Window trickle vents for secondary glazing?

[malcolm]

I'm going for a change of use for a disused pub in a conservation area (which is like being listed on the outside). We've got some lovely very big 1920s leaded bay windows that I want to preserve.

As in previous topics change of use means building control is going to require we upgrade thermal performance (as we want to), conservation officer is going to want to keep the lovely windows (as we do). I don't fancy ruining the inside with ugly aluminium secondary glazing or even magnetic stuff but do want decent thermal performance. There is likely to be a trade off in insulation with building control as I don't want to make the 17c wood frame rot any more than it has already and will resist wall insulation. Can put as much as they like in the roof but very good windows might help negotiations.

We've got a good window frame thickness to work with and the windows are fixed and not opening. I'm tempted to fix single or even double glazed units (possibly thin ones) in the frame to act as primary windows behind the existing but am worried about condensation between those and the existing leaded windows.

Would old and poorly maintained leaded windows offer enough ventilation to keep the cavity dry or will I need to add some trickle ventilation from the outside? And how to do it nicely? The question is prompted by a load of research and a desire to plan ahead - I wasn't going to contact building regs until the planning application is done with.

 

[88v8]

My experience of secondary glazing is that unless the cavity betwixt original and secondary is pretty small in volume, it will mist up at the bottom and trickle-type venting will not clear it. With a succession of cold days, the mist will become beads, and then begin to run down the window.
The affect has been ameliorated by drilling a hole in the frame to the outside, but this is not entirely effective and leads to the inner pane becoming colder, so that one can end up with both inner and outer pane misted.

However, our house is by most standards pretty damp. Our rh is in the 70s. In a dryer house, this may not be an issue. In our previous house - 1920s - our secondary DG consisting of sealed units fitted inside the original leads scarcely misted at all, even though I knew nothing about trickle-type venting and installed none, and the sealing around the secondaries was not great.

What rh do you have?

vor

 

[malcolm]

The relative humidity near the big windows is about 60 to 65 at the moment. It is tempting to fit sealed double glazed units behind the existing leaded window with a gap of maybe 0.5 inch.

I guess if the double glazed units are well sealed in place then there shouldn't be much moisture getting from the inside to the existing window, but sealing them well would make them difficult to remove to vent the existing window from the inside. Venting from the outside could work but I can't think of a way to do it neatly. There might already be enough venting through the leaded window. I could do one panel for a test and see what happens.

 

[88v8]

For external venting here, I have drilled holes, slanted downwards, obliquely through the frame. Somewhat less than quarter inch.

Venting from the inside is useless - it's the warm/damp air from the inside that's the problem. The sealing needs to be pretty much hermetic. Not easy. The windows that come nearest to working here are quite small - about 12" x 20" - with an air gap of less than half inch.
I'm using this to create a seal between the inner glass and the interstitial wood bead
http://affixit.co.uk/industrial-tapes-expanding-foam-tape-c-2_57
Various widths, and thicknesses, very compliant.

Between inner glass and inner bead of these windows, no seal. The inner bead is fastened on with brass screws, I can remove it if needs be although I hope not to.

For opening windows, I've made oak cover bead, will have to remove it all in summer.

At our previous house, I increased the depth of the existing frames by nailing/gluing 2" x 1" to them, then inset the sealed units set back about 1.25" (ie the width of the existing stop bead). The units were in solid brass frames made of u section, all hinged. It was quite a palaver to fit, but in our dry house it worked pretty well.

Ivor

 

[OriginalGlass]

Do you have any Pictures Malcolm?

 

[malcolm]

Thanks for the advice Ivor. Your approach sounds very similar to what I would like to do.

Here are a couple of pictures. Initially I thought the moulding on the inside was beading but I'm starting to think it might be the actual frame. It's tricky to tell for sure as there is a lot of paint on there.

Here is the window I'm talking about (photographed in the middle of renovation) . It's about 6 feet tall and maybe 7 or 8 feet wide. The centre panels have nice old glass and the outer panels have been replaced with stick on leading.

I might have to resort to secondary glazing on the inside instead of a proper job to avoid changing the existing frame. The metal glazing bars supporting the leaded glass extend towards the rear of the beading.

 

[malcolm]

Searching through older threads about secondary glazing and thought this one could do with an update. Building regs wanted wall insulation on solid walls and secondary glazing throughout but accepted the technical difficulty in insulating the old timber frame walls before I get around to fixing them.

A chap from Storm Windows came around today to look at secondary glazing. I was impressed by the complete lack of any of the high pressure sales techniques you might expect from glazing companies. The system they generally suggested for fixed windows was 4mm glass attached to an aluminium frame via magnetic strips at the side, a channel at the top, and a seal at the bottom. Quote is due next week. I could probably do something a bit similar myself if the quote is high.

I can't see the system being completely air tight from the inside (though the guy said it would be). I certainly can't see the existing windows suddenly becoming air tight from the outside. Drilling holes in the frame from the outside would reduce thermal performance and reduce payback time for me beyond the current 100 years. I guess as the panels are removable I can wedge them open from time to time during the day to allow the cavity to dry and I expect that would work reasonably well. That's more than I can do with the 10 year old double glazed units I have which have all failed.

