Ha. Who said being a gas man was easy :lol:
Having a sound knowledge of construction methods comes in handy. New (ish) roofs have been built to be fully ventilated for around 30 years or so now. Look up at the soffits for either a continuous vent strip (usually plastic) or round vents fitted about 2ft apart. If there is a dry ridge system on it (these have a continuous vent built in) look at the apex of the roof from inside to check the felt has been cut and folded back at the ridge (it should be). Tyvek is breathable. Just look at it. It is full of microholes. The only time it could theoretically become airtight is if it became heavily frosted over.
Apart from all that "common knowledge" :smile: if you look in BS5440 it says this
6.2.2 An air vent shall not communicate with a ventilated roof space or underfloor space if that space communicates with other premises. Where an air vent draws air from a ventilated roof space or ventilated underfloor space, the total effective free area of the grilles or louvres into that space shall be at least equal to the total effective free area of any air vents drawing air from that space. The ventilated roof space shall be visually inspected to confirm that it conforms to the minimum unrestricted ventilation requirements in BS 5250.
COMMENTARY ON 6.2.2
To avoid condensation and associated problems that can result from low ambient seasonal temperatures in the roof space, adequate ventilation should be provided. Existing air vents, grilles or louvres that have previously been installed to meet the requirements for roof space ventilation and that communicate directly to the outside air may have apertures of less than 5 mm, and are deemed to be acceptable if the total free area is satisfactory and openings are checked to ensure that they are free from obstruction. In order to prevent condensation occurring within a roof space that can result from low ambient seasonal temperatures, ventilation is normally required and further guidance can be found in BS 5250:2002, 8.4.2.1.2 and 8.4.2.2.3.
A roof space constructed in accordance with the ventilation recommendations in BS 5250:2002, 8.4.2.1.2 and 8.4.2.2.3, may be considered to be outside air.
It is important that any ventilation provision is not blocked at the eaves and at the point where the ventilation passes through the ceiling insulation into the dwelling. This can be caused by poorly installed roof/cavity wall insulation, for example. Some modern construction methods employ the use of a “breathable” roofing felt designed to allow similar ventilation of the roof space. Where this method has been used, the roof space should not be considered to be outside air for the purpose of gas appliance air supply, due to its propensity for reduced performance of the membrane over time.
It is also important that the requirements of Approved Document B regarding fire safety [10] are taken into consideration and that any ventilation provision for a gas appliance provided through the roof space or loft does not compromise fire safety.
Further guidance on ventilation and fire safety can be found in BS 5250.
A roof space not constructed to conform to BS 5250 can only be regarded as another internal space and therefore should be treated as such when calculating air vent free-areas.
BS 5250 is about preventing condensation in roofs (ventilation). You may find this interesting.
http://www.glidevale.com/downloads/condensation_control_bs5250_white_paper.pdf
A breathable membrane will not pass IMHO