How widely are they used and in what applications ?
A2 Breather membranes have been used abroad in construction since the 1970’s.
Membranes were first Certified by the BBA in 1982 as sheathing membranes for timber frame
walls and as roof tile underlays in conventionally ventilated cold roofs. They were
subsequently approved as roof tile underlays in warm roof constructions in 1982 and most
recently in cold non ventilated roof constructions in 1999. At the time of writing this article,
Certificates for roof tile underlays used as part of a cold unventilated pitched roof system in
dwellings have been issued as shown in Table 1
Table 1 – BBA Certificates issued for cold unventilated roof applications (at 15.03.04)
Certificate No. Product name Certificate holder
99/3648 Daltex Roofshield Don & Low Ltd Nonwovens
00/3749 Permo Forte Klober Ltd
01/3842 Global Breather Roof Tile
Underlay
EBC UK Ltd
03/4017 Breathline Vapour permeable
Underlay
Knauf Insulation Ltd
03/4062 Monarperm 700 Icopal Ltd
04/4078 - DS3 VapR-free underlay Mercury Building products Ltd
04/4101 Tyvek Universal Underlays DuPont de Nemours (Luxembourg) Sarl
Details of approved products can be found on www.bbacerts.co.uk or by phoning the BBA
hotline on 01923 665400. Users are advised to check the validity of Certificates and any
limitations on the scope of the approval.
Q3 Can they be used simply ‘instead’ of conventional eaves ventilation ?
A3 Only if a number of measures are taken to limit the ingress of water vapour into the
unventilated space. In this regard, the roof system must be considered as a whole from ceiling
to roof tiles, including the following design points:
- all penetrations into the roof space must be properly sealed
- loft hatches must incorporate effective compressible draught seals
- the rooms below the ceiling must include provision for the dispersal and rapid dilution of
water vapour in accordance with the Building Regulations, including extractor fans in
rooms that may experience high humidity
- All water tanks in the loft space must be covered
- Any vent pipes should be arranged so that they do not discharge water vapour into the loft
space
See also Q&A10 to 15 and the relevant BBA Certificate for conditions of approval.
Q4 What are the advantages of using the unventilated system ?
A4 One of the main advantages is reducing the amount of heat lost by air leakage through the
ceiling and from the loft space to the outside. Up to 25% of the heat lost through a
conventional roof system is by this mechanism. The unventilated roof ‘system’ will reduce
this mechanism of heat loss, more so if the underlay laps are sealed.
In addition, not having to provide eaves and ridge ventilation can help to save time and cost
on site and can give a more desirable appearance to the finished roof.
Q5 What are the disadvantages of using the system ?
A5 Extra time and care is needed to ensure that penetrations into the ceiling are properly sealed
and that the loft space is protected from sources of water vapour, in accordance with the
requirements of the Certificate. See also A3.
If the ceiling is constructed without due attention to sealing joints and around penetrations, the
opportunity for moisture ingress into the loft space and consequent risk of condensation
occurring is increased. This is also true of conventionally ventilated constructions where the
effect of wind movement over the roof can reduce the air pressure in the loft space and ‘suck’
moisture laden warm air from the living space into the loft space. However it is recognised
that the rate of moisture removal via the ventilated and unventilated approach is not the same.
It is therefore important that the ceiling is sealed to an appropriate level. BBA computer
modelling of the behaviour of roofs has assumed effective ceiling ‘defect’ areas of up to 10
square mm per square metre of ceiling. See also Q/A 6, 7 and 11.
There are reports concerning ‘flapping-noise’ in roofs incorporating some types of permeable
roof tile underlay membranes in certain wind conditions. The BBA has, however, only
received a very limited number of reports in this regard and therefore do not view it as a
fundamental problem.

 

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引用

Breathable membrane for house roof

Breathable membranes have been used in buildings for many years now and, more recently, in cold
pitched roof constructions without traditional eaves ventilation. The benefit of reduced heat loss and
not having to incorporate ventilators has seen their use grow but at the same time has attracted
noticeable comment both from supporters of the application and from detractors who favour traditional
ventilation solutions.

 

What is a breather membrane and how does it work ?
A1 The particular membranes in question typically comprise spunbonded polypropylene or
spunbonded polypropylene/polyethylene laminated either side of a micro-porous
polypropylene or polyethylene film.
Monolithic spunbonded membranes are also used. They have a structure that is sufficiently
fine to prevent liquid water penetration in service conditions, but not too fine to prevent the
transfer of water vapour (molecules of water in air), see figure 1.
All materials, including vapour control layers, have a finite permeability to water vapour
transfer. However, in the context of this Article, a breathable membrane is a material which, in
service conditions, is sufficiently permeable to water vapour transfer to adequately limit the
risk of condensation in roofs.
Current published definitions include the following:
BS 5250: 2002 section 3.2 “Breather membrane” – vapour permeable membrane with a
vapour resistance less than 0.6 MNs/g. The Standard goes on to describe the use of ‘Breather
membranes’ in walls.
BS 5250: 2002 section 3.26 “Vapour permeable underlay” – Pitched roof underlay which
meets the recommendations of BS 5534-1 and NFRC TB6 and is additionally water vapour
permeable with a vapour resistance of less than 0.25 MNs/g
These two figures can also be expressed as water vapour transmission rates of at least 341 or
820 g/m2/day respectively, when tested under standard conditions of 25o C and humidity of 0%
rh and 75% rh on each side of the membrane.

The function of these membranes is to provide all of the normal demands made on a roof
underlay and at the same time allow water vapour egress without making specific provision
for ventilation.

 

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