Adding internal insulation to a Cavity Filled wall. Notifiable or Not

[mctb62]

Question just renovating a 1969 Detached house with a 50mm cavity. The cavity has been filled but i was looking at adding another 50mm insulation internally. I know the regs say its notifiable if it was a Solid wall as its were hot meets cold. But i can see not reference to Cavity filled walls as hot and cold are already separated.

 

[^woody^]

This type of work is normally caught by Part L1 - renovation of more than 25% of a thermal element, and would require an application.

 

[Ivor Windybottom]

Agree with the above. But who's going to know or care if you don't notify? You could download part L1 for free and check it's to standard for yourself.

I'd say it will need a vapour barrier, you need to treat the inner leaf as cold as it will now be coldER than the room so there's potential for condensation forming.

 

[mrrusty]

IMHO even if you apply for a BC application I bet you will still struggle to get a confirmed specification on what to do. The risk I see is the filled cavity, which means there is no airflow inside the wall to take away any interstitial condensation. If you make the internal brick much colder by putting it behind insulation there is an increased condensation risk.

As @Ivor Windybottom says you could install a vapour barrier, but can you actually manage to completely seal off the room air from the building structure? If the walls do get damp from interstitial condensation, their insulating properties will drastically reduce. Personally, as you already have cavity wall insulation, and if it is problem-free, I would leave it alone.

 

[^woody^]

A vapour barrier on a masonry wall?

 

[mrrusty]

Y. If you installed insulated PB it has the VCL already built in......

 

[Ivor Windybottom]

A vapour barrier on a masonry wall?

Who knows really, it's unusual.

I'm equating it to a floating timber floor over concrete. There you have, from top-down...

Floorboards
Vapour barrier
Insulation
Concrete

It's pretty much the same scenario, except plasterboard is replacing the floorboards. The vapour barrier needs to be on the inside of the insulation, before the point at which it cools so condensation could form.

As pointed out above, insulated plasterboard is effectively waterproof anyway so may be the most practical method.

 

[robinbanks]

as you already have cavity wall insulation, and if it is problem-free, I would leave it alone.

50mm of wool is not adequate insulation in the modern sense of the regulations. Even 50mm of wool + 50mm of PIR is inadequate

there's potential for condensation forming.

If the dewpoint is inside a block of PIR, no problem. If moist air can't reach a cold surface, no problem

 

[^woody^]

Who knows really, it's unusual.

I'm equating it to a floating timber floor over concrete. There you have, from top-down...

Floorboards
Vapour barrier
Insulation
Concrete

It's pretty much the same scenario, except plasterboard is replacing the floorboards. The vapour barrier needs to be on the inside of the insulation, before the point at which it cools so condensation could form.

As pointed out above, insulated plasterboard is effectively waterproof anyway so may be the most practical method.

No. You don't have a vapour barrier on a masonry wall, or a concrete floor, there is no purpose.

 

[Ivor Windybottom]

Pretty sure a timber floating floor over concrete should have a vapour barrier. Why not? It's not fundamentally different to a timber wall, you keep the damp interior air away from anything cold, e.g. the concrete floor slab.

Tell me what I'm missing.

 

[robinbanks]

Cold concrete floor slab should have insulation somewhere, then timber's not cold

 

[Ivor Windybottom]

Of course there's insulation, as there should be everywhere. But there's a point somewhere along the temperature profile where the warm air from the room could condense. My understanding is that a barrier should block this interior air from reaching this point, to prevent condensation.

My floor has (top-down)...

Chipboard
Thin polythene
Celotex
Skimmed kiln-dried sand
Concrete
Damp proof membrane
Sand
Compacted rock
Planet earth