No room for air bricks - how can I ventilate my sub-floor?

[Border89]

Hi all, long time DIYer but first time posting here, I have searched for similar threads for the last hour or so but nobody seems to have faced the same issue on here just yet!

I bought my first house last year, c.1890 mid-terrace 2 up / 2 down with a roof conversion and a single storey extension out the back for the kitchen and bathroom. The house is on a hill leading down to a canal (the waterway is around 30m from the front of the house and ~8m down). The ground is pretty damp around here all the way along the street; damp marks pop up sporadically all the way along the pavement outside people's houses every now and then.

When I got stuck into cosmetic renovations I lifted the old laminate floor and found some damp floorboards. In the past someone had taken out a chimney breast and swept the lot under the floor, bridging the gap below the joists and letting it all get damp.

Work (/overkill) I've done so far to avoid this happening again:

• ripped the lot out (floorboards, joists and the crumbly sleeper walls) across the whole bottom floor, front to back
• dug down to a depth of a foot or so, and covered with damp-proof membrane
• rebuilt honeycomb sleeper walls with DPC below top course
• Fitted additional airbricks below front threshold (so there's now 6 airbricks at the front!)
• Laid new joists throughout, insulated between joists with 100mm kingspan held in place with garden netting
• Boarded with 22mm P5 T&G chipboard up to three quarters of the way to the rear of the house - and now the issue starts

Here's my problem: it's a mid-terrace house so no chance for airbricks at the sides (boo), there's plenty of airbricks at the front (yay!) but none at the back (damn.) With the house on a hill, the floor level at the rear of the house is ~400mm higher than the floorboards, so no easy way to get an airbrick in the rear wall!

BEFORE ANYONE SUGGESTS A PERISCOPE AIRBRICK - the next problem is that the rear extension sits on a concrete pad (without any ducting buried for ventilation) and this extends across two thirds of the house width. The final third is taken up by a door, so there isn't any free external wall available to squeeze even one airbrick into the rear of the house!

It looks like I may need to duct either under the rear path and up to either a stack or fan, with ductwork under the floor to ensure airflow across the whole width of the room. I can't think of any other possibilities or easy solutions - is there an obvious solution I'm missing?

Thanks in advance for any help!
Tom

 

[Border89]

Picture for clarity - the kitchen extends behind the wall with the radiator, and the external wall (perpendicular to the one pictured) sits around 80mm away from doorframe for the back door

 

[freddiemercurystwin]

You could just accept that you can only practically get air bricks at the front and then inspect the underfloor void during the wetter months/winter and in the spring. If there's no issues then your front vents are enough.

 

[Border89]

You could just accept that you can only practically get air bricks at the front and then inspect the underfloor void during the wetter months/winter and in the spring. If there's no issues then your front vents are enough.

When I pulled up the floor at the rear of the house it was pretty damp in that area - I'm guessing partially due to the high water table (can barely dig any further than I have without striking water!) but also there was nowhere for the air to go. There had been at least one set of repairs to the joists at the back of the house so I'm guessing it's been an issue in the past which makes me worry that without any cross-flow it'll just get bad again!

I'm planning on selling this place in a couple of years but I don't want to do half a job and then have some poor sod cursing me 10 years down the line when he falls through the floor!

 

[bernardgreen]

Is the area outside the back door exposed to rain ? If not then it may be possible to vent to a grill in set in the ground directly outside the back door.
Or dig down outside the back door deep enough install air bricks ( above the normal water table ) line the hole with brickwork ( or blockwork ) and then have a small area of decking over the hole.

 

[Border89]

Is the area outside the back door exposed to rain ? If not then it may be possible to vent to a grill in set in the ground directly outside the back door.
Or dig down outside the back door deep enough install air bricks ( above the normal water table ) line the hole with brickwork ( or blockwork ) and then have a small area of decking over the hole.

Thanks Bernard, we're looking into having a small porch (~6ft length) added to the rear of the house so that might be the time to bite the bullet, dig down and lay some ducting through to the outside before building above the path. Looks like there's an old 4" clay pipe down there already which I think is the kitchen drain from the original layout - it's been concreted up by the back door but a single storey extension for the kitchen was added around the '70s and the drain trap from there looks like it tees into the same pipe. There isn't much room width-wise so this might get fun!

 

[Border89]

Having thought about this a bit more, what's the best approach to take when burying ducting? Would 4" PVC be up to the job? depth will be around 300-400mm, say a 7 foot straight run then turning 90 degrees to a vertical stack.

What sort of covering should the pipework have? is it ok to bed in gravel then concrete over the top?

 

[Border89]

Having thought about this a bit more, what's the best approach to take when burying ducting? Would 4" PVC be up to the job? depth will be around 300-400mm, say a 7 foot straight run then turning 90 degrees to a vertical stack.

What sort of covering should the pipework have? is it ok to bed in gravel then concrete over the top?

Anyone? Had building control out this week for some of the other work I've had done and they didn't reckon I'd need any ventilation at the back now there's vapour membrane down across the whole subfloor - does that line up with people's experience on here?

 

[ree]

In your first post are you saying that the GL (ground level) in the back yard is 400mm higher than the FFL inside the kitchen & rear room?

Why not post a pic pointing at the base of the new PVC door from outside in the yard?

Do you have any c/breasts left in the ground floor?

When you dug out the sub-area did you go below the wall footings?

 

[Border89]

In your first post are you saying that the GL (ground level) in the back yard is 400mm higher than the FFL inside the kitchen & rear room?

yep, the outside ground level is ~400mm higher than the finished floor of the rear room. The kitchen is built at the same height as the back yard though i.e. 2 steps up from the rear room level, on a solid concrete floor

Why not post a pic pointing at the base of the new PVC door from outside in the yard?

Will do when I get a chance, not got one to hand sorry

Do you have any c/breasts left in the ground floor?

There is one left in the room in question, doesn't seem to flow well from the opening at about knee height though

When you dug out the sub-area did you go below the wall footings?

Nope, I made sure there were still at least a couple of courses left buried (I checked that far in a small area anyway, could be deeper still!)

 

[Border89]

found a picture actually - hope this helps!

 

[ree]

Thanks for the pic.

1. You could take on the task of dropping the back yard GL and running the falls towards the back yard gate?
As it stands, water could eventually penetrate below the PVC frame.

If the yard level is dropped you would have space for ventilation below the door. It would involve quite a bit of work.

2. Or, inside the room, close to the rear rad wall, you could cut two or three floor grill openings in the boarded floor - (google "floor grills"). This would provide a constant thro ventilation. Some floor grills are "Hit & Miss" for closing when desired.

What do you mean in reference to the c/breast: "doesn't seem to flow well ..."?

 

[Border89]

Right, just updating this in case anyone else needs to do the same...

I took the hit and got a trench dug all the way out for 110mm ducting. This joins the house at a periscope airbrick and sends fresh air under the floorboards without needing twice the trench depth.

The ducting comes up to an 8 foot stack with a decent cowl on the top to keep rain out and bring plenty of air in (or out, depending on which way the wind is blowing!)

Will get pictures when the new slab has cured enough for me to walk on it! Thanks everyone for your help and I hope this might be useful for anyone in similar circumstances in future

 

[JohnD]

so, have you got any chimneys at the back?

 

[Border89]

so, have you got any chimneys at the back?

one near the back of the house yes (just out of the bottom left-hand corner of the photo up at the top of the thread), although not sure how clear it is as it's not capped up top but the vent brick down on the ground floor doesn't seem to have any sort of air flow in or out