Membrane cut or not?

[junglejim]

It’s time to make the cut across the ridge of the breather membrane… cassette roof construction. However it just doesn’t quite feel right after working so hard to protect the building and keep water out. I realise that I need airflow from the eaves and above the insulation to the top of the roof but with this design there isn’t av ridge beam so air can actually flow from eaves, over insulation upwards and back down to eaves on the other side… I’m wondering if this is sufficient to avoid the bed to cut the membrane and help me sleep easy knowing that the roof is watertight…. I’m sure this is a can of worms but any advice welcome. Thank you 👍

[junglejim]

bed = need   🙄

[nod]

No I never have Some BC insist in this 

But I can’t see any good reason 

I’ve asked the last two BC and they have said it’s up to you 

[scottishjohn]

vented dry ridge system ?

then it can come out at the tiop which is where the heat generated between tiles and mebrane will wnat the air to go 

[junglejim]

5 minutes ago, scottishjohn said:

vented dry ridge system ?

then it can come out at the tiop which is where the heat generated between tiles and mebrane will wnat the air to go 

Thanks yes I have a vented dry ridge and airflow between tiles and breathable membrane. The question is about airflow beneath the osb as it’s a cassette roof. I have 50mm air gap that runs from eaves on one side between rafters and to eaves on the other. Not sure if I should also be cutting the membrane at the top as well but would rather not if it’s not necessary.

[junglejim]

29 minutes ago, nod said:

No I never have Some BC insist in this 

But I can’t see any good reason 

I’ve asked the last two BC and they have said it’s up to you 

Thanks…. Hoping I can do that.

[nod]

If BC insist You can always put a slit in afterwards 

[junglejim]

1 hour ago, nod said:

If BC insist You can always put a slit in afterwards 

Thanks. Yeah not so easy as would need to remove the ridge tiles and ridge tree. (Doable but a bit of a faff). I’m not too worried about BC as don’t think they’ll have an issue but want to make sure that I don’t create future problems.

[junglejim]

5 hours ago, nod said:

No I never have Some BC insist in this 

But I can’t see any good reason 

I’ve asked the last two BC and they have said it’s up to you 

Was this on a solid roof ie; osb cassette?

[scottishjohn]

13 hours ago, junglejim said:

The question is about airflow beneath the osb as it’s a cassette roof.

what is a cassette roof 

[junglejim]

10 hours ago, scottishjohn said:

what is a cassette roof 

Factory made in sections built around rafters; osb - air gap - insulation - osb

[scottishjohn]

1 hour ago, junglejim said:

Factory made in sections built around rafters; osb - air gap - insulation - osb

so basically a warm roof  as if it was sips panels 

 so there should be little or no moisture inside it 

and making holes wil surely induce a warm flow of air where moisture can condense from heat inside house 

If i understand you correctly ?

 

 like it has vaulted ceilings as in a room like a loft which is heated from house?

 

 

 

 

[scottishjohn]

On 15/12/2024 at 10:17, junglejim said:

I have 50mm air gap that runs from eaves on one side between rafters and to eaves on the other.

I may be being thick 

but why would you have ,as you say a gap 

 if you have filled space between rafters with insulation ,the osb can be hard aghianst it both sides   

the eaves gap is to vent the cavity walls to exterior ?

I am confused 

is this a cold or a warm roof system 

 

a drawing please 

[junglejim]

thanks @scottishjohn sorry I’m not being very clear basically a normal roof but sections made in the factory and then assembled on site. It’s a cold roof. yes vaulted ceilings. 

 The roof layup from inside to out is: plasterboard - batten - ybs insulation - osb - insulation between rafters - air gap - osb - breather membrane - counter batten - batten - tiles 

Guidance from lots of sources is that airflow is needed between insulation and top osb. I’m just not sure whether I also need to cut the top membrane or because there isn’t a ridge which means air can flow across, whether I can get away without cutting. 
 

thanks for all your thoughts.

[scottishjohn]

It is not really a cold roof 

that is where all the occupied space is insulated from all of the loft and roof --eg flat ceilings

I am guessing as it has pB then it is occupieed below the insulated roof section 

I would call that a wram roof 

 so back to your question 

 I do not see any need to slit it 

as there will always be low pressure on one side of the house due to winds ,heating of roof  etc etc 

 so it should cause enough of a draft to pull from one side to the other,just make sure there are no cold bits where the eaves meets the walls 

Edited December 16, 2024 by scottishjohn

[Redbeard]

It's essentially a cold roof over room(s)-in-the-roof. A Warm Roof has all the insulation *above* the rafters. Whether or not the roof void is used as a room or just a cold void does not necessarily change whether it is a Cold roof - it is simply a Cold Roof on different planes. Then we can get onto hybrid Warm Roofs and interstital condensation risks, but we won't now!

[scottishjohn]

9 hours ago, Redbeard said:

A Warm Roof has all the insulation *above* the rafters.

or below  them

change the Pb for insulated PB  and loose maybe 35mm in head room,and you got a warm roof as it already has insulation above and only rafters are not insulated,but to be fair wood is a pretty good insulator 

 

[scottishjohn]

you only show  one baton

far better to use cross batons ,then there is no ledge for moisture or crap to build up on over the years

single batons with osb sheeting caused my house to rot  as all winter the dust  etc were damp ,,so 25 years on i had a problem  on the north side of the roof 

the extra cost of double batons is very small and also gives you a bigger gap 

 

[junglejim]

38 minutes ago, scottishjohn said:

or below  them

change the Pb for insulated PB  and loose maybe 35mm in head room,and you got a warm roof as it already has insulation above and only rafters are not insulated,but to be fair wood is a pretty good insulator 

 

Thank you for the advice. Yes I have counter batten and then batten… sorry not very good at drawing 😂

 

 that was my thought on venting and several people have said the same which is reassuring. My concern was whether Warner air would accumulate at the top and condense but my assumption is like you say the differential pressure one side to the other will ensure air flow.

[Redbeard]

 

Perhaps we can agree on another term for a roof with insulation all above the rafters?

 

I wrote:

 

13 hours ago, Redbeard said:

A Warm Roof has all the insulation *above* the rafters.

 

and @scottishjohn wrote:

2 hours ago, scottishjohn said:

or below  them

change the Pb for insulated PB  and loose maybe 35mm in head room,and you got a warm roof as it already has insulation above and only rafters are not insulated,but to be fair wood is a pretty good insulator 

 

If we change the 'title' of 'my' description (only 'my' in terms of my having used it above) to 'Warm Roof *Structure*' - meaning one where all the supporting timbers (or steels or whatever you like) are 'tucked up warm' below the insulation layer I think we've cracked it and avoided confusion. This means we can then also have a hybrid 'Warm Roof Structure' where some of the insulation is on top and some between the rafters.

 

Thoughts and comments welcomed.

 

 

[Redbeard]

Just seen @ToughButterCup's post and reference in this thread too. Very clear.

 

[Post and beam]

Yep that was my question from the other day. I have a lack of understanding of the role of the ceiling membrane in my cold roof .