Inalling DG units in Wooden windows

[Antwerpman]

I am currently restoring the original oak windows in my period property here in Belgium. Unfortunately conservation does not seem to have taken off in this part of the world and the usual modus operandi is to rip all the old stuff out and replace with new

Anyway one concession I have to make is to install double galzed units in to the original frames as they are very large windows (original wavy glass all gone except in a couple of small areas) and the heating bill for the house is rather large.

there is plently of depth to the original frames to rout out the recess for the panes deeper to allow for the DG units, but I havea couple of questions.

• What are the common combinations of glass thickness/air gap for DG units?
  
  I have heard that where noise supression is a requirement to use laminated glass for one of the panes, is this true and if we used this what would the thickness combinations then be?
  
  what is the 'normal' way for mounting DG units in to wooden frames, considering that they will be treated with linseed oil and painted with Linseed oil paint. Holkam say that the oil can cause the seals on DG units to fail - how do you prevent this?Are the panes dry mounted and held in place with a bead or is putty the best method?
  
  How much deeper does the rebate need to be than the DG units? by this I mean if the DG unit is 20mm thick, does the rebate need to be 25mm? 30mm?

thanks in Advance for the help

 

[biffvernon]

The ideal air space for double glazed units is about 16mm, so with two sheets of 4mm glass that makes a total of 24mm. When I make windows with such units I use pvc foam glazing tape that squashes down to about 2mm each side and externally it is held in with a wooden bead of about 12mm so the total rebate needs to be 40mm. This is often too much to create in an existing old window.

If sound insulation is important to you then a larger space is better - perhaps use secondary glazing. Yes, laminated glass as one of the layers in double glazing gives a significant noise reduction. One can get laminated glass with an especially thick plastic layer that works even better than the standard laminated.

To get the best of all three worlds, reglaze the original frames with real cylinder glass and add secondary glazing with laminated glass. It maintains the historic fabric while improving sound and thermal insulation.

I don't know about Belgium but I have seen this done in many old houses in the Netherlands.

 

[skier-hughes]

We had a future N coating and argon filling to help give us a better U value to our double glazed units. Although as they are open all day long I don't think that makes much difference

As for the painting, you should have had an instruction leaflet from Holkham. But if your paint hasn't come yet,
You paint with linseed everywhere.
Paint rebate and beads with shellac.
Glaze
Paint with linseed paint.

 

[Mark]

You can get very thin dg units which have a vacuum 'filling' they have very very small metal splints holding the panes apart - that shows when you are right up at the glass as a number of specs on the glass.

dont know who / where though.

 

[Antwerpman]

thanks for the tips everyone, much appreciated.

is the 16mm air gap in response to the latest round of building regs and u values? I dont seem to remember it being so large in my last house (Maybe I just have a bad memory)

Anyway I measured the thickness of the frame yesterday instead of guestimating from memory and it is 43mm thick, so 4-16-4 seems out of the question. I also found some reference to replacing the traditional 4-16-4 with 10mm float glass and 6.3mm laminated to reduce the transmission of sound (reducing the air gap slihtly) but it seems that this will not be feasible for me.

Looking at the construction of the window I think I can get the rebate to about 33mm which is a little less than Bif recommends

Can I ask Biff, how do you seal the frame to the pane when you use glazing tape? do you not get water ingress between the beading and the pane if there is a 2mm gap where the glazing tape is? Why not use Linseed putty instead? what are the advantages/Disadvantages of each method and do you know how much depth you need to leave for the putty to be effective?

I thought of secondary glazing, but the window openings are so big that it is difficult to imagine how I could place the secondary galzing in a way which was attractive, not too costly and still allowed the windows to function. Luckily being Belgium we have (wooden) roller shutters on the outside of the window for night time which helps keep the heat in as well.

Skier, thanks for the info on the paint, I have received my paint but being a typical man the instructions were quickly put to one side :roll: I would have thought I would have learnt by now

Finally, do the DG units sit directly on to the bottom of the frame, or is something used to pack out between them?

Cheers

 

[skier-hughes]

Speak to your d/g manufacturer, they should be able to give you U-values for different gaps etc.

Biff's 16mm air gap is the most efficient, you can get larger air gaps, but the u-value doesn't increase significantly for the size increase. Sound noise reduction is better though.

Different fillings like air/argon/vacuum effect the U-values, so you can get smaller gaps.

Different glass coating also effect the U-value.

We had 4 mm toughned/4mm normal glass/futur N coating/argon filled and 8mm gap and came in at a U-value of 1.6, so well within current spec.

There isn't a 2mm gap, this is how much the "tape" compresses to, it's double sided sitcky foam.

Graham

 

[biffvernon]

is the 16mm air gap in response to the latest round of building regs and u values?

Thermal insulation increases with size of air gap but it's not a linear relationship. 16mm is a lot better than 12mm but not much worse than 20mm. Above about 20mm convection cells start up and the insulation then gets worse. So 16mm is a good compromise and has become the industry standard. Older units were often 8 or 12 mm.

I also found some reference to replacing the traditional 4-16-4 with 10mm float glass and 6.3mm laminated to reduce the transmission of sound (reducing the air gap slihtly) but it seems that this will not be feasible for me.

