Cavity walls - making vents into 'tunnels' and other changes?

[mrcharly-YHT]

Our house is built of concrete blocks with a cavity walls.

There are numerous vents below the floor (good, because free air keeps the floor timbers in decent condition).

However, these vents are just grills on the outside wall and holes through the outer and inner blockwork. When it is windy, the wind blows up the cavity. There has been an attempt to insulate with those polystyrene beads, but this has poured out of the holes in the blockwork to the space under the floorboards.

So I was thinking of changing the vents to make them 'tunnels' through the blockwork to the space under the house. This would enable me to refill the insulation, and reduce the draft that currently screams up the cavity and around the upstairs (which is uninsulated, just plasterboard).

Also considering semi-blocking up the cavity at the top, to reduce the draft. Maybe with something like rockwool or similar.

I'm a bit concerned that the latter would start causing condensation issues in the blockwork, what does the panel think?

[Andy W]

The grills that you see on the surface of the blockwork are usually sleeved through to the far side of the cavity wall. Usually a two piece affair. The grill you can see is normally 75mm deep and the sleeve
abuts this and travels across the cavity, sitting on the inner leaf. Without examining your paticular house its difficult to comment further. Im a retired bricklayer BTW.

[mrcharly-YHT]

These ones definitely aren't sleeved. They are just a grill on the outside, then a rough hole knocked through the outer and inner blockwork.

They are the full height of a block, erm, I'd guess the grill part is about 6" and maybe 8" wide.

House built in the early 70s, and local standards are . . .  odd. Or missing.

[Andy W]

They should always be sleeved in order to provide clean airflow through to the inside of a building.
Fitting airbricks was often carried out retrospectively in the 70s and 80s because with the advent of sealed double glazing combined with either a coal fire or gas fire people died or became unwell due to both lack of oxygen and or carbon monoxide. My parents easily outwitted this by sellotaping a piece of cardboard over the vent cos it was draughty

[rogerzilla]

These are underfloor airbricks and serve a different purpose, i.e. stopping the floor from rotting.

[mrcharly-YHT]

Yes - there wasn't ventilation into the room with an open fire. The floor was thickly carpeted, the door snug on the carpet; I think it was a deathtrap, tbh. Certainly didn't draw well at all.

Carpet gone now, insulated under floorboards with a membrane. There is a section of flooring next to the stove (I replaced the fireplace with an inset stove) sans membrane that provides low-level airflow for the stove. I did contemplate putting in a grill if it seemed necessary, but the stove draws well.

[rogerzilla]

Off topic, I know, but my stove has outside secondary air (primary air is rarely used for wood). Makes it more efficient as it doesn't pull cold air in through every nook and cranny in the house.  It's a bit like a balanced flue, except that the air inlet is separate and low down.

[rafletcher]

Off topic, I know, but my stove has outside secondary air (primary air is rarely used for wood). Makes it more efficient as it doesn't pull cold air in through every nook and cranny in the house.  It's a bit like a balanced flue, except that the air inlet is separate and low down.

And our Baxi fireplace (open) has an underfloor duct from outside, and a flap valve on the front of the grate to regulate it. Used for initial draw to get it going.

[HTFB]

I'm a bit concerned that the latter would start causing condensation issues in the blockwork, what does the panel think?

I'm supposed to understand building physics, but interstitial dew points inside walls always defeat me. Handwavingly, the inner block is mostly close to room temperature and should stay well above the dew point if the room is at a habitable humidity. Decreasing airflow in the cavity should raise the temperature in the block so actually decrease the risk of condensation on the inner leaf (and although it would raise the risk in the outer leaf, that is allowed to be wet). Even if that reasoning is wrong, cavities aren't supposed to have howling gales through them and even with the sleeves there will still be free drainage for any surface condensation into the soil. So I can't imagine there's a problem.

[mrcharly-YHT]

I'm a bit concerned that the latter would start causing condensation issues in the blockwork, what does the panel think?

I'm supposed to understand building physics, but interstitial dew points inside walls always defeat me. Handwavingly, the inner block is mostly close to room temperature and should stay well above the dew point if the room is at a habitable humidity. Decreasing airflow in the cavity should raise the temperature in the block so actually decrease the risk of condensation on the inner leaf (and although it would raise the risk in the outer leaf, that is allowed to be wet). Even if that reasoning is wrong, cavities aren't supposed to have howling gales through them and even with the sleeves there will still be free drainage for any surface condensation into the soil. So I can't imagine there's a problem.

Thanks for that input.

I've tried looking this up, but have found 'definitive' sources saying that cavities should be capped, and others saying the exact opposite.

It is unclear to me whether 'weep points' (small airflow holes high up) are required. They would be difficult to fit to this type of blockwork (apart from being thick, the stone in it is incredibly hard and difficult to drill or cut).

I think I'll start by converting the vents to 'tunnels' somehow, and see what effect that has.

Doing that will also enable collection of the vast amount of polystyrene beads that have poured into the underfloor space, and re-addition of that to the wall cavity.

[Ian H]

There are caveats about cavity-wall insulation in exposed areas.

[rogerzilla]

There are caveats about cavity-wall insulation in exposed areas.

If you choose not to fit it, you can say you have caveaty wall insulation.

[HTFB]

There are caveats about cavity-wall insulation in exposed areas.

If you choose not to fit it, you can say you have caveaty wall insulation.

VG, will snarf.