Anyone had experience of living with commercial secondary glazing using glass? Any more tips or thoughts would be very handy.

 

[88v8]

Update on my previous replies.
My 2013 secondary glazing campaign here has on the whole been successful. But.....

In all, I did eight windows.
The air gap varies from a half inch to about 3 inches.
in all but one case the cavities have misted, despite what I believe is very good secondary sealing. It only needs a tiny weeny gap for the temperature variation in the cavity to suck air from the house through volume variation.

To the south - six windows - the misting has been intermittent and has always cleared during the day. If the sun appears, these windows are in full sun.
The window to the west mists quite badly but just about clears. It gets sun late evening. Sometimes it is more misted than a sister window in the same room that I did not double-glaze.
The window to the north has been misted pretty much every day until this week, and does not clear. I have had to remove the glazing twice, and clean the mould that began to take root.

In all cases there is what I would regard as adequate ventilation to the outside. Without that, I think the windows would become a vertical aquarium.

I do not believe this winter has been much of a test; we had no successive days of sub-zero. In a hard winter, I suspect the north and west misting might become endemic.

As I said before, it also depends on how 'damp' your house is. Our rh was over 70% at the start of winter. With two wood-burners going it fell to about 45%, but now it's warming and we're only running one stove or the other, it's crept up to 50% again.
If you have <50% rh you are less likely to encounter problems.

I would try one window and see how it goes. Choose one on the north side, that will be the greatest test. See how much you have to ventilate the outer window to make it work. If the glazing doesn't happen until summer, wait until well into winter before deciding that it's OK or not and proceeding with any more.

I won't be doing any more northerly secondary glazing. I might try some slimline units, but we have leads and I fear the aesthetics may be too negative. For me, aesthetics trump insulation.

As to payback, one has to reckon the cost of capital. The money you spend could presently be invested at 7% tax-free, so if your return is less than that, the payback is.... never.
Otoh, it could make the house more comfortable by reducing the thermal gradient across the room, and of course helps to save the planet blah blah.

But no point kidding oneself about financial payback. Beyond the obvious and cheap measures of lagging the hot tank and the attic, most so-called cost savings are a bust. Radio programmes where they blather about 'economy' measures while ignoring the cost of capital, and often not even knowing how much of their energy bill is for hot water and how much for space heating... doh.

Ivor

 

[malcolm]

My big bay windows face west and I have 3 smaller east facing windows. I need to replace the north facing window with something else for fire egress reasons so likely that will end up double glazed. Sounds like I will have a problem with condensation.

Did you once post pictures of your installation on here?

It's kind of a box ticking exercise to fit secondary glazing. Needed because of change of use so I don't really have much choice other than opening as a pub. Though it's good to try and avoid creating trouble. I'm planning to internally insulate a cavity wall rather than fill the cavity. Costs a lot more but is reversible.

Might fit the best I can and suck it and see. It's only the bedroom windows that get any condensation at the moment. Possibly the others 'breathe' enough.

 

[Keithj]

We had the same problem with a large leaded light window in the front of the hall. It was cold and draughty! Normal double glazing was a no-no: the CO is helpful, but not THAT helpful

With the COs approval, we had our carpenter fit secondary glazing held in with guides and latches (or whatever the correct technical term is). It misted within a couple of days, and got steadily worse. I just happened to have some industrial desiccant (used for electronic equipment in long-term storage). M made a cloth "sausage" to hold half a kilogramme of that, resting along the bottom of the frame between the inner and outer glazing. That absorbed about 200ccs of water a month, at which point it was saturated and the windows misted up again. Out with the sausage, weigh it, microwave it on "low" until it returns to dry weight, and back in it goes.

That worked for a couple of years, until the repeated cooking killed the cloth bag and there was a desiccant/floor moment. We produced the Mark II sausage, a kilogramme of better spec stuff (£8 on Ebay) in a larger fabric tube. It's been in there all winter, and there's still no sign of misting. It's not a thing of great beauty, but is the same colour as the window frame and not too auffällig.

 

[malcolm]

Does £7,500 seem a lot for aluminium framed secondary glazing with 4mm glass for 27 window panes? The big bay windows account for 15 of the panes and there are about 7 more normal windows. It's about £300 per pane. They have suggested a U value of 2.0 for the system including existing glass which would make building regs happy.

One acrylic glazing seller is suggesting a U value of 2.7. I'm tempted to price up acrylic magnetic secondary glazing and argue with building regs that 125 year payback won't save the environment.

 

[malcolm]

I've got a cunning plan!

Following several visits from secondary glazing companies with expensive and ugly solutions I'm looking at DIY again.

Given the windows are fixed non opening I'm tempted to buy glass of the appropriate size and secure it with a wooden moulding such as the Astragal moulding in the picture below from Wickes. A fixed installation with plenty of sealant on the secondary glazing and a little draft from the leaded windows might give me half a chance at reducing condensation.

I've not found any suitable beading for the edges. Can anyone recommend beading and moulding supplier?

