Ventilation for loft conversion

[wwwebber]

Hi,

I am currently in the planning stage of of enhancing an old loft conversion (better insulation, reinforced floor beams etc). At the moment air whistles up through the floorboards via the eves.

This of course needs to be sealed off if the loft is to become fully habitable but I am conscious of having to ensure the rooms are still ventilated to prevent condensation etc.

For info, the roof doesn't have a membrane and I will be inserting celotex in between the rafters ,covering with a vapour barrier and then finally I will re-install the existing insulated plasterboard over the top. So the rafters will be well insulated due to no membrane.

So what else do I need to look at here regarding ventilation ?

Much appreciated. Thank you.

 

[FRAN1870]

Airflow needs to be retained between the rafters, so leave a gap between celotex and roof tiles, think its 25mm min....
You will also need to insulate between the floor joists and purlin walls.

I assume you know it can't be an habitable room until it's signed off by BC

 

[wwwebber]

I don't need an air gap apparently as there is no membrane but a small gap has been recommended.

Purlin walls ? you mean leave the space behind them un-insulated ?

Its already a habitable room - its an 1820s house and the loft used to be servants quarters

 

[FRAN1870]

I’ve got a breathable membrane and was told the gap was mandatory but hey ho.
If it’s already a room then I think you still need building control approval as it changes a thermal element?

Anyway I assume the room has side walls? you need to insulate these too. You could probably get away with rockwool to block up drafts between the joists. I added mine all the way across to stop noise.

So rockwool between joists, rockwool or kingspan for side walls and kingspan for between rafters.

Otherwise it I’ll be cold in the winter and boiling in the summer

 

[wwwebber]

I’ve got a breathable membrane and was told the gap was mandatory but hey ho.
If it’s already a room then I think you still need building control approval as it changes a thermal element?

Anyway I assume the room has side walls? you need to insulate these too. You could probably get away with rockwool to block up drafts between the joists. I added mine all the way across to stop noise.

So rockwool between joists, rockwool or kingspan for side walls and kingspan for between rafters.

Otherwise it I’ll be cold in the winter and boiling in the summer

Most of it is non compliant anyway due to its age etc. I am just making some small improvements which will still leave some elements non compliant. For that reason I will not be seeking BC approval.

Yes there are side walls - I was hoping to make the areas behind them available for storage etc so I will probs need some insulated doors or alternatively insulate all the way to the back to and including the eaves.

 

[FRAN1870]

Ok cool.... BC are pretty leanient with older buildings though tbh.

Just curious but if it’s already a room then how come you need to reinforce the floor beams?

 

[wwwebber]

The floor beams are only 3x2 across 3m spans :O . They've survived this long without issue but I want to do something about it whilst the floor boards are up etc.

 

[FRAN1870]

Mine were the same, bit piddly for a floor given that my first floor ones are 10x3 monsters .... there were a few patches of floorboard in my attic so I think there may of been a room up there in days gone by.

I found the remains of a bell system too ... wife told me where to go when I said about reinstating it lol