Re: Re: New Part L Revision re Applications lodged after Nov 201
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Just as a matter of interest;
Post occupancy, what happens when the owner/occupier puts up shelves, pictures, flat TVs, curtains or new lights on various walls. Surely the perfectly assmbled, detailed and airtight proofed environment is compromised. Worse still because of the semi pressurised environment any compromise will focus the movement of vapour to a critically vulnerable location in the wall system.
this isnt so much an issue with a masonry house, as the plaster layer is the airtightness layer. simple eduction to the occupier as to how best finish drill holes would suffice ie plenty of caulk
in a TF house it can be an issue but one that should be designed to minimise any risk of puncturing the air tighness membrane. this is usually done by incorporating a service cavity of say 75 – 100 mm between the inner face an dthe timber frame. The air tightness membrane is affixed to the frame so there is a depth protection by the service cavity. Also any good TF spec should include a strong OSB inner lining prior to plaster boarding. This can easily act as a ground for most domestic applications. In cases where there is doubt, the spec should be altered to include cementious board.
basically, fore sight and education is required. it would be a very foolish occupier to pay extra for an air tightness strategy and MHRV and then not know how it affects the dwelling.
as for the pressurisation point, i think you put way too much credit into the ability of a HRv system to create pressure differentials.
A simply day like today where it is say 20 degrees internal climate and 4 degress external would create a vast greater pressure differential than any HRV system could. The investigation of condensation issues should be carried out regardless of any percieved differential anyway. Most problems actually occur when there is negative pressure in summer times when its warmer outside thus creating a situation where condensate moisture is drawn back into the building rather than being allowed to exhaust outward. The use of non breathable materials along the inner face is what causes problems in this situation eg drylining with polythene DPMs etc.