Full fill and IWI ?

[Packman]

I’m renovating an uninsulated 1950s detached house, with a normal cavity masonry buildup of 

 

100mm facing brick

50mm empty cavity

100mm block

sand/cement plaster

paint 

 

I was originally planning to fill the cavity with graphite eps beads and then externally insulate with 100mm graphite eps then finish with brick slips on ground floor and render on upper. 
 

However the current external brickwork is actually very good, and from a cost saving exercise i’m considering IWI instead so that I don’t need to touch the outside. 
 

I know that EWI is going to be better performance, but if I ruled this out and went down the internal route, I have a few questions:

 

1. Is insulated plasterboard any good?

2. If using IWI or insulated plasterboard, should I still fill the cavity or should I leave it empty to stop potential moisture / damp / mould problems?

3. How / where do I create the air tight layer in this build up? On top of the IWI/ insulated plasterboard or between the IWI/board and the blocks?

4. Will it still be possible to get an ok u-value (under 0.2) from my walls without losing any more than max 100mm internally?

5. should I go on top of the existing sand/cement plaster, or hack it off and take it back to block first?

Thanks for your help!
 

[tonyshouse]

Leave plaster on, use it as the airtightness layer. Make sure that it goes down between the joists through the floor void.

 

fill the cavity graphite eps is breathable - insulated plasterboard is ok - no battens, no voids, no air leakage paths 

 

slight bridging at internal walls, need to window reveals, heads and sills 

 

stick or fix sheets directly to plaster, no voids or air gaps 100mm follow the insulation layer through the floor void 

 

I would add false skeiling ceilings to mitigate thermal bridge at the wall, plate

 

insulate gable and chimney/party wall above the ceiling 

[gmarshall]

I'm very interested in this as we are currently buying a 60s detached and so about to start something very similar.

 

The biggest issue with insulated plasterboard as far as I am concerned is the cost compared to buying the same materials seperately. It's more faff to use sepearate insulation but in my opinion worth it cost wise if DIYing. I'm planning to do that, and it gives you the option of using the foil facing of the insulation as an airtightness layer- although using the wet plaster as suggested seems very sensible if it's in good condition. Biggest challenge as said above is the detailing around joists and first floor void.

 

Our cavities are apparently already insulated- just praying they aren't poorly filled with damp mineral wool! If you have insulated cavities at ~0.5 u value I think you will be pleasantly surprised how much internal insulation you need to get decent performance overall- I'd reccomend playing with some of the online calculators to see the different options. 

[Packman]

On 20/02/2021 at 07:37, tonyshouse said:

Leave plaster on, use it as the airtightness layer. Make sure that it goes down between the joists through the floor void.

 

fill the cavity graphite eps is breathable - insulated plasterboard is ok - no battens, no voids, no air leakage paths 

 

slight bridging at internal walls, need to window reveals, heads and sills 

 

stick or fix sheets directly to plaster, no voids or air gaps 100mm follow the insulation layer through the floor void 

 

I would add false skeiling ceilings to mitigate thermal bridge at the wall, plate

 

insulate gable and chimney/party wall above the ceiling 

 

Thanks Tony. Thats really helpful.  So you would fix the insulated plasterboard directly to the plaster? Not with battens and no dot and dab?

 

What would you do re insulation in the roof space? The loft is just going to be used for typical storage, not for rooms or anything. Will probably put the MVHR unit up there though, and maybe the megaflo cylinder. We were planning on just laying normal loft floor insulation e.g. 300mm knauf, across the floor, except for one section where we plan to vault the ceiling above the master.  Is that any good, or are we better off insulating in between the rafters rather than across the floor? 

[tonyshouse]

Insulation in the loft is best across the ceiling and ventilate the loft, build an insulated box round cylinder but better to locate in the house than the loft 

 

minimising the size of your heated envelope will reduce energy use and the opportunity, which is high in a loft, for air infiltration and draughts 

[Packman]

1 hour ago, tonyshouse said:

Insulation in the loft is best across the ceiling and ventilate the loft, build an insulated box round cylinder but better to locate in the house than the loft 

 

minimising the size of your heated envelope will reduce energy use and the opportunity, which is high in a loft, for air infiltration and draughts 

Thanks. If I can find space for the cylinder in the house, what about the MVHR unit? That ok in the loft, or better in the house too?