Venting a Cold Attic in New Build

BarneyMc

Does a cold attic need vents in the roof/soffit/etc. or is it ok to just use a breathable membrane under the slates on a pitched roof?

BryanF

Can we see the detail drawing?

BarneyMc

Stonework to the front (right hand side of picture).

Won't be using fink trusses but the "open" ones to allow movement in the attic.

sydthebeat

BarneyMc wrote: »

Does a cold attic need vents in the roof/soffit/etc. or is it ok to just use a breathable membrane under the slates on a pitched roof?

I did a detailed thread on this a few years back. On my phone so i can't search for it.

BryanF

BarneyMc wrote: »

Does a cold attic need vents in the roof/soffit/etc. or is it ok to just use a breathable membrane under the slates on a pitched roof?

I would put in traditional soffit vents and 50m air gap.

BarneyMc

sydthebeat wrote: »

I did a detailed thread on this a few years back. On my phone so i can't search for it.

What were the basics? Vents as BryanF suggests?

archtech

Did you ask your professional for his/her opinion advise ?

Personally I would alway maintain a ventilation gap with breather membrane.

PROJECT K

Hi barney, using a vapour permeable breather membrane negates the need for traditional ventilation - the ventilation is required to remove excess moisture but if the membrane is vapour permeable then it can exit the building that way.

Typically in Ireland vents and min. 50mm airspace is adopted as a typical "to be sure to be sure" attitude but this is not required. I would consult the membrane supplier for their specific advice in any case.

Theres a lot of research by the BRE on the impact of rapid air movement on the performance of all insulation (but particularly fibrous/loose fill), and as a result "windtightness" as distinct from airtightness is becoming an increasing concern...

BarneyMc

archtech wrote: »

Did you ask your professional for his/her opinion advise ?

Personally I would alway maintain a ventilation gap with breather membrane.

Plan was to go ahead with a ventilation gap as everyone here suggests so will stick to that.

The only reason why I ask is that I was speaking to someone this week who suggested not allowing for a ventilation gap and just let the breather membrane do the breathing.

sydthebeat

PROJECT K wrote: »

Hi barney, using a vapour permeable breather membrane negates the need for traditional ventilation - the ventilation is required to remove excess moisture but if the membrane is vapour permeable then it can exit the building that way.
..

No it doesn't

http://www.boards.ie/vbulletin/showthread.php?p=68250588

please read

4Sticks

sydthebeat wrote: »

No it doesn't

http://www.boards.ie/vbulletin/showthread.php?p=68250588

please read

+1

I have seen too many "breathing" felts sweat like a sauna. Always ventilate.

4Sticks

BarneyMc wrote: »

The only reason why I ask is that I was speaking to someone this week who suggested not allowing for a ventilation gap and just let the breather membrane do the breathing.

Dont believe the hype

BarneyMc

Thanks everyone for your time and knowledge. I will be ventilating!

declan52

BarneyMc wrote: »

Thanks everyone for your time and knowledge. I will be ventilating!

if you have used a dry ridge the felt should be cut to allow ventilation at the ridge. For the gap if its breathable felt you still have to leave a gap but this can be as little as 10mm all depends on felt manufacturers guidelines. It has to be below a certain value something like 0.25 vapour something can't remember fully. Building control checked mine out and were happy as long as the roof was vented at the soffit and ridge and minimum of 10mm under the felt.

PROJECT K

http://www.dorsetforyou.com/media.jsp?mediaid=153063&filetype=pdf

See above link for an independant overview by BBA - i admit that care is needed to minimise moisture ingress to the attic but this is critical either way. If the ceiling does not have adequate vapour resistance the effect of wind blowing through your attic will create a negative pressure and suck moisture (perhaps more than even the designed ventilation can deal with) and heat through the ceiling element if not properly sealed

Note also the research indicates up to 25% reduction in thermal performance compared to theoretical U-Values due to the impact of this air movement. This is backed up by numerous research docs and case studies.

I have seen issues where 'breather membranes" as opposed to LR type (as per BS 5250) Vapour Permeable membranes are used - moisture build up can be an issue as vapour resistance can be much higher than 0.6MNs/g. VP membranes must be less than 0.25MNS/g and have been simulated, tested and approved by the BBA (and of course 'verified' by our very own IAB/NSAI for what its worth!).

Some of the points on the above links are valid (although i do despair at the reference to the TGDs as "the Building Regulations"), particularly where low permeable insulations are used tight to the underside of the membrane as this concentrates water vapour in a very small area (immediately beside the rafter) and this I would not recommended (apologies if this is what you intend - I should have clarified that with you at the outset).

Good luck with the build anyway, i am not trying to sway you either way - just making you aware of the options!

(PS i am not in the business of selling membranes or have no vested interest either way!):)

Regards
PK

JD6910

i put the following layers:

150MM rafters filled with 150mm metac with an airtight membrane on the underside along with 62.5mm insulated slab.
tyvek supro plus felt
batten and counter batten creating a 50MM gap
slate

the tyek supro plus is breathable, but i still created the 50mm air gap also.
i did airtightness within the house.
i left 2 slate vents - one at either end of the house to allow cross ventilation of the attic space. no air can come down into the living space because of the airtight membrane. i have no trap door, because my attic is absolutely tiny - only circa 900mm high.

sydthebeat

PROJECT K wrote: »

http://www.dorsetforyou.com/media.jsp?mediaid=153063&filetype=pdf

See above link for an independant overview by BBA - i admit that care is needed to minimise moisture ingress to the attic but this is critical either way. If the ceiling does not have adequate vapour resistance the effect of wind blowing through your attic will create a negative pressure and suck moisture (perhaps more than even the designed ventilation can deal with) and heat through the ceiling element if not properly sealed

Note also the research indicates up to 25% reduction in thermal performance compared to theoretical U-Values due to the impact of this air movement. This is backed up by numerous research docs and case studies.

completely agree with you, and thats a warm roof construction is scientifically superior.

PROJECT K wrote: »

Some of the points on the above links are valid (although i do despair at the reference to the TGDs as "the Building Regulations"), particularly where low permeable insulations are used tight to the underside of the membrane as this concentrates water vapour in a very small area (immediately beside the rafter) and this I would not recommended (apologies if this is what you intend - I should have clarified that with you at the outset).

firstly 'building regulations' is phrase most lay people refer to the TGds as. Professionals understand the difference between the regulation and the guidance.

Secondly, unless a dynamic condensation risk analysis is carried out on the construction, then its very difficult for anyone to claim that the breather felts will work ok. These felts are moisture resistant, which means once condensation forms it cannot get out. The dew point will be somewhere within the vapour permeable insulation therefore will form on the underside of the felt and will cause problems.

My own personal opinion is either go fully cold or fully warm, but this neither/nor hybrid is a disaster waiting to happen and shouldnt be risked.

4Sticks

PROJECT K wrote: »

http://www.dorsetforyou.com/media.jsp?mediaid=153063&filetype=pdf

See above link for an independant overview by BBA - i admit that care is needed to minimise moisture ingress to the attic but this is critical either way. If the ceiling does not have adequate vapour resistance the effect of wind blowing through your attic will create a negative pressure and suck moisture (perhaps more than even the designed ventilation can deal with) and heat through the ceiling element if not properly sealed

Two either / or options can offset ( not eleimiante ) this

1. use board insulation only , not fibrous

2. Used extended vent cards to encourage wind flow up and over the fibrous insulation