MVHR and warm roof loft space

[Crofter]

Hopefully this is a simple question to answer.

When you have an uninhabited attic space that is within the heated and vapour lined envelope of the building, should it be served by the MVHR system?

My concern is that without some form of ventilation, warm humid air will collect up near the ridge.

 

If I were to fit an extract at ridge height, would I also need to fit a supply somewhere to ensure cross-flow? The access to this space will be by a simple DIY ceiling hatch so will not be particularly airtight.

[jfb]

I personally wouldn't worry about it - I have what you describe but no issues with moisture.

 

[A_L]

3 hours ago, Crofter said:

My concern is that without some form of ventilation, warm humid air will collect up near the ridge.

 

The roof/wall insulation will keep it sufficiently warm to prevent any condensation. If the MVHR/outside world exhaust/supply pipes run through the space they should be insulated with water vapour impermeable insulation (also if elsewhere).

[Crofter]

Thanks, that's reassuring. My MVHR is a bit oversized for the house so I had been toying with ventilating the loft space, however it's extra complication and prevents the system from doing such a good job of ventilating the main space, I presume.

 

A followup question: If you had the choice, would you make the loft space well sealed up, or encourage free ventilation from the habitable areas?

[jfb]

I'm not sure how much free ventilation I would as it might increase the sound levels in the bedrooms of the MVHR.

[AliG]

You said that the attic would be within the vapour lined space.

 

I think in this kind of build up, it would be normal that the vapour impermeable layer would be the ceiling and the roof would have a vapour permeable layer to allow moisture to leave.

 

Then moisture cannot rise up into the roof space and if it does it can get out through the roof.