CWI only vs CWI and EWI

[allthatpebbledash]

Hello. Done lots of reading till now but not quite come across an answer regarding my own specific situation.

 

I am planning a detached house retrofit. Part of works will include taking the whole roof off and placing a brand new one on (we’re restructuring basically). Also, suspended timber ground floor is going and we are filling with insulation and a slab most likely. Crawl space is 400mm leading to 800mm as ground slopes. There will be all new windows and doors and MVHR. But I’m struggling with understanding the walls.

 

Currently external walls are covered in pebbledash. Ideally we want to remove or cover this.

 

Wall build up is brick cavity brick, 70mm cavity and (fully) filled with blown wool many years ago. As the house is stripped out, can confirm it is still in place on 1st storey and not slumped. A part of the house (previous extensions) has 50 PIR in the cavity.

 

I was thinking to avoid IWI to not reduce footprint inside, so my options were CWI only or CWI and EWI. But concerns about condensation and air tightness.

 

Option 1. Extract current CWI wool, refill with EPS bead. Wet plaster internally for air tightness. CorkSol or render over pebbledash externally.
 

Option 2. Leave wool CWI in place, 80-100mm EWI with weather tight render which will become new air tight layer. New roof and ground insulation will meet EWI to create envelope. EWI will go straight over pebbledash too.

 

Cost wise, whilst CWI is cheaper than EWI, the additional labour of extracting the wool CWI, and re rendering over the pebbledash, and wet plaster internally makes option 1 more or less same as option 2. Hence why I’m looking at this route. 
 

I am concerned about condensation and convection within the cavity. Or can this be mitigated by other means?

 

EWI runs down below ground closing off air bricks? Or new ground insulation/slab can close it? Or cap cavity at bottom and top?

 

Looking for advice and input from anyone who’s done something similar or can advise on the merits of the above really. 

Edited March 11 by allthatpebbledash
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[LiamJones]

Have a look at ubakus, you can model it all out and have a look at u values, interstitial condensation risks and decrement/phase delay.

 

Im in a similar situation to you. Ripped suspended floor out, added 200mm PIR and 100mm concrete. Blocked up air bricks. Current cavity filled with loose EPS beads, will top up with glued EPS beads. Adding 100mm EPS EWI and render.

[allthatpebbledash]

21 hours ago, LiamJones said:

Have a look at ubakus, you can model it all out and have a look at u values, interstitial condensation risks and decrement/phase delay.

 

Im in a similar situation to you. Ripped suspended floor out, added 200mm PIR and 100mm concrete. Blocked up air bricks. Current cavity filled with loose EPS beads, will top up with glued EPS beads. Adding 100mm EPS EWI and render.

The architect has been working on PHPP modelling and data suggests extracting of blown wool CWI and refill with EPS bead would give us the target value needed. But with adding EWI we can get that number lower of course. Given the added benefit of nicer exterior finish, and better cold bridging across the envelope, is why I’m debating the extra cost.

 

Can you share a rough section detail of your floor to wall junction/build up?