Loft Insulation Question

PoGee

I'm finally getting round to clearing out the left over rubble from when the roof was done and putting in new loft insulation. Property is a semi built 1960s. No cavity wall insulation, and will keep it that way. The cavity area is open, when looking in from the attic. Should loft insulation cover this gap (wall cavity)? The old insulation that's been there since probably the 1980s, stops at the wall cavity. I know not to cover air vents but don't know about wall cavity bit. Hope someone can help.

greyteam1959

In general the advice is to leave the cavity uncovered by the loft insulation.
This is so the air in the cavity & the loft can move around preventing condensation.

stuart45

The main thing is to leave the eaves ventilation clear. The cavity is usually better off being closed. You can get a cavity sock to close it. Years ago they were sometimes left open, but realised that the convection currents made the inner skins too cold. They have been closed when built for quite a few years now.

Chickereeeee

Well, that's the two opinions.

If left open, warm(ish) from your cavities can rise up, carrying any moisture it might pick up, into your loft. This is not ideal, as the moisture could condense there, but if you have a a new roof with a proper breathable membrane, you probably will not get condensation. BUT you ARE dumping a lot of your expensive warmth into the loft, for it to blow away.

If you close the cavities at the top, you stop the warmth escaping. Any moisture in the cavity should condense on the cold, outer leaf of the wall, and fall down the cavity.

So, answer B, close the cavity, is current best practise.

ThisIsWeird

The insulation values of these walls will be abysmal. Pretty much - and certainly if they are vented - just a single solid block's worth.

Covering the gap at the top will likely help, but even then only marginally.

PoGee, you clearly have a reason for keeping it unfilled - are you in a driving-rain area? Do you have any other insulation plans? Lining the insides of these external walls will be the single biggest improvement that you can make - genuinely transformative. As well as draught-proofing the floors.

FFHillbilly

my house (built in 1975) has a closed cavity at the top in the form of a course of bricks half on the outer course, hanging over into to cavity, between the trusses. looks original from what I can tell, thy are all the same bricks as the rest of the house was built with

I would insulate the celling as far as you possibly can towards the outer walls, rockwool or fibreglass insulation is still not a vapour barrier no matter how well you install it, and it's difficult to get it right into the corners anyway so even if you tried you probably wouldn't block airflow into the loft

PoGee

Thanks for all advice.. I lined gable end internal wall with insulated wallpaper type roll (just hallway as was always freezing) and it's actually made the hall feel warmer. I'll cover wall cavity with loft insulation on gable wall and maybe leave other 2.

ThisIsWeird

Interesting. May I ask which insulated paper you used?
I can fully accept it helps.

(I recall an auntie from my childhood who lined her house with thin polystyrene sheet - 1/8th inch thick - and I was fascinated by how warm it felt to the touch. What a hazard, I realise now!)

FreeBear

ThisIsWeird said: (I recall an auntie from my childhood who lined her house with thin polystyrene sheet - 1/8th inch thick - and I was fascinated by how warm it felt to the touch. What a hazard, I realise now!)

Had a small room that had polystyrene tiles stuck to the ceiling and external walls - Didn't make the slightest of difference. And when I stripped the walls, the plaster behind was sodden wet.

PoGee

ThisIsWeird said:

Interesting. May I ask which insulated paper you used?
I can fully accept it helps.

(I recall an auntie from my childhood who lined her house with thin polystyrene sheet - 1/8th inch thick - and I was fascinated by how warm it felt to the touch. What a hazard, I realise now!)

It was from b n q. I think it's called Diall. I didn't use wallpaper paste because thought it would make a mess when taking off so .....put it up with double sided tape which was very awkward to work with but manageable.