ducting airbrick through solid floor

[sm_jamieson]

Building new solid floor against wall with suspended wooden floor and airbricks. The top of the airbricks are on a level with the top of the floor slab (which is due to have 70mm screed on top for levelling purposes). The floor slab will be 150mm thick, and is on top of 75mm celotex and hardcore. An air path will be vented through the new cavity wall using periscope ducting. So to join the air bricks via ducting in the new solid floor.

I plan to cast a 2 to 3 inch high duct into the top of the floor slab when it is layed to join the old and new airbricks, then cover this with, say, a paving slabs before the screed goes down. I'll put some metal into the concrete under the duct area since it will only be around 3" thick there. Slab not thick enough to use 4" soil pipe as is commonly done.

Do folks think this this will be OK ? Yes, I'll end up asking the BCO of course.

Only other option is to use a perisope on both ends and duct it much lower down (cut though insulation layer or hardcore layer), but this is a more convoluted air path and does not seem ideal to me.

Thanks, Simon.

[Hugo Nebula]

On Mon, 23 Jun 2008 02:21:40 -0700 (PDT), a particular chimpanzee, sm_jamieson randomly hit the keyboard and produced:

The duct should be at least 100mm diameter, it should be covered by the insulation, and by at least 100mm of concrete. It needs to go through the hardcore. One hopes your original floor has a sub-floor at least 5" (125mm) below the bottom of your (at least 100mm deep) joists, so there should be plenty of space to take the pipe horizontally through the existing wall.

[sm_jamieson]

Is what you have said in the building regs, or just good practice ? What part of the building regs is it ?

I would just have to make an opening through 9" wall and block up the current air bricks. All possible. Sounds like you just treat it like a drainage pipe (ish). No problem with the vertical space for this.

Thanks, Simon.

[mail]

I have been doing this sort of specification professionally for over

30 years and Hugh is dead right on all counts. I, personally, would go to more than one 4" and preferably 6" pipes in the hard core. (Allow for insect mesh at the outer end and put some form of vermin mesh at the inner end of the pipe and leave a trap in the floorboards to gain access to the pipes in the future if you need to) Well done for realising the need for this I make a living out of those who dont bother or whose architects forget to put this detail in - it is usually wet rot in the plate abutting the new floor about 20 years later! chris

Chris

[Rod]

Many years ago I suffered from someone having done this job badly. The suspended floor slowly deteriorated until it lost strength and started to collapse.

Not worth skimping - at all - even if you can/could get it past the BCO.

[sm_jamieson]

There is currently two air bricks venting the back room. If I put two

4" pipes to replace, I think this should be enough, since there will be more area due to the pipes not having the restriction of the old clay airbricks. I'll have to go around a 90 degree bend to get the pipe to the correct position on the outside wall though. If this is in the regs I guess it would be part F ? Simon.

[sm_jamieson]

It turns out the sub-floor / original foundation is not low enough to have a pipe go out through the house wall and under the insulation in the hardcore layer of the new slab, without descending down a little way against the outside of the house wall. So it looks like I'll have to use a periscope style duct on both ends of the 4" pipe, one end in the new cavity, the other end against the house wall. A bit like:

at the house wall the new floor slab and old suspended floor will meet, I cannot have vertical insulation here anyway, like you do between a floor slab and exterior wall, because I need the solid floor to butt right up to the wall - there is no "edge" to hide the insulation board with skirting etc. I guess I could put celotex on the inside of the old house wall in the sub-floor void to keep the slab partly insulated from the cold underfloor void. There will still be cold bridging from the ground to the new slab, but not much can be done about this. Simon.

[Hugo Nebula]

On Mon, 23 Jun 2008 05:12:41 -0700 (PDT), a particular chimpanzee, sm_jamieson randomly hit the keyboard and produced:

Approved Document C, Section 4.14(b).

[sm_jamieson]

Hum. Looking through part C, noticed section 5. Says there should be 225mm clear vertical cavity above the cavity infilling. Good job I noticed that. I've always been told fill sloping down to ground level, when it should be sloping down from ground level, such that the slope ends 3 courses down. I've done a small stretch (with some spare concrete I had) one brick too high according to that. I'm sure (hope) I won't be made to change it (drilling out

75mm would be a pain), but you find some things out rather late. Part C was about the only doc. I hadn't scrutinized ! Cheers, Simon.