Loft Flooring: Ventilation Queries

[papafett]

Hi all,

To try and make it as easy as possible for anyone willing to offer advice, I've knocked up a diagram of the building:

I've bought an old single storey stone carriage house which has been converted in to a home (so very thick stone out walls, 2 or 3 inch gap then plasterboard mounted on a frame for heat/ventilation. As the building is difficult to heat, there is presently insulation between the ceiling rafters and a second layer across the top (perpendicular to the rafters) so its pretty deep.

I want to convert the loft space in to a storage area and had a couple of questions:

1) From what I head read online, ventilation is everything and so the flooring I put down should be suspended above the insultation. It's going to be quite a bit more work and cost - is it necessary given all the other ventilation?

2) I want to cover the loft ceiling. If I board it, air will only be able to move up and down between the rafters - is that enough or do I need to raise the boards off the ceiling a but to allow 'side to side' airflow too?

3) I wanted to cover the loft ceiling in 1/4 plywood but was wondering if there was lightly insulated version to trap a bit more heat in the space? I understand that insulating between the rafters is a no go (ventilation).

Any other tips or advice hugely appreciated - thanks!

Bob

 

[freddiemercurystwin]

Is the roof membrane a breathable one?

You don't have to worry about any air or ventilation gaps between the joists for the ceiling joists.

 

[papafett]

Hey Freddie,

Thanks for the reply. I'm 99% sure its felt but I'll have to check - is that breathable? When you say ventilation gaps between ceiling joists, do you mean the loft ceiling (AKA the roof verticals) or the actual ceiling (so ensuring space between the ceiling and new loft floor).

Bob

 

[papafett]

I was planning on trying to mount the loft ceiling plyboards on magnets so I can easily get behind them if I need to be, these magnets take 5kg per magnet (or 1kg shear) so four per panel should work great:
http://www.amazon.co.uk/gp/product/...iveASIN=B00TACK28Y&linkCode=as2&tag=spasto-21
This will actually provide a 3mm gap between the ceiling panels and joists anyway thinking about it!

I'm also keen to have the floor panels reasonably easy to lift - any tips on that anyone? All of the chipboard flooring is tongue and groove which could be a nightmare if I need to get to the underneath.

I've updated my diagram to show what I'm hoping to do (in red):

 

[freddiemercurystwin]

Unfortunately if you have traditional felt then that is not breathable so the only way you can safely board under the rafters is to fit continuous ridge vents or a tile vent between each rafter at high level. Otherwise you will prevent the timbers from being able to breath and you risk problems with condensation as shown in the following pic:

I appreciate you have a floor in the loft but this is a good pic to illustrate the roof venting principle you would need to achieve. Though ff you have felt then why do you want to board the underside of the rafters anyway?

When I say ceiling joists I mean the horizontal joists you would stand on in the loft, or lets call it the loft floor. No need to vent that space. Typically you would fit counter joists perpendicular to your existing ones and then insulate between those too. Then board on top. If you are that worried about being able to access parts of the ceiling then just don't used T&G boards but just ensure any joints occur over a joist/noggin. You're not making a proper floor to have a party on just creating a storage space.