Breather membrane sweating

[Moggaman]

Hi. I am in the middle of insulating the rafters on my self build. The windows and doors are in . I have most of the rafters done with a 50mm air gap behind. I have not the ceiling slabbed or insulated yet. 
it was brought to my attention that the membrane behind the PIR board is sweating a lot.. fine where there is no insulation but sweating alot where there is. I have no hearing yet. I used corotop blue breather felt… pls someone put my mind at ease… thanks 

 

[Dave Jones]

what ventilation do you have above that ? Needs to be open at eaves and ridge.

[Cpd]

Do you have soffits vents, what’s above the membrane, is there continuous ventilation from soffits to ridge. More information would be good. 
I have the 50mm gap behind the insulation with soffit vents and a vented area below the ridge, all bone dry. The photo shows the the open vented area below the ridge, I have roof vents and an open air path from one side of the building to the other . 220mm of PIR was then put up to close this area off and link into the insulation in the rafters. 

Edited November 26, 2021 by Cpd

[ADLIan]

To be expected. House drying out, ceiling not finished, no plasterboard, no AVCL…….It’ll be ok once ceiling etc finished correctly.

[Dave Jones]

remember its not sweating the dew layer is being pulled inside the insulation. You have a cold roof so make sure there is a LOT of ventilation all over the roof, open eaves or vented fascia and open ridge (dry).

[Moggaman]

17 hours ago, Dave Jones said:

what ventilation do you have above that ? Needs to be open at eaves and ridge.

It is open at eaves … eaves not finished yet and open into attic space which is uninsulated 

[Moggaman]

15 hours ago, ADLIan said:

To be expected. House drying out, ceiling not finished, no plasterboard, no AVCL…….It’ll be ok once ceiling etc finished correctly.

Thats what people are telling me!!

[Moggaman]

16 hours ago, Cpd said:

Do you have soffits vents, what’s above the membrane, is there continuous ventilation from soffits to ridge. More information would be good. 
I have the 50mm gap behind the insulation with soffit vents and a vented area below the ridge, all bone dry. The photo shows the the open vented area below the ridge, I have roof vents and an open air path from one side of the building to the other . 220mm of PIR was then put up to close this area off and link into the insulation in the rafters. 

Soffit is wide open currently!.yes 50mm air gap behind insulation into cold attic with vents on attic roof..

there was frost yesterday as well 

[Dave Jones]

1 minute ago, Moggaman said:

It is open at eaves … eaves not finished yet and open into attic space which is uninsulated 

 

in that case you have an airflow problem, can air circulate under the ridge or is it blocked off with a ridge beam ?

 

Some tile vents at the top can help to create a draft over it.

[Moggaman]

Some more pics.. 

[Moggaman]

Just now, Moggaman said:

Some more pics.. 

This is outside 

[Moggaman]

[Jilly]

Did you use battens to stop the celotex touching the membrane, to maintain the 50mm gap? There was much discussion on my site about this as I have F1 bituminous because of bats. A lot of people rely on the friction fit of the cellotex between the rafters, but I wanted to be sure that they didn't get knocked accidentally and impede the air flow. 

 

Do you have a dry ridge? If so, it should give an airflow down each bay to the eaves (our builder/roofer put the ridge onto the felt with no gap tho). The other option BC wanted was a tile vent in every bay (32!).  It was mooted that we could cut the felt, but in the end we made a little cold roof and this satisfied them. Ie the air can come up either eaves and communicate across. This doesn't actually make any sense to me as hot air rises and needs to escape or it will condense, which is presumably what is happening to yours? I'm going to add more ventilation tho  and use a humidity monitor, as I'm a bit of a worrier! 

Edited November 27, 2021 by Jilly

[Moggaman]

23 hours ago, Jilly said:

Did you use battens to stop the celotex touching the membrane, to maintain the 50mm gap? There was much discussion on my site about this as I have F1 bituminous because of bats. A lot of people rely on the friction fit of the cellotex between the rafters, but I wanted to be sure that they didn't get knocked accidentally and impede the air flow. 

 

Do you have a dry ridge? If so, it should give an airflow down each bay to the eaves (our builder/roofer put the ridge onto the felt with no gap tho). The other option BC wanted was a tile vent in every bay (32!).  It was mooted that we could cut the felt, but in the end we made a little cold roof and this satisfied them. Ie the air can come up either eaves and communicate across. This doesn't actually make any sense to me as hot air rises and needs to escape or it will condense, which is presumably what is happening to yours? I'm going to add more ventilation tho  and use a humidity monitor, as I'm a bit of a worrier! 

The rafters are 175mm deep and my insulation is 125mm deep so the 50mm gap is definitely there. I have spend a lot of time putting in insulation and it is more than friction fit… it’s hard to remove !.