Help with Insulation in Garage Conversion Window!

Long story short, having a nightmare with approval from the building control inspector and it's really holding up the build.

We're quite far into a garage conversion project, thought we were at the stage where we could plaster, but booked in a visit with the building control inspector. He pointed out a number of issues with the insulation on the front wall where the garage door used to be. A window and brick wall is now in its place. Please note, the garage has a newly constructed warm roof (150mm PIR and felt finish).

The builders have built a stud frame with a breathable membrane backing which stands off the single skin brick wall (25mm off the piers on either side and a lot more in between). They have filled between the studwork with 100mm celotex.





The inspector has pointed out that there are too many gaps in the insulation, which will be addressed, filling gaps with additional insulation and taping them up.

His main problem is with the window frame. He says it isn't appropriately finished/insulated but won't provide guidance on how he wants it done. At the top of the frame is a steel hollow girder, with the sides and base sitting on brick.

Should the void between the studwork and brick be filled with additional PIR as otherwise the window cill will allow thermal bridging? Should the brick/steel surround have insulated plasterboard? Please just looking for advise on how to properly finish this to satisfy the inspector.

The window should be much further back, roughly so the centre of the window is in line with the rear of the brick wall, with a suitable closer in the cavity, either pre-manufactured to suit the gap or you can make your own by wrapping some dpm around some celotex. This is pretty standard stuff and any decent builder should have got this right. Your problem is with the builders not the inspector.

freddiemercurystwin said:

The window should be much further back, roughly so the centre of the window is in line with the rear of the brick wall, with a suitable closer in the cavity, either pre-manufactured to suit the gap or you can make your own by wrapping some dpm around some celotex. This is pretty standard stuff and any decent builder should have got this right. Your problem is with the builders not the inspector.

Click to expand...

Thank you, appreciate the reply. Can I ask, you say the window should be much further back, why is that? If it were any further back it wouldn't be resting on the brick, or have I misunderstood?

Looks like window fitted into 9" solid wall/pier from old garage - probably wants a cavity forming.

The window should be in line with the wall insulation to minimise cold bridging, the window is resting on the inner timber frame. It can be strapped back to get fixings into the timber frame wall at the base. The bulk of the window fixings should be into the jambs.

freddiemercurystwin said:

The window should be in line with the wall insulation to minimise cold bridging, the window is resting on the inner timber frame.

Click to expand...

Just so I understand, that would mean there would be no window cill on the inside and on the outside there would be a sort of brick shelf. Is that correct?

The window is in the correct place.

Potentially you could wrap insulation + plasterboard around the jamb to meet the window, same at the head but you'll only end up with a token 10mm of insulation or thereabouts, hence why the window should be further back.

JackRusselll said:

Long story short, having a nightmare with approval from the building control inspector and it's really holding up the build.

We're quite far into a garage conversion project, thought we were at the stage where we could plaster, but booked in a visit with the building control inspector. He pointed out a number of issues with the insulation on the front wall where the garage door used to be. A window and brick wall is now in its place. Please note, the garage has a newly constructed warm roof (150mm PIR and felt finish).

The builders have built a stud frame with a breathable membrane backing which stands off the single skin brick wall (25mm off the piers on either side and a lot more in between). They have filled between the studwork with 100mm celotex.

View attachment 378615

View attachment 378616

The inspector has pointed out that there are too many gaps in the insulation, which will be addressed, filling gaps with additional insulation and taping them up.

His main problem is with the window frame. He says it isn't appropriately finished/insulated but won't provide guidance on how he wants it done. At the top of the frame is a steel hollow girder, with the sides and base sitting on brick.

Should the void between the studwork and brick be filled with additional PIR as otherwise the window cill will allow thermal bridging? Should the brick/steel surround have insulated plasterboard? Please just looking for advise on how to properly finish this to satisfy the inspector.

Click to expand...

Maybe a bit late now, but we ask for 25mm knock-on pieces fitted to the frame sides and top so that we can fix insulated plasterboards to the jambs.
The other solution is to fix the window further into the room so that the external masonry jambs are not visible from the inside.

cdbe said:

The window is in the correct place.

Click to expand...

For the window fitters. They don't always appreciate the consequences of visible external masonry and condensation...

Looks spot on in this case. It appears to be sat correctly on the new masonry outer skin - 30mm back. Not sure what bodgerry is allowed when dealing with the solid walls at the sides. Think I'd have just have cut the back out and built a standard block inner skin.