Draught from cavity - insulation between ceiling and floor?

[johnsheffield]

The house we've bought is a split-level 2 bed made in the late 70s. Over the last couple of months, our first winter, a considerable draught has been felt in the back and front bedrooms - as bad as having a window open with a moderate wind - with the windows closed.

The floors in these two rooms are badly damaged and I will replace them in the next month or so, like for like i.e 22mm chipboard. I unscrewed a piece which an electrician had cut out to install a socket and found that the draught appears to be coming from the cavity, through the unplastered blocks and travelling in the 'channels' created by the joists, from the windows to the centre wall of the property.

My question therefore is about how do I go about insulating the perimeter of the rooms below floor level (i.e between ceiling and floor)(both with two outside walls)? I've checked the outside of the property for obvious signs of wind ingress like air bricks (none), pipes, cables, taps, vents, cracks between the bricks, badly installed windows etc and have remedied the couple of problems I did find - the silicone sealant around one window wasn't good and the outdoor tap had a small area around it that wasn't secure. The draught actually intensified after that.

We enquired about cavity wall insulation but the guy who came out to survey the property said we had dirty cavities, lots of debris and snots so said he wouldn't recommend the work to be done.

When I change the floors later this year would parging/plastering the blocks below floor level be OK, that might stop the draught egress from the cavity into the voids below floor? The draught only appears to be where the walls aren't plastered, i.e below floor. Can that be done or is there a reason not to do it? I really don't have any other ideas about what to do.

Can anybody offer help/suggestions?

John Sheffield

 

[JohnD]

1) rip up the old chipboard and put down WBP ply instead 18mm is fine unless you need 22mm for the levels. It is stronger, more rigid, quieter, and does not crumble and fall apart. As it is stronger you can lay it without T&G but nog any unsupported edges.

2) You can fill gaps between bricks with mortar or expanding foam. You can get a fireproof grade of foam which I think gives an extra benefit.

3) You can pack against the wall and between the joists with insulation. You can also take up windowboards and pack here with wool or expanding foam, and inject foam round the windowframes. I am inclined to the Knauf mineral wool with the Ecose treatment that prevents it shedding dust or loose fibres. If you pack the entire floor it will also improve sound insulation to some extent (though the heavy, dense wool slabs are more effective) as well as preventing heat loss and draughts.

Loft insulation is heavily subsidised this winter and you can get the big rolls very cheaply. You can cut them with a breadknife. I use gloves and mask anyway as I am prone to coughing.

 

[ajrobb]

I used a mortar gun to inject mortar between the joists and the bricks. Some of the gaps were over 20mm. This also made the floor less bouncy. I've read a local authority modern building manual says to fill the gaps with mortar leaving a groove 10mm deep to take a bead of mastic to allow for shrinkage. I reckoned that after 30+ years, there wasn't going to be much more shrinkage.

You might be interested in Tony tray for some background reading.

The joists usually have noggins or herringbone bracing mid-span. If there is no end block between the last joists and the walls, consider putting one in mid-span.

Regarding wall insulation; new rules came in last October (look on planningportal.gov.uk for Approved Document L1B). Apparently this work is notifiable to the Local Authority Building Control. A typical cavity wall has a U-value of 1.7 W/m²/K. If you insulate existing walls internally (or externally), you HAVE to take the the U-value down to 0.3 at least (Table 3) unless there are technical factors that prevent this. That needs something like 65mm of PIR (Kingspan K17).