Insulation and ventilation question

[rosswilliams21]

Hi guys, 

 

currently converting my loft conversion - 1965 semi with a cut roof. and I have some questions around the ventilation between the non-breathable felt and foil backed insulation.

 

I have bulked out the current 4x2 rafters with 4x2 and will be filling the void with 150mm celotex PIR and leaving the 50mm ventilation gap. I am wondering whether I will require a ridge vent or will it be sufficient to leave a 50mm continuous gap including under the ridge ridge so the 50mm runs from one side of the rood to the other?

 

I imagine this may be seen a a silly question, but every day is a school day.

 

Thanks in advance 

[rosswilliams21]

Also just noticed I have posted in the introduce yourself section! so hi i'm Ross - 26 years old and have been consistently renovating my first house since 2016

[MikeGrahamT21]

Minimum 25mm gap, so 50mm absolutely fine, and then put some wedges in the overlaps of the felt to open it up and allow air to flow freely.

[Jeremy Harris]

I found that offcuts of plastic waste pipe, about 6" long, worked well as wedges to provide better ventilation through sarking felt overlaps.  These were light enough to stay firmly in place just from friction.

[rosswilliams21]

Thanks guys, so I just need to wedge the starking open? does this mean i do not require a gap at the ridge and can insulate it fully?

[MikeGrahamT21]

As long as there are plenty of openings on both sides of the pitch, there will be plenty of airflow, I can't personally see the need for ridge ventilation, just make sure you leave a gap all the way to the ridge, and a gap from one side of the pitch to the other, so air can move right the way across. If you can't achieve this, then you may require ridge vents to ensure air gets all the way up, in which case dry ridge is your answer.

[rosswilliams21]

Brilliant thanks mike your a star, I should be able to provide a continuous gap from pitch to pitch and will aim to put an wedge in the starking at the bottom and top of the pitch between each rafter. 

[Russell griffiths]

Have you looked at a vapour control layer, as you will want to keep as much moisture and heat from escaping. 

[Big Jimbo]

Be careful, re venting at the ridge, or not as the case may be. I can't remember the technical, but something to do with the pitch, and the lenght of the rafter. It is not just as simple as saying, "I have gable ends, and vents at the front and the back"

I recently designed some trusses, and the engineer said if the pitch was 1 degree higher, i would have to vent the ridge.... So in my opinion, i would either vent the ridge, or have it checked. You do not want a soggy roof.

Edited March 4, 2020 by Big Jimbo
Missed a bit

[Temp]

Some BCO provide this advice of a version of same..

 

https://www.eden.gov.uk/planning-and-building/building-control/building-control-guidance-notes/roof-ventilation/

 

[Jenni]

Our BCO insisted on (what I considered) an excessive number of ridge vents, also he said minimal 50mm air gap under the felt. 

 

Edited March 4, 2020 by Jenni

[rosswilliams21]

Russle - do the foil backed boards not act as a VCL if I use foil tape of the joints and rafter? 

 

regarding ridge vents then it may be a case of asking BCO, I can just jump up and fit a dry ridge system when i fit the velux so its not a massive inconvenience but I would rather not if I didnt have to.

 

Thanks for the advice really appreciate it 

Edited March 5, 2020 by rosswilliams21

[Mike]

Most vapour-permeable sarking membranes have agreement certificates that enable them to be used with no ventilation gap between the felt and the insulation, and no ridge or eaves vents either, provided there is a vapour control layer on the inside of the roof / ceiling. Solitex Mento, Tyvek Supro, etc.