 

[Feltwell]

Could you do something similar to what I've got on this window? See below - hopefully you can see the secondary glazing, bottom picture shows the window without it:-

 

[malcolm]

That's a very neat system. I might look at doing something like that on the windows upstairs. It might be a bit bulky for the big bay window downstairs but it would be lice to be able to lift them out. Did you make the sections for the wood frames or are there ready made parts on the market? They look a bit like the sections of wood used to hold the glass in my greenhouse.

 

[Keithj]

But how do you stop them misting up? We have one at the front of the house which made for a serious cold spot, so we fitted secondary glazing to it. It promptly misted up.

I fixed it by getting M to make a long cloth sausage, which I filled with about two pounds of industrial desiccant (came in the box of a piece of military radio equipment that I bought decades ago). That stops the mist for about three months, then I have to microwave it on low for an hour or so. Is there an easier way?

 

[MatthewC]

I have to say that my acrylic sheet DIY secondary glazing does not mist up at all. I can only guess that this is due to the draught through the sash windows, both around the sashes and through gaps in the old putty, which keeps it well ventilated. Imagine what it was like before I did the SG! I really must refurbish these windows next year (as I have been saying for the last three years).

I have removed the SG from our bedroom window as J thought this was making it stuffy and not helping her breathe. Of course, when it's cold outside, the glass does then mist up completely during the night.

 

[Feltwell]

That's a very neat system. I might look at doing something like that on the windows upstairs. It might be a bit bulky for the big bay window downstairs but it would be lice to be able to lift them out. Did you make the sections for the wood frames or are there ready made parts on the market? They look a bit like the sections of wood used to hold the glass in my greenhouse.

I didn't make that frame. The previous people here must have had it made for them - it's quite well made, and their own DIY attempts were pretty poor, luckily they didn't attempt much! Mind you they did get the window cleaner to do various DIY jobs for them - and lovely chap as he is, he's better suited to being a window cleaner than a handyman.....

Anyway, the frame looks to have been made with some standard PSE rectangular-section softwood. Someone has ran this on a router table I'd guess to remove a bit from one corner to form a rebate. They've made it into a frame and reinforced the corners from behind with metal L brackets, routing them into the frame so they're flush.

They've then glazed the frame, holding the glass in with a bit of silicone, and then pinned a very small quadrant moulding to face out the rebate, but one that is very slightly wider then the glazing rebate - then stained the lot to match the window frame. A neat job, except that for some reason they chose to stick some draught-proofing foam behind the frame to seal it against the window - a good idea but they stuck it to the face of the window frame rather than the back of the secondary glazing frame. Doesn't show when it's fitted though.

 

[Feltwell]

On this frosty morning, I thought some pictures on the effectiveness of secondary glazing against condensation might be of interest to some of you considering fitting it.

Firstly, a window which has been renovated and is now well draught-proofed but has no secondary glazing - as you can see it is absolutely dripping wet:-



Secondly, and more tellingly, two windows side-by-side in the same room. Neither has been renovated yet - both are draughty - but one has secondary glazing, I haven't finished the other one yet.



A bit of a difference as you can see! The right window is not as dripping wet as the first picture though, which may be just because it's in a different room, or maybe the draught from the window itself having an effect. What I don't know yet is if those windows would mist up inside the secondary glazing once they've been properly draughtproofed.

However, in our small en-suite bathroom the window is both draught-proofed and secondary glazed, although it isn't a sash and is difficult to get fully effective draughtproofing on. This only mists up slightly inside, despite the best efforts of my daughter to turn the room into a sauna by having very long, scalding hot showers and always "forgetting" to turn the extractor fan on!

Secondary glazing is 3mm acrylic held on with magnetic & steel strips in this case. I have found that some additional mechanical fixing, by means of clips, is necessary on the windows that have not been draughtproofed otherwise, in strong winds only, the draught causes enough air pressure to build up in the gap to push the acrylic off it's magnetic strip. The glazing is a right pain to store in summer, which is it's main downfall. However, with the style of windows I have options are very limited - as I've no internal window reveal the more permanent types where you effectively have a secondary window mounted in the reveal internally are not an option.

 

[Flyfisher]

Impressive results there Feltwell. I have some windows that could benefit from a similar thing and I do like the ease of removing them during the warmer months.

Though I must say . . . .

The glazing is a right pain to store in summer, which is it's main downfall.

. . . . I find that hard to believe, given that aircraft hanger of a 'garage' you built :wink:

 

[Feltwell]

I find that hard to believe, given that aircraft hanger of a 'garage' you built :wink:

I wish! The "aircraft hanger" now has a top floor, in addition to it's main floor area and the sub-floor "cellar" storage. I still run out of room

What's that, "Too much stuff!" I hear someone cry? Never!

Actually the main problem with storing the perspex is it's size (large windows are getting on for 8' x 4' - very unwieldy trying to handle a flexible sheet that size) and trying not to scratch it. Smaller panes would be a lot easier to store. I have got a storage hidey-hole that even the big sheets fit in, tucked behind the staircase inside the garage, but the route to that is usually blocked by something.