That would certainly give better sound insulation. The thicker the glass the better but, importantly, having the glass layers at different thicknesses cuts out a broad spectrum of sound frequencies and reduces the possibility of resonant frequencies getting through.

how do you seal the frame to the pane when you use glazing tape? do you not get water ingress between the beading and the pane if there is a 2mm gap where the glazing tape is?

The tape is double sided sticky so fills the gap between the glass and the rebate on the one side and the gap between glass and beading on the other. The manufacturers claim that the tape is waterproof. Adding a bead of clear silicone sealant on the top edge of the tape would add braces to the belt.

Why not use Linseed putty instead?

Interesting question. The double glazing industry is unanimous in the opinion that one should never use linseed oil putty with dg units. I did eighteen years ago on a window I put into my own house before I knew better. It's still doing just fine! I think the theory is that linseed oil putty sometimes cracks and that this might allow water to reach the sealed edge. Then if the sealed edge failed water could get into the unit. I've also heard that putty reacts with the butyl seals buy this may have more to do with urban mythology than chemistry.

Finally, do the DG units sit directly on to the bottom of the frame, or is something used to pack out between them?

Little plastic packing pads are often used for the unit to sit on. This creates an air space under the unit. The biggest danger of a unit misting up is when the unit sits in a puddle. Having a space keeps the bottom of the unit clear of a puddle in the event that the seals have failed and the water does not have a means of escape. In PVC frames there should be drainage holes to get rid of such water and allow air in to keep it dry. In wooden windows, at least where the rebate is not painted, any water that might get in should soak into the wood and dry out through the wood, so long as it is not painted with a paint with low vapour permeability. The philosophy to keeping dg units from misting up is to put in as many defensive layers as possible to keep the water out and then allowing it to dry out if that doesn't work.

 

[Antwerpman]

Thanks once again for the replies, most useful and informative

I found this link if anyone is interested

Fitting sealed DG units in Wooden windows

It gives the authors opinions on why DG units fail in wooden windows and how to prevent this. The main reason he quotes is ingress of water in to the rebate. Do you think the use of Linseed oil paint and its breathable nature will help to avert these problems?

A quick question for Biff - when you use silicone sealant around the beads, do you do this before or after painting with linseed oil paint? Will the Linseed oil paint actually adhere to the silicone?

The discussion forum from which the above link comes can be found here

window forum

Mostly about plastic but interesting reading all the same to see what problems these windows have

 

[skier-hughes]

Although I'm not Biff, I did speak to Holkham to clarify a few points, so it's paint with linseed oil, seal with shellac fit with whatever - silicone or d/s tape.

Graham

 

[biffvernon]

A quick question for Biff - when you use silicone sealant around the beads, do you do this before or after painting with linseed oil paint? Will the Linseed oil paint actually adhere to the silicone?

Before or after painting? I don't think it matters but I can't see what is to be gained by putting paint on top of silicone. I don't know how well it would stick and, anyway, I'm not too sure how long the silicone really lasts. It's the sort of thing that might be worth keeping an eye on and maybe replacing after many years if it looks like it's coming adrift. But as I said, it's the braces and the belt is still there.

That Window man article has some interesting things to say. His theories are fine but I'm not convinced about some of the details in practice. He points out that the drainage holes have to be big enough to allow water to drop through unimpeded by its own surface tension, and then worries about insects getting in and so he suggests stuffing some fibre glass up the hole to keep the bugs out. But this would stop the water draining out. We used to have a half timbered Morris Traveller what had drain holes in the wood under the windows. They were forever getting blocked with earwigs and spider nests so I'm not convinced that drainage holes in the bottom of window frames actually do much good after they have been neglected for a few years.

 

[Antwerpman]

Biff, you noticed the same contradiction as me - I also thought it a bit strange. It is also like saying that unless there are 6mm holes in the beading then the water will never get in to the rebate also as the surface tension will prevent it

I think his suggestions are perhaps worth some consideration even if his scientific reasoning for them is somewhat lacking. certainly he advocates raising the dg unit up from the wood in the same manner that you do. Hopefully though the use of linseed oil paint will allow any trapped vapour to be shed due to its breathability. I will of course also try to make sure I have belt and braces by trying to prevent any water ingress in the first place.

The reason I asked about painting the silicone is because I thought that painting over the beading and on to the pane would provide an additional seal - I guess I was thinking of how you do it when you use putty where you want to paint on to the pane to ensure a fully weathertight seal

cheers

Antwerpman

 

[biffvernon]

I guess the point about the hole size is that water will soak through a very small gap by capillary action but only drop down a drain hole under gravity if the hole is quite large (large enough to be blocked by an earwig).

His suggested spacing for the drain holes seems rather large - I can imagine a puddle sitting in one place with a drain hole 20cm further along. Maybe the 'drain' hole is more effective as a ventilation hole. It seems as though our windowman is taking the good design features (oxymoron?) of uPVC windows and applying them to timber frames.

I think that puddles are less likely to remain if the wood in the rebate is not painted/varnished/knottinged or otherwise made waterproof. Allow the water to soak into the wood - but make sure that it can dry out again by not painting with a vapour impermeable coating.