Installing UPVC windows

[Rob broom]

Hi all, I have a couple of queeries with installing UPVC windows.



1: Fixing the top of the window into a Catnic.

I have read that the windows should not be screwed up into a catnic, which does make sense regarding water and damp getting through the holes.

Can i get conformation that i should just screw the Lef and Right sides and the bottom into the blockwork, then use expanding foam along the top.

In theory, the render on the outside and the plasterboard on the inside will help to fully secure it?



2: How far back into the cavity should i install the window?

The UPVC frames will be 70mm wide.

I am expecting to fix the windown onto the outer leaf blockwork, with the rear face of the fram being flush with the inside of the outer leaf blocks. Tha will mean they will be about 30mm recessed from the outside of the blockwork.

Is that the correct postiton to install them?

Many thanks

 

[Swwils]

Based on research for my own house and I'm in no way an expert:

1. It depends.

In general if you determine a mechanical fixing is required (often it isn't) then the usual recommendation is a suitably drilled and tapped hole that can then accept a machine screw of correct depth. But often people with just blow a masonry bolt through it regardless. (A leak through a head fixing as you can imagine is rare).

2. This will depend on the construction and if your reveals are insulated or not etc. A modern construction detail and the cavity closer specification will likely say at least a 30mm overlap over the insulated part of the cavity closer not from the outer leaf face, so the resulting setback will be deeper and perform better - some older recommendation are confused in this regard. Some cavity closers have flexible pvc bits that are meant to sit inside the uPVC frame, not used as a stopguide. This may mean that standard depth cil can not accomdate the desired depth so a deeper one should be ordered to suit. The "zero carbon hubs thermal bridging guide" has good illustration of this - as does the ACD details and also the cavity closers instructions.

There are several good practice documents online that will follow BS 8213 to help you out.