Chicken or egg: do we render or install the windows first?

[laurenco]

We've finally approved the drawings for our timber frame house and the erection is slated for January.  Time to order our windows. The supplier and the price are both sorted, but something that cannot be agreed upon is whether the windows are installed before or after the house is rendered. MBC and Velfac are adamant the property should be rendered first. Our renderer says not. What has everyone else done in this situation?

[nod]

4 minutes ago, laurenco said:

We've finally approved the drawings for our timber frame house and the erection is slated for January.  Time to order our windows. The supplier and the price are both sorted, but something that cannot be agreed upon is whether the windows are installed before or after the house is rendered. MBC and Velfac are adamant the property should be rendered first. Our renderer says not. What has everyone else done in this situation?

You can do them afterwards Then pick up the unredered reveals when the frames are fitted

But much better to get all the window frames in prior to refer 

[lizzie]

we had Internorm windows and they went in before the render...also an MBC frame.

 

I think Velfac may have their own particular requirements but its odd to render first and then try and get the windows in

[the_r_sole]

.

 

Edited September 26, 2019 by the_r_sole

[laurenco]

Thanks @the_r_sole. It's alu-clad triple glazed with an MVHR system. My understanding was that no ventilation was required at all, and the house would be hermetically sealed by the air test.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[laurenco]

ah I see..

[lizzie]

I seem to remember from when we were looking that Velfac had different requirements.  I think their fitting guide is available to download from the website if not I might have it somewhere.

[lizzie]

https://velfac.co.uk/domestic/technical/download-centre/10424/interface-drawings

 

here is the link

[laurenco]

6 minutes ago, lizzie said:

https://velfac.co.uk/domestic/technical/download-centre/10424/interface-drawings

 

here is the link

amazing thanks @lizzie

[Barney12]

I actually dumped Velfac exactly for this reason. I could never get a straight answer out of their technical team as to why they require a render first approach and I asked more than once! I have a trail of emails on the subject. 

 

If it is as per @the_r_sole says then this seems a design element not required by any of the other manufacturers I came across. 

 

If you’ve got an MBC frame and you’re trying to minimise any potential thermal bridge then you want the windows set into (or partially into) the timber frame with a damn good air tight seal. Velfacs design just makes that a lot harder IMHO.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Nickfromwales]

19 minutes ago, the_r_sole said:

 

Tbh a lot of the manufacturers do require it, they just don't really push it and they leave the responsibility for measuring up to contractors so it's their fault of they haven't read the guides properly. We used to size window openings so that the outer leaf brick/block would have a check for the window to sit against but it doesn't work with most modern windows if you get into the installation guides.

The idea is that the external cladding gets ventilation, some of them now vent into the cavity rather from the front but they all do it. As it's only the external face of the windows it really shouldn't have any kind of impact on air tightness

If ph windows benefit from a cork / other thermal break in their construction to reduce the huge cold bridging, then leaving the face gapped at the sides would throw that straight into the bin. 

Ventilating to the cavity is great if you’ve got a cavity, but the thought of driving wind and rain not being deflected / otherwise arrested would not be a good thing imo, so for me it would have to be sealed. If those gaps were fully backfilled with foam after installation I fail to see how condensation could occur ? Afaic leaving the gap is what would cause / promote the problem, not cure it  

#confusedofwales

[Mr Punter]

With the Velfac system the outward opening ali sash is the same size as the fixed internal timber frame, so if you rendered up to the window you would not be able to open it and nor would it drain correctly.  Their detail is to render the returns first, then the window size is the render-to-render size minus 12mm per side and top, and 20mm for the bottom.  The window is then fitted with a 15mm cill packer at the bottom and fitted in the opening.  The gap is sealed with foam on the inside and Compriband externally.

 

Double check all the measurements as it is easy to mess up.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[nod]

7 minutes ago, the_r_sole said:

 

true enough, forgot about the opening action on the velfac outward openers

Nearly always ends up with a patch up job

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Nickfromwales]

My 2 cents says if you manufactured with a flaw, eg a cavity between the Ali and the wood where interstitial condensation is promoted, then that a poor design. 

My point about cold bridging was that ‘cold’ shouldn’t be able to get ‘around’ ( beyond ) the thermal break. ?

Agree with @nod as when the fruits bead up on the external angles and they end up fatter than the opening, then good luck when you go to open the window ( after the fitters have squeeze them in ) as they’ll hit the reveals   

Good practice would be to discuss this with the plasterers well in advance and flare the reveals sufficiently to take the depth of the corner bead.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[Barney12]

............So in summary; Velvac design means incredibly careful detailing of openings. Those very openings can be influenced by a number of factors/contractors.

No thanks, there's already enough to go wrong on a build

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole