Fine tuning my IWI Solid wall (Warm Batten) design

[Mike]

1 hour ago, Annker said:

Should my airtight layer continue up from the external brick wall and across my 1st floor ceilings; or can it terminate at the top of my external wall?

I'd run a timber underside the rafters (yellow below) and fix the vertical studs to the underside of that. The red would be the VCL but, since the wall isn't vertical, leave a little slack so that the insulation can push it flush with the wall. Where you joint the VCL isn't vital, as long as you can access it to make the join.

 

[Annker]

15 hours ago, Mike said:

I'd run a timber underside the rafters (yellow below) and fix the vertical studs to the underside of that. The red would be the VCL but, since the wall isn't vertical, leave a little slack so that the insulation can push it flush with the wall. Where you joint the VCL isn't vital, as long as you can access it to make the join.

 

In the photo I had only offered up that uncut 4x2 to help illustrate the detail.

You are correct Mike, I will be running a top plate along the studs as you've expertly shown in isometric!

 

Just to clarify, you would opt to continue the VCL below all the entire ceiling area, or just ~300mm or so into the ceiling area proper?

 

From my research it seems that where the loft space is a cold roof, installing a VCL at the ceiling level has its pros and cons.

It is suggested by some that the ceiling/cold roof space interface may provide a function similar to a dehumidifier removing moisture from the habitable room; installing a VCL would prevent that function from operating. 

Although in a bathroom perhaps it it best to have a VCL in the ceiling as the required mechanical extraction will remove moisture at a good and controlled rate.

 

Perhaps in terms of ensuring the IWI assemble is air tight, a good option would be to just wrap the top of the studwork in Intello Plus; taped to the wall and tape to the top plate. (As illustrated by the blue line in the mark up below)

Seems to tick the boxes of building with the princple of vapour open and air tight to the forefront.

 

 

 

Edited October 5 by Annker
Spelling

[Mike]

6 hours ago, Annker said:

you would opt to continue the VCL below all the entire ceiling area, or just ~300mm or so into the ceiling area proper?

I would add an airtight barrier across the full ceiling on the the warm side of the insulation in the position of that red line, probably using Intello (which is suitable for the task according to Pro Clima; 'traditionally' it would have been polythene sheet). The main reasons are the second and third ones that I gave for requiring a parge coat: it prevents wind blowing through any cracks in the ceiling, and stops air escaping out due to the 'stack effect' (warm moist internal air is more buoyant than the outside air, so will rise and escape through any gaps, drawing outside air in as it does so).

 

Ideally you'd have a service cavity below the Intello and above the plasterboard.

 

I'd also want eves ventilation trays above the insulation at the eaves, to deflect wind that would otherwise blow into the insulation - the eaves insulation is very exposed without that.

 

Edited October 5 by Mike

[Annker]

17 hours ago, Mike said:

I would add an airtight barrier across the full ceiling on the the warm side of the insulation in the position of that red line, probably using Intello (which is suitable for the task according to Pro Clima; 'traditionally' it would have been polythene sheet). The main reasons are the second and third ones that I gave for requiring a parge coat: it prevents wind blowing through any cracks in the ceiling, and stops air escaping out due to the 'stack effect' (warm moist internal air is more buoyant than the outside air, so will rise and escape through any gaps, drawing outside air in as it does so).

 

Ideally you'd have a service cavity below the Intello and above the plasterboard.

 

I'd also want eves ventilation trays above the insulation at the eaves, to deflect wind that would otherwise blow into the insulation - the eaves insulation is very exposed without that.

 

Thanks Mike.

Rationale for installing a continuous VCL makes sense. 

As with any renovation, each room ceiling will have its own challenges to overcome; Plan was that 1x existing ceiling only needs reskiming and 1x ceiling only needs overboarding. Elsewhere 1x ceiling needs a new MF ceiling installed to get it back to level, remaining ceiling are in a state in which a VCL can be install as is.

 

I have the ideal princple in my head so just have to tailor it to each situation.

 

Understood on the eves tray. A full re-slate of the roof will be done next year so ventilation of the roof space will be entirely dealt with then.   

 

[lookseehear]

On 04/10/2025 at 10:14, Annker said:

Yesterday was a particular humid day I had left a rear sliding door open for a while and the humidity meter was reading 95% (matching the weather report).

 

I just wanted to pick up on this point. It's often not useful to compare internal and external RH's because of the temperature difference. You may know this already, so sorry if I'm teaching you to suck eggs!

 

95% RH at 15 degrees is roughly 70% RH at 20 degrees, which is why opening windows to allow colder external air in is almost always going to help dry a house out. If your internal warm air is 95% RH then that's much more likely to be a sign of a problem (water coming from another source). I've lost count of the amount of times during the winter I've seen people posting online saying that they won't open their windows because the weather shows it's 100% humidity outside, when in reality 100% humidity during a cold spell will give lovely dry air once warmed up to room temperature.

[Annker]

On 12/10/2025 at 20:58, lookseehear said:

 

I just wanted to pick up on this point. It's often not useful to compare internal and external RH's because of the temperature difference. You may know this already, so sorry if I'm teaching you to suck eggs!

 

95% RH at 15 degrees is roughly 70% RH at 20 degrees, which is why opening windows to allow colder external air in is almost always going to help dry a house out. If your internal warm air is 95% RH then that's much more likely to be a sign of a problem (water coming from another source). I've lost count of the amount of times during the winter I've seen people posting online saying that they won't open their windows because the weather shows it's 100% humidity outside, when in reality 100% humidity during a cold spell will give lovely dry air once warmed up to room temperature.

That is a good point to make, and yes I am broadly aware of the relationship between RH and temp.

 

Its worth mentioning that atm my house is very much a bare bones building and I believe at a lot of times during the day it is colder inside the house than it is outside it.

 

My observation was moreso a generally one ,that on humid days (not necessarily rainy days) the patches appear darker.