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Loft extension - what should be done to ensure adequate ventilation?

(8 Posts)
Hollyhocks7 Mon 26-Jan-15 10:16:17

Hello, looking for a bit of advice as I think I have seen some discussion on this board about importance of ventilation in lofts to prevent condensation in the house. We are about to embark on a loft extension and while I assume the loft company will do a competent job I would like to understand for myself what the issues/potential problems are and the solutions. The l

Hollyhocks7 Mon 26-Jan-15 10:17:34

Sorry, didn't quite finish, the loft will be heavily insulated - am I right in thinking this can cause probs if not properly ventilated also?

PigletJohn Mon 26-Jan-15 10:52:01

insulation is not relevant to condensation.

What matters is:

the amount of water vapour in the air

the temperature of the air and the things it touches

Water vapour is lighter than air, so it naturally rises within a house until it either escapes, through ventilation, or finds a cold surface to condense on.

The envelope of your habitable space in the loft conversion must be fully insulated, especially the roof slopes. The Building Regulations approval will include the designers specification of how this will be done, it must be followed. The construction, especially the sloping ceiling, must prevent water vapour passing through (the design might include foil-backed plasterboard with taped joints or some other layer). Holes, especially for downlighters, are very pernicious.

The cold space outside your habitable envelope can be fully ventilated. Probably the roof will be built with a permeable membrane under the tiles which, unlike roofing felt, allows the passage of air and water vapour, so it can escape. Ventilating tiles can be added by the roofer if there is any doubt that this space is generously ventilated.

The habitable space must be well ventilated, for example with vents on at least two sides so there is airflow. Windows should have trickle vents. Some occupants have an aversion to ventilation and delight in closing or blocking vents and windows.

Anything that adds water vapour, especially a shower room, must have additional ventilation, such as a ducted fan. It must vent to the outside world. The approved design will include this, but to ensure that it is actually used, it is necessary to wire it to come on with the light switch and run on for long enough to dry the room. People with an aversion to ventilation will try to defeat this.

Because water vapour rises, any major sources, such as wet washing draped around the home, or unventilated bathrooms, or holes in ceilings below, will cause it to rise into the loft conversion.

Hollyhocks7 Mon 26-Jan-15 11:02:00

Thank you very much this is very helpful! So in terms of the habitable space itself, would trickle vents on the windows (plus opening the windows daily) be sufficient, or should we have air bricks too?

PigletJohn Mon 26-Jan-15 12:27:03

if there are trickle vents at each end, that would probably be enough. Window salesmen might tell you that the law currently does not force you to have trickle vents, which is true, but modern windows without them seal very tight.

If you do have air bricks just in case, you can put hit-and-miss vents on the inside to shut them when not required. If the windows look misty, that's a sign it is too humid (you will only see mist in the winter though). Velux or similar roof lights will be closed a lot to keep rain out.

TeddyBee Mon 26-Jan-15 12:29:50

We just have trickle vents in ours and leave the window open when we shower (sorry PJ but I hate extractor fans and have no issues with opening windows). A year in and we have no damp problems. The uninhabitable bit of loft is massively ventilated, roofing membrane under the tiles, a vent file on each side of the roof and quite a lot of daylight visible in there. It's absolutely freezing in the loft but toasty in the room below, which I assume is desirable? We got an insulated loft hatch with built in ladder fitted too.

PigletJohn Mon 26-Jan-15 12:36:19

yes, lofts and underfloor voids should be cold and breezy.

Hollyhocks7 Mon 26-Jan-15 12:56:39

Thanks to you both for your help, that's great - I can talk to the contractor about it now.

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