Insulation and warm roof

[tonytonys]

hello

i have 100mm full fill cavity bats in the walls of a new extension.
the roof is a shallow pitched roof.
the insulation in the roof is going to be a warm roof design of

profiled tiles, breathable membrane, 50mm air gap (i know only 25 is needed), 75mm kingspan between rafters, 50mm under rafters, 25mm batten, plasterboards + skim.

is there any point in putting in over fascia vents? i know they are not actually needed but ive had mixed answers even from manufacturers. the pitch of the roof is below 15 degrees and so a top abutment vent cant be used and so if the fascia vent is used it wont have proper through ventilation however it would create a bit more air flow than not having one.


also, how far up should the cavity bat come or be cut off. should it just come upto the top of the wall plate and then then sloping kingspan go 100mm past the wall plate to touch the back edge of the cavity bat, keeping a gap over the kingspan right to the fascia/soffit. or should the cavity bat come right up past the wall plate and go right upto an eaves roll installed over the rafters to keep the 25mm air gap? or doesnt it really make any difference?

 

[geraldthehamster]

hello

i have 100mm full fill cavity bats in the walls of a new extension.
the roof is a shallow pitched roof.
the insulation in the roof is going to be a warm roof design of

profiled tiles, breathable membrane, 50mm air gap (i know only 25 is needed), 75mm kingspan between rafters, 50mm under rafters, 25mm batten, plasterboards + skim.

is there any point in putting in over fascia vents? i know they are not actually needed but ive had mixed answers even from manufacturers. the pitch of the roof is below 15 degrees and so a top abutment vent cant be used and so if the fascia vent is used it wont have proper through ventilation however it would create a bit more air flow than not having one.


also, how far up should the cavity bat come or be cut off. should it just come upto the top of the wall plate and then then sloping kingspan go 100mm past the wall plate to touch the back edge of the cavity bat, keeping a gap over the kingspan right to the fascia/soffit. or should the cavity bat come right up past the wall plate and go right upto an eaves roll installed over the rafters to keep the 25mm air gap? or doesnt it really make any difference?

In a warm roof the insulation is above the roof joists; yours sounds like a cold roof design, which would need ventilation.

Cheers
Richard

 

[tonytonys]

hi, its ventilated by breathable membrane. no further vents needed

 

[^woody^]

You have a cold roof

Whether it needs venting will depend on what membrane you used, as some aren't certified for that design unless vented

 

[tonytonys]

rubbershield is being used. i have spoken to manufacturers who agree that no extra ventilation is needed, however they were 50/50 on weather fascia vents would help or not. they confirmed that the breathable membrane and tiles being used was enough on its own.

the argument for fascia vents is if they are put in it may give some extra ventilation to help with moisture however without vents at the top abutment flashing then you could also argue that there isnt any through ventilation and so a bit of a waste of time.
also if the room below is insulated with kingspan style boards with all joints taped to form a vapour control layer then the breather membrane on its own should be enough to deal with any little moisture and adding vents may not be a good idea because it will add in colder air.

they are not actually required, but im thinking of adding in the over fascia vents. then it will be eaves tray, breathable membrane and eaves comb because of profiled tiles.

 

[geraldthehamster]

rubbershield is being used. i have spoken to manufacturers who agree that no extra ventilation is needed, however they were 50/50 on weather fascia vents would help or not. they confirmed that the breathable membrane and tiles being used was enough on its own.

the argument for fascia vents is if they are put in it may give some extra ventilation to help with moisture however without vents at the top abutment flashing then you could also argue that there isnt any through ventilation and so a bit of a waste of time.
also if the room below is insulated with kingspan style boards with all joints taped to form a vapour control layer then the breather membrane on its own should be enough to deal with any little moisture and adding vents may not be a good idea because it will add in colder air.

they are not actually required, but im thinking of adding in the over fascia vents. then it will be eaves tray, breathable membrane and eaves comb because of profiled tiles.

I'm not clear what your roof looks like. Is it a double pitched roof, a mono pitched roof, or a flat roof with a fall on it?

Cheers
Richard

 

[^woody^]

The manufacturers can't have it both ways. The membrane either needs venting or it does not. It can't be a case of some vents might help.

The membrane should have a BBA certificate, and it should be installed in accordance with that, as that is what a BCO will be checking

 

[tonytonys]

hi
its a lean to roof with gable and 1 abut wall.
shallow pitch of 12.5 degrees with correct tiles to suit pitch.

rubershield trade doesnt need any extra vents, the BBA confirms that and their technical dept.

unvented cold roof, profiled tiles, 25mm batten, breathable membrane, min 25mm air gap, 75 insualtion between rafters, 50 under rafters, joints taped with foil tape, 25mm batten, 12.5 plasterboard, 3mm skim.

it doesnt have to have over fascia vents or soffit vents, the question is will it be of any benefit or help if they are added. the cost is pennies to buy them, and various conflicting advice is flying around as to weather they will add in a benefit to vent the roof or not.

 

[^woody^]

If the membrane does not need venting, then there can be no benefit in fitting any vents

 

[ey143]

Could someone please explain the criteria for deciding on a cold or warm roof design? Is one better or warmer for the room than the other? Is cost an implication or convenience?

 

[AronSearle]

hi, its ventilated by breathable membrane. no further vents needed

Breather membranes do not create ventilation, they simply allow the passage of vapour it whilst maintaining air tightness.

Rubbershield is a low resistance membrane, which means ventillation may be provided by gaps in the tiles, but you still need to allow for drainage (draping the membrane between rafters, or batten and cross battens).

Non-ventilated Cold Roofs

For design purposes, the resistance to water vapour transmission of Rubershield-Trade may be taken as not
more than 0.25 MNs/g. This value can be used in roof designs shown in Section 8.4 of BS 5250:2002. For roofs
designed in accordance with BS 5534:2003. When installed in an unsupported system, the product can be fixed
in the traditional manner for roof tile underlays, i.e. draped between the rafters, providing that the intended tiles
or slates are sufficiently air open (if in doubt, seek advice from the tile / slate manufacturer and see note below).
However, it is recommended that Rubershield-Trade is fixed to the rafters using counter battens with no drape
(ie. pulled taut) prior to installing the tiling battens. This construction provides a naturally vented batten space
between the underlay and the tiles or slates.

NOTE: Some artificial slates and tiles are close fitting which may impede the passage of water vapour through
the roof covering. In these circumstances, it will be necessary to ventilate the space between the underlay and
the tiles. This can be achieved by using counter battens and providing appropriate ventilation apertures at the
eaves to provide a naturally ventilated batten space. Alternatively, sufficient vent tile/slates can be used to
provide adequate ventilation to the batten space. Guidance should be sought from the vent tile manufacturer
and BS 5250.

The minimum thickness for tiling battens and counter battens must be 25mm

 

[freddiemercurystwin]

I wish people would check the dates and not drag up old threads (ey143!), it really doesn't help.

 

[AronSearle]

Balls!