Aluminium Cill Depth

[Barney12]

I'm sure this must be a really simple question but...........

 

our build is:

 

300mm Twin Wall timber Frame

50mm Cavity

100mm Dense Concrete Block

Render (10mm?)

 

As I understand it best practice is for the window to be installed into the timber frame to minimise the cold bridge (makes sense). This actually has another advantage for us in that we have a planning condition that windows have to be recessed by at least 100mm.

 

So, a standard 150mm cill is clearly not going to be deep enough. So what depth should I use, is there a preferred overhang to prevent water run off onto the render?

I assume patio/front doors have the same detailing requirements but it does feel like there will be a lot of cill to step over?

 

Note we are also not permitted any form of belcast arround windows.

 

[IanR]

My Cills are 260mm deep.

 

The outer face of the windows are 75mm back from the TF Structure, outside of which I have a 94mm vent gap plus 38mm cladding.

 

This is for Internorm windows where the Cill attaches to a Cill spacer under the frame which is at least 10mm (maybe 15mm) under-flush to the front face of the windows. The Cills have a drip feature on the outer edge where there is a return flange back underneath the outer edge of the Cill making the edge of the Cill at least 10mm thick, and there is at least 10mm clearance between the back edge of this return and the outer surface of my cladding.

 

Edited to add:

 

[Cladding not yet complete into reveal]

 

For Full height windows and doors, on ours the Cills are narrower since there is no cladding and vent gap, just plinth wall below DPC. Wouldn't that be the same for you?

Edited January 27, 2017 by IanR

[Temp]

http://www.nhbc.co.uk/NHBCPublications/LiteratureLibrary/Technical/StandardsExtra/filedownload,25277,en.pdf

 

See page 5. Says 40mm overhang.

[Stones]

40mm overhang is what we went for on both our rendered and timber clad walls.

[TerryE]

4 hours ago, Barney12 said:

Our build is: 300mm Twin Wall timber Frame / 50mm Cavity /100mm Dense Concrete Block /Render (10mm?)

 

As I understand it best practice is for the window to be installed into the timber frame to minimise the cold bridge (makes sense). This actually has another advantage for us in that we have a planning condition that windows have to be recessed by at least 100mm.

 

Barney we have a similar profile except that our outer skin is stone ~125mm thick.  What we did was to mount our windows in a narrow mounting frame (marine ply is typically used) so that the face of the windows sit some 45mm in front of the frame and largely close the cavity.  This overhang is insulated by the cavity closers that surround each window.  This makes finishing off the windows in term of weatherproofing, cills, etc a lot easier.  If you look at the thermals is makes bugger all difference to heat losses and the slightly increased psi effect is still far above the thresholds where condensation might become a problem.

[Barney12]

2 hours ago, TerryE said:

 

Barney we have a similar profile except that our outer skin is stone ~125mm thick.  What we did was to mount our windows in a narrow mounting frame (marine ply is typically used) so that the face of the windows sit some 45mm in front of the frame and largely close the cavity.  This overhang is insulated by the cavity closers that surround each window.  This makes finishing off the windows in term of weatherproofing, cills, etc a lot easier.  If you look at the thermals is makes bugger all difference to heat losses and the slightly increased psi effect is still far above the thresholds where condensation might become a problem.

 

Thanks @TerryE did MBC provide that detail and the mounting frame? Or did you have to retrofit?

[TerryE]

@Barney12, no, MBC weren't proactive here.  It was an issue that I picked up on eBuild chit-chat before we spec'ed the frame so we added 12mm  on all sides to our frame window openings to give us room to insert the "shims", that is the framing.  I've you haven't done this and the window openings are already agreed, then there is an alternative approach that either @ProDave or @Stones described (sorry, can't remember which; it was nearly 2 years ago) and this was to use something like the 38×44 tanalised construction timber to make up box frames which you screw to the outside of the frame openings once the frame is erected (depending on your detailing, then you might need to have an inverted U instead of a box) -- then you mount your frames in this.  Most high-spec window frame profiles are 80mm+, so positioning them like this is fine.

 

We got this right more by luck than by judgement.  It's a pity that MBC (and other framing companies) don't go through this as part of their design check list. If you want to talk it through then PM me your voice details and we can do. That's if one of the other guy doesn't get there first.

[Barney12]

8 hours ago, TerryE said:

@Barney12, no, MBC weren't proactive here.  It was an issue that I picked up on eBuild chit-chat before we spec'ed the frame so we added 12mm  on all sides to our frame window openings to give us room to insert the "shims", that is the framing.  I've you haven't done this and the window openings are already agreed, then there is an alternative approach that either @ProDave or @Stones described (sorry, can't remember which; it was nearly 2 years ago) and this was to use something like the 38×44 tanalised construction timber to make up box frames which you screw to the outside of the frame openings once the frame is erected (depending on your detailing, then you might need to have an inverted U instead of a box) -- then you mount your frames in this.  Most high-spec window frame profiles are 80mm+, so positioning them like this is fine.

 

We got this right more by luck than by judgement.  It's a pity that MBC (and other framing companies) don't go through this as part of their design check list. If you want to talk it through then PM me your voice details and we can do. That's if one of the other guy doesn't get there first.

 

Thanks @TerryE makes sense. The order was only signed with MBC a week ago so still plenty of time to specify openings. We've decided on Internorm HF310 windows which have an 85mm construction depth so no issues there.

 

So keeping the questions in the thread (for the benefit of others too). Answers open to @ProDave and @Stones too! 

 

1. If I've understood your description properly the marine ply shims act as the cavity closer? 

2. Did you further insulate the shims (effectively creating a closer in the same style as the ones you can get off the shelf Link)?

3. Did you still install the windows prior to the stonework?

4. Did you use something like a compriband tape to create a weather seal between the exterior window face and the stonework? 

 

My my one last thought is would fibreglass sheet be better than marine ply? Proberly overkill as the frame is wood anyway. 

 

Glad I asked what I thought was a basic question now! 

[TerryE]

@Barney12 I am more than happy to go through all this, but it doesn't really belong in a topic about Alu cill depths.  It's really about all the issues to do with adding a stone / blockwork skin to an MBC-style frame.  I'll do a blog post over the weekend.  Did you check your private mail? 

[Barney12]

5 hours ago, TerryE said:

@Barney12 I am more than happy to go through all this, but it doesn't really belong in a topic about Alu cill depths.  It's really about all the issues to do with adding a stone / blockwork skin to an MBC-style frame.  I'll do a blog post over the weekend.  Did you check your private mail? 

 

OK thanks, much appreciated.

I cant see I've had a mail from you?

[Stones]

I think what @TerryE may be referring to re previous discussion is the normal way windows are fitted into timber frame houses in Scotland. A 50mm x 50mm batten (firestop) is fixed to the outside of the window frame to close the cavity between timber frame and rainscreen (be it block wall, timber cladding etc). The window itself therefore sits partially inside the timber frame protruding 50mm from the outer face of the timber frame. 

[TerryE]

On 28/01/2017 at 11:03, TerryE said:

@Barney12 I am more than happy to go through all this, but it doesn't really belong in a topic about Alu cill depths.  It's really about all the issues to do with adding a stone / blockwork skin to an MBC-style frame.  I'll do a blog post over the weekend.  Did you check your private mail? 

@Barney12, see my latest blog entry