IWI or fully fill cavity (or both)

[richard_scotland]

Hi BuildHub, I've gained a lot of knowledge reading this forum but I have a question about how best to proceed with an upcoming installation of a wetroom into an existing bedroom -  whether to add IWI to a partially filled cavity wall, or fully fill the cavity wall

 

Current build
early-2000s pitched roof single story extension wrapped around the end of 19th century sandstone single story cottage

3 bedrooms with external partial fill cavity walls

external wall length (area in brackets)
bedroom 1 - 10 m (2 windows) (23 m2)
bedroom 2 - 5.4 m (1 window) (12.4 m2)
bedroom 3 - 3.5 m (1 window) (8 m2)

current u-value estimated at 0.54 
wall construction (from some old plans we found) is plasterboard, 25 mm cavity, 100 mm concrete blocks, 35 mm insulation board (from what i can see looks like PIR), 50 mm cavity, 100 mm concrete block + cement render. 

 

We need to subdivide bedroom 1 to create a disabled access wetroom for one of our children (so there will be a fair amount of disruption to this room regardless). Given this work is taking place anyway, I am contemplating whether now is the point to add internal wall insulation. 

 

We are rural and near the fife coast fully exposed to the north sea on the east (just open fields between us and the sea), but somewhat sheltered to the west by trees. These rooms are on the west and north side of the house. Overall its a windy location, but not especially cold (-5 is the coldest we have had)

 

These are the options I can think of, would be great to hear the collective wisdom from the forum members. Loft is already topped up to 300 mm. 

1. Fully fill the cavity wall e.g. with EPS beads (all 3 rooms)

pros
-less disruption (and cost?)
- more of the house improved at an earlier point

cons
- u-value still not great
- worry about creating damp issues where none exist?

- will I be able to find an installed happy to install a partial fill wall?

 

2. Internal insulation wood board 60 mm (just room 1 initially)

an architect involved at an earlier stage proposed this option (to help with managing moisture + thermal mass perhaps) but I am not sure what the benefit is of this approach on what is essentially a modern concrete wall? Seems a lot of disruption for modest improvement in u-value?

 

 

 

3. Internal insulation PIR board (just room 1 initially)

- loss of thermal mass?

 

 

Options 2 and 3 would only be in room 1 initially. We would hope to also do room 2 and 3 at some point but we have 3 small kids so its possible it may not happen for a few years... and it will involve loss of space in what are already quite modest rooms. 

 

Any thoughts welcome

thanks

 

[MikeGrahamT21]

Option Number 2 needs a VCL behind the plasterboard to remove condensation risk

[IGP]

I would go option 1. It’s simple, effective and with EPS beads, unlikely to cause damp issues as the beads themselves cannot get soaked. 
 

You could always add IWI (or EWI???) later. 
 

 

[richard_scotland]

14 hours ago, IGP said:

I would go option 1. It’s simple, effective and with EPS beads, unlikely to cause damp issues as the beads themselves cannot get soaked. 
 

You could always add IWI (or EWI???) later. 
 

 

Thanks, that makes sense. I guess the next question is, if I also add IWI (as the room is having work done anyway for the wetroom), how much should I add? EWI might be trick as there are no overhanging eaves

[richard_scotland]

My only concern about beads is that the cable for the car charger crosses the cavity - its an outside rated cable so perhaps thats provides more protection from plasticiser migration? if not, I could try and wrap it with some sort of barrier tape (foil tape?) where it crosses the cavity.

[Mattg4321]

3 minutes ago, richard_scotland said:

My only concern about beads is that the cable for the car charger crosses the cavity - its an outside rated cable so perhaps thats provides more protection from plasticiser migration? if not, I could try and wrap it with some sort of barrier tape (foil tape?) where it crosses the cavity.

Wouldn’t worry about it. 

[richard_scotland]

Thanks. There is a junction where the cavity wall meets the old sandstone corner of the house. 

 

Would IWI be worth doing to help with this thermal bridge and potential cold corner? Or something else?

[richard_scotland]

bumping in case anyone has ideas on whether it would be worth doing IWI to help with the thermal bridge at the corner, or even if I just did a small section of the wall in IWI (we would like to put a fitted wardrobe here)

[Redbeard]

Yes. I would recommend never leaving thermal bridges un-'cloaked' if you can help it.

 

Take care if you are doing any IWI inside CWI. Get a condensation risk ass't done.

 

 

[richard_scotland]

thanks. Would the internal part of the gable wall in the loft count as a thermal bridge, if we didn't do IWI? in which case might it be worth putting a little bit of insulation up on the wall in the loft?