Sub Floor detail

Villain

Hi,

A mate of mine has given a sub floor detail, which is attached, I can understand it and think it will work well the only one question I really have is I'm going with a HRV system and want to make the house as air tight as possible and I was wondering will the area circled in red cause me problems with air tightness?

I will be putting down a breathable membrane over the joist's

Thanks

ircoha

see details here:
http://www.planningportal.gov.uk/england/professionals/en/1115314255968.html

u will see where the additional insulation is fitted as well as the flexible sealant.

Try see the Homebond manual for where the dpc is fitted in relation to the finished floor level and also under 'wallplate' on dwarf walls.

I would not be happy with the current position of the dpc in the wall

What flooring material are u using?
If sheeting as u will see in the link all edges need to be supported

Villain

Thanks thats a great link

ircoha

No problem.

see the attached, courtesy of Rockwool, for where they show the dpc.

http://www.rockwool.co.uk/sw47799.asp

Also would be interested in what the others think about the 'breathable membrane' idea

ircoha

ircoha wrote: »

Also would be interested in what the others think about the 'breathable membrane' idea

Would it allow moisture to rise from the sub-floor?

muffler

The DPC should be along the line of the underside of the joists. You will also have to allow for ventilation of the joists which will then give you air movement. A mastic sealant applied at the point you circled in red will help in this regard.

Although not shown in the sketch do ensure that the top of the sub floor is above the finished ground level externally.

ircoha

muffler wrote: »

The DPC should be along the line of the underside of the joists. You will also have to allow for ventilation of the joists which will then give you air movement. A mastic sealant applied at the point you circled in red will help in this regard.

Although not shown in the sketch do ensure that the top of the sub floor is above the finished ground level externally.

The last point is interesting:
following the Home Bond manual would suggest that the following applies
18mm flooring
100mm by 44 Class C14 joists at 300mm centres
150mm min from top of subfloor to underside of joist


so FFL should be a minimum of 268 above external ground level.

I don't this is happening in the attached picture.

From the same manual I take it that a concrete floor should follow the same rules
18mm flooring say
150mm concrete slab
100mm rigid insulation
mm DPC/radon
mm blinding
150mm hardcore.

Mellor

A concrete floor doesn't need to follow the same rule. It should be level with the external, the reason the the suspended timber floor is above is because the level of the concrete subfloor must be level. If you have a concrete subfloor, it is the finished floor that must be level. Im not sure if this is what you meant or not, as the build up for concrete has no subfloor, but included for reference.

sinnerboy

sub floor ( below timber floor ) must be at least same level as ground , so it does not become like a little paddling pool with surface water ingress . so when you start to buildind tasslw walls , vent space wall plates and joists over you end up with about 300mm FFL above ground level

top of concrete floor slab ( i.e. concrete only - not timber floor construction ) should be 150mm above ground level ( i.e. at same level as DPC course )

a nice thing to do with a refurb is to remove the timber floor entirely replacing with concrete and thereby and drop the FFL - increasing the headroom . you have to watch details around DPC and add a step to the stairway , but the affect in the internal spaces can be really good