Window installation details to minimise thermal bridging

[Mark Siddall]

Did the Vale's get it right?

In posing this question I am in the first instance ignoring certain constructional issues and focusing purely on the thermal bridging detail. The Vale's detail has the front face of the window aligned with the front face of the insulation. Where as the Stamford Brook data (see elsewhere on this site), and this Passive House information, suggests that the thermal bridge detail can be optomised further by placing the glazing centrally within the insulation plane.

As the Vale's were using masonry they were constrained by that technology. Using timberframe (as attached) a refinement of the thermal bridge seems to be possible.

Any thoughts?

Mark

[Mark Siddall]

Chris,
I've seen PH detail you mention i.e where the window is fixed back to the face of the masonry via a discontinuous unequal angle bracket; the longer length of the angle is used to offer the window into the center line of the insulation. I have only seen this detail used where render is applied to the insulation (Sto or similar).

Thinking about cavity wall construction, and the way that it is built in 'lifts,' this detail could be problematic as a number of lifts are required to form a window opening. Whilst it is possible to form the detail it would mean that the internal leaf masonry would have to be built in advance of the external leaf. Such a detail will be expensive and time consuming.

Mark

[Mark Siddall]

David,
Your technique agrees with mine. As it is hardly the most efficient means of construction the cost of building in such a manner remains a concern (but that's up to the depth of ones pockets and the planning constraints).

Mark

[Nick Grant]

Issue that came up for me recently when suggesting window moved back into insulation plane in timber wall was issue of rain proofing. Easier with window in line with rain screen as we did for our house.

Good metal cills sorts the bottom but does anyone have a fail safe detail that doesn't rely on sealants for the rest of the frame?

Pics or sketches would be good.

Nick

[David Olivier]

Thermotech windows (Canada) supply standard aluminium trim which overlaps the (assumed to be timber) cladding all the way around the window opening.

I didn't use it as I wanted to recess the windows.

David.

[Nick Grant]

David is this the typical US style flange that normally gets taped to the sheathing?

ie does it all window to be set back as I assume you wanted to do?

The justified fear is that any sealant dependent detail will fail and water will go straight into the middle of the wall.

I can imagine possible details but was hoping to find some standard examples of good practice rather than starting from first principles.

Nick

[Mark Siddall]

I have seen a similar detail to the one David describes on Passive House windows (just the cill mind.) The cill is fixed back to the frame via few screws and an EDPM gasket that abutts the frame. The PH window in question was a composite frame alu/timber frame so the weathering face of the alu provided what amounted to drip onto the cill. I guess this meant the EDPM would only get wet if there was driven rain thus making the detail less vulnerable than may have otherwise been the case.

(Hope all that makes sense.)

Mark

[Nick Grant]

Here's a cill pic.

Yes it's the head and jambs I'm worried about. The pic shows inward opening window with some tidy timber but I'd be very interested in any fail safe drained and vented arrangements people have.

Nick

[Andy Mitchell]

Just to be geeky…this is a Gutmann sill fitted to a Sigg PH window, but pretty generic detail.

[Mark Siddall]

Keeping with the cavity wall, earlier Chris asked: How to detail deep external reveals?
Has anyone considered a rendered solution to the external window reveal? This detail would be similar to that found on Georgian/Victorian terraces.
Perhaps such a detail could cover the brickwork and the insulation without leading to a brick return (and the resultant cold bridge/reduction in thermal performance.) A few extra cavity ties may be required around the opening though!
An added bonus would be that the render, if painted white (as Georgian/Victorian examples) would help to minimise glare etc.

Chris asked: Is there any way windows could be moved out into the insulation?
Perhaps the unequal angle bracket, if long enough, could fix to the internal face of the internal leaf (rather than cavity side) this would allow the window to be offered into the opening after the opening in the cavity wall has been formed (avoids the problem with the discontinuous lifts of masonry discussed earlier.) I think that if using membranes pre-applied to the window frame airtightness issues can be managed if suitable thought is given.
NOTE: The advantage that the German rendered wall detail (see attached) has is that the window is placed prior to the insulation installation. As a consequence the window is built into the insulation in such a way as to avoid the need for the 10mm construction tolerance that is normally left around the perimeter. In the detail I describe above the builder will need to remember to pack the void with insulation if the cold bridge is to be avoided.

Nick talked of rainscreen reveal details: Easy answer is to turn the rainsceen into the reveal. As long as the cill is wide enough it can be lapped under the reveal-rainscreen and any water finding its way down inside the rainscreen will be pushed back out by the cill. …..aaahhh just described the detail in Nick's photo!

Mark

[Nick Grant]

What cill is that then Chris??

[Mark Siddall]

Peter,
If I have understood you correctly the window frame aligns with the internal face of the external leaf. As a consequence of this detail the Psi value is higher than it would ideally be for a Gold standard home (see the start of the thread.) The Stamford Brook thread has a Psi value graph that is informative and confirms reasoning behind the German diagram shown at the start of this thread.
The essence of what you say though would seem to confirm that the rendered reveal option should be workable.
I am interested that you have pre-installed the windows in some cases. Were these simply built straight into the cavity wall as it progressed, or did you form the opening first? Did this instalation process cause any problems? (See my concerns about lifts and impact upon programme.)

Mark

[Nick Grant]

Photo of window in fairly new budget hotel in Copenhagen.

It felt spongy but I didn't hang out of the window to look properly, just held camera out there. Looks like a drain hole in corner?

Are we thinking some sort of external cavity closer/trim to fill gap between brickwork and window?

For render on external insulation it's no problem.

[Andy Mitchell]

Looks like window is between brickwork, not behind??

Maybe something like an external cavity closer, yes.

[Nick Grant]

Sure, not the detail we are after but the spongy sealing detail is nice.

[Author]

Posts