How to - insulate and air seal this….

[BadgerBodger]

Thoughts on a postcard…

 

I have a number of concepts which I’m sure will work but looking for fresh takes on how to take airtight AVCL line from door frame to Iinternal insulation (inner face) - gap circa 415 to traverse but only 20mm or so to work with…. 
 

Current lines of thought are to AVCL to frame behind build-up and use jacko/marmox board to form reveal with stop end bead to frame to restrain the board. 
 

Seems like a lot of work though. So if anyone has a cleaner solution from experience it’s welcome. 
 

Maybe the more slight frames were a bad choice. doesn’t leave much to work with. 

Problem occurs 13x to varying degrees albeit this is one of the tighter ones 

 

Edited Thursday at 15:51 by BadgerBodger
Clarification

[Iceverge]

That's very tight to get insulation in there but anything would be a great improvement to prevent condensation etc.

 

How many mm do you have to play with, it looks to be only about 15mm? 

 

I would get Marmox board and shim it level and screw it to the brick reveal, holding it back 5mm from the window frame. Squirt behind it with FM330 or other low expansion, airtight expanding foam. Then use a white acoustic/airtight sealant to seal the 5mm gap between the marmox and the window frame. 

 

The room side of the Marmox can be taped to the PIR. 

 

3MM Gypsum skim over the marmox then. 

 

Consider adding some kind of check strap or other stopper to the window sash to stop it hitting the reveal and cracking it afterwards.

[BadgerBodger]

Nice. Looks like I was half way there. 
 

Yeah, it’s deceptive but could be at little as 15mm so I was thinking 12 mm marmox and skim. 
 

Was hoping to avoid shimming though 🤣

[Iceverge]

That lead to yet another amusing and informative visit to urbandictionary.com

[Nickfromwales]

On 27/08/2026 at 17:58, BadgerBodger said:

Nice. Looks like I was half way there. 
 

Yeah, it’s deceptive but could be at little as 15mm so I was thinking 12 mm marmox and skim. 
 

Was hoping to avoid shimming though 🤣

Just dot & dab them on, using flexible, extended set tile adhesive, as I did for a previous similar project. Forget shims / screws, you'll be there forever trying to get all those reveals and heads plumb / level / square etc.

[BadgerBodger]

18 hours ago, Nickfromwales said:

Just dot & dab them on, using flexible, extended set tile adhesive, as I did for a previous similar project. Forget shims / screws, you'll be there forever trying to get all those reveals and heads plumb / level / square etc.

Yes, this is the way. And airtight with silicone/sealant? 

[Nickfromwales]

3 hours ago, BadgerBodger said:

Yes, this is the way. And airtight with silicone/sealant? 

Silicone has a very short service life, and tapes & foams are for airtightness, mastics etc are for sealing and weather-tightening etc.

 

Mastic sealant such as CT1 or (cheaper option) Stixall for fundamental frame sealing, then you tape / foam to get airtight. 
 

As you’ve boxed yourself in very, very tightly with the windows etc completely filling the spaces I would mask the windows up with about 8-10mm on show, and mastic seal to get your AT in check, smoothing this to nearly nothing. Remove masking and leave to dry for 24hrs minimum. 
 

Then when using the tile adhesive you apply a full bed, up to and over the mastic, and whilst that is still wet you dab the reveal boards on, pushing them tight up to the frames.

 

Use stop beads to ‘picture frame’ the reveals to the frames, and skim to those to get sharp, straight, consistent lines. 

[BadgerBodger]

I see. I have both these products already. Just have a habit of still calling them “silicone”. Thanks for clarity. 
 

And bonding compound for drywall not suitable? Tile adhesives is preferred? 

[Nickfromwales]

31 minutes ago, BadgerBodger said:

I see. I have both these products already. Just have a habit of still calling them “silicone”. Thanks for clarity. 
 

And bonding compound for drywall not suitable? Tile adhesives is preferred? 

Yes, it seems to grab and hold, and never let go. Dry line adhesive seems to just go off and then you can knock the board off without any real effort.

 

Extended or standard set adhesive will give you 2 hours of open time, so whatever you do, do NOT use rapid set!!

[Nickfromwales]

This is one masonry project, an EnerPHit type of standard. 20mm boards used, and still retained the 10mm that Norrsken asked for. 
 

Done with tile adhesive, and stuck on there like shit to a blanket.

 

I flew these through prior to the windows going in, so they created frames for the EWI to be dressed into. These projected out around 160mm, for 150mm EWI + render etc, as you can see in the next pic with windows in. 
 

 

You can use tile adhesive with complete confidence, it’s remarkably good for this application, and way better at staving off damp bridging.