Internal insulation understanding check

Barmy_Army_92

Hi, 

I have made a post before considering CWI vs IWI for my Victorian Terrace. I have unfilled cavities, I am not sure of what thickness etc - just as a sanity check, if I were to dot and dab insulated plasterboard to the inner leaf of brick as my method of IWI, there are no additional concerns as a result of having an unfilled cavity? 

Often I see IWI discussed for properties with a solid brick construction, so am struggling to find info relative to my type of property. 

Thankyou

FreeBear

Personally, I wouldn't dot'n'dab or use insulated plasterboard. Instead, Put a 50mm thick sheet of Celotex/Kingspan type PUR board up. Fix some 25x50mm battens using 120mm hammer fixings through the insulation boards into the brick. Squirt a bit of expanding foam in to any gaps, and then tape the joints with aluminium foil tape. Fill the space between the battens with 25mm thick Celotex, and again, fill gaps with expanding foam & tape the joints. Finally, fix plasterboard to the battens and optionally use some grab adhesive - Taking a break from doing just this on a solid brick wall..

Material costs will be a marginally lower at the expense of a bit more labour. But you get multiple layers of vapour control membrane (VCM) thanks to the foil coating on the insulation boards. If you have any sockets to install on these walls, the conduit & 35mm back boxes will sit nicely on top of the first layer of insulation boards. And with the 25mm board plus 12mm plasterboard, the back boxes will be flush with the finished surface.

Having an unfilled cavity will be advantageous as it will reduce the risk of interstitial condensation building up behind the insulation.

Why a sheet of 50mm+25mm of Celotex ?

This will get you well under the current u-value of 0.30 W/m²K required by Building Regulations - If you use EPS (polystyrene), you'd need to go to 100mm at least. And if you use insulated plasterboard, you don't get the free VCM.

If you can, put a 25mm thick layer of Celotex around the window & door reveals - This will reduce cold spots in these areas.

Barmy_Army_92

FreeBear said:

Personally, I wouldn't dot'n'dab or use insulated plasterboard. Instead, Put a 50mm thick sheet of Celotex/Kingspan type PUR board up. Fix some 25x50mm battens using 120mm hammer fixings through the insulation boards into the brick. Squirt a bit of expanding foam in to any gaps, and then tape the joints with aluminium foil tape. Fill the space between the battens with 25mm thick Celotex, and again, fill gaps with expanding foam & tape the joints. Finally, fix plasterboard to the battens and optionally use some grab adhesive - Taking a break from doing just this on a solid brick wall..

Material costs will be a marginally lower at the expense of a bit more labour. But you get multiple layers of vapour control membrane (VCM) thanks to the foil coating on the insulation boards. If you have any sockets to install on these walls, the conduit & 35mm back boxes will sit nicely on top of the first layer of insulation boards. And with the 25mm board plus 12mm plasterboard, the back boxes will be flush with the finished surface.

Having an unfilled cavity will be advantageous as it will reduce the risk of interstitial condensation building up behind the insulation.

Why a sheet of 50mm+25mm of Celotex ?

This will get you well under the current u-value of 0.30 W/m²K required by Building Regulations - If you use EPS (polystyrene), you'd need to go to 100mm at least. And if you use insulated plasterboard, you don't get the free VCM.

If you can, put a 25mm thick layer of Celotex around the window & door reveals - This will reduce cold spots in these areas.

Thats interesting, I think I would prefer doing it that way anyway! I assume this also helps if you have to hang a radiator, as can fix into the battons? 

FreeBear

Barmy_Army_92 said: I assume this also helps if you have to hang a radiator, as can fix into the battens? 

If you are planning on hanging radiators on an external wall after insulating, yes, put battens in place - Although, with a well insulated & draught free house, there is no need to put a radiator under the window (assuming double/triple glazed). As I renovate each room, radiators are being moved to an internal wall - This cuts down on the amount of pipes and reduces the amount of water slightly in the system.

It is also a good idea to fit battens above windows so that you have something solid to attach curtain poles to. Just make a note of where they are before you're finished.

FreeBear

Barmy_Army_92 said: I think I would prefer doing it that way anyway!

Work in progress, top of the stair well...

Will remove the little random strips once the foam has hardened. Cut back smooth, then tape the joints with aluminium foil tape.