will start using these solutions. However, I think you should be open to the idea that your truth is not the only one. Much of what you describe I have never encountered in reality.

 

[människa]

Since it is an older house, I believe you should do this with ventilated eaves from both sides as well as a ventilated ridge.
Use a vapor retarder. A vapor barrier construction is based on a "plastic bag effect," which requires all walls and roofs to be airtight, which in your case will not be achieved as the old part of the house will still be very drafty.

Thank you for your opinion.
In the end, we will have renovated the whole house, and then the vapor barrier will be tight. But it may take several years before we tackle the attic floor, and by then a lot of moisture might have had time to build up in the construction.

 

[eleson]

Personally, I would choose to look at diffusion-open roofing. It can be installed without an air gap and ensures that the roof is ventilated and that moisture from the inside has somewhere to go.

 

[människa]

What does undertak mean?