Secondary glazing on old windows

Energy prices have finally pushed us into doing something we’ve talked about for years – installing removable secondary glazing on the windows. In our case, this is worthwhile for draft proofing as much as for improved thermal efficiency, because we have 7 leaded windows, which are leaky, and impossible to seal any other way.

Secondary glazing needs to be light, easy to carry and hard to break, making it safer if we want to take it off for storage in spring and reinstall in the autumn. It turned out to be quite simple to do, using sheets of solid polycarbonate cut to size by the supplier.

The polycarbonate sheet is hard to see on this mullion window

The great advantage of polycarbonate in a crooked old house is that it’s far easier to drill and cut to shape than glass. In our case, that required several curved cuts to fit against ancient oak posts and beams, which I’d hate to try with glass.

Polycarbonate is also half the weight of glass, just as transparent, and very tough indeed. The disadvantage compared with glass is that it is easily scratched by rough handling or careless cleaning if you don’t follow guidelines. But those problem are far outweighed in our case by the advantages.

There is another choice, clear acrylic. This is similar to polycarbonate, which was not far short of twice the price, at just under £50/square metre against around £26 for acrylic.

Polycarbonate is much the strongest. It’s a bit less easy to cut and drill, but its UV resistance and much greater strength – needed for regular removal and reinstallation – made it worth spending more.

At first, we thought we’d need to build wood frames to support the polycarbonate. We bought a 3mm thick sheet for one window as a trial, and it was clear immediately that the polycarbonate was stiff enough to mount without a frame.

Thought needs to be given to how to fix the sheets and that will vary with preference and type of window.

Nine windows in the old part of our house are old and irregular, eight of them with oak mullions, one of which is already well insulated.

Polycarbonate on a 19th century window

After playing around with the trial sheet, much the easiest solution was simply to screw it to the window frame, accepting that the small holes would be visible when the glazing is removed. This worked with both the mullion and the more modern windows.

A complication is that it is advised not to drill holes less than 40mm from the edges of the sheets, or it could produce cracks. So where the window layout does not allow a good overlap of the sheet with the frame, another method is needed. The simplest is to hold the polycarbonate against the foam with removable battens.

This short piece of wood was enough to hold one side firmly against the draught-proofing foam

Given the sheets’ stiffness, the battens do not need to be continuous all the way round. A few short ones or small, swivelling toggles would do the trick, depending on the exact design of the window. Others may come up with different solutions.

Next question is how to make a seal between smooth plastic and the frame. Our solution was ordinary foam draftproofing strips with adhesive backs, stuck all the way round the edges of the sheets.

Applying draught excluder to the edge of the sheet. While working, leave most of the protective film on the polycarbonate sheet till the last minute, peeling back just near the edge to apply the draught excluder.

For the Victorian and more recent windows, where the timber surface of the frames is flat, white 5mm foam was enough. For the irregular oak timbers framing the mullion and other windows, we used brown 10 mm draftproofing strips.

The technical advice sheets say use new and very sharp drill bits for the holes to avoid cracking or overheating.

Tighten the screws evenly, each one a little at a time, as you go round the perimeter, several times. Tighten as little as possible, or the polycarbonate will distort. They need to be just tight enough for the foam to make contact all round.

Using a fine saw to trim the sheet, leaving in place the protective film on both sides. A scribing tool was needed on this sheet to mark a gentle curve.

Some trimming of the sheets may be needed if the window frames are not square; if done with a power jigsaw, then a specialist blade for plastic is required, to avoid melting the material. In fact, it was easier to use a new, sharp handsaw, as we did not have much cutting to do.

This saw cut shallow curves quite well
..and this very fine-toothed saw worked well for straight cuts.

Fine adjustment of the shape of the perimeter can be done with a sharp flat file, though make sure the sheet of polycarbonate rests on a clean, flat surface while you do it.

The sheet comes with a plastic protective film on both sides, to peel off just before final installation – it is important to leave it on as long as possible. The type we bought has ultra violet protection on one side, and that should face outwards. The correct side is marked on the film. Some later sheets had UV protection on both sides.

Sheet with protective film still in place, ready to screw to window frame. Here black draught excluder has been applied all round the edge.

Because polycarbonate is stiff, we found 6 x20mm screws at around 250mm to 300mm centres were an adequate size on windows where the sheet rested on the window sill and 5mm foam was used.

Where the sheet used 10mm foam (on rougher oak surfaces), and had no support from below, we used 8x25mm screws. We used zinc-plated steel screws on white-painted windows, and round-headed black japanned where we screwed into dark oak timbers.

An old oak window with polycarbonate fitted using black draught excluder. (Three mullions are missing from this window).

The cost averaged at around £60 a window, including a modest amount for draughtproofer and screws. Our supplier, which delivered accurately cut sheets on time, was

Be very careful cleaning polycarbonate, because it scratches easily, even though it is hard to break. I made a mistake on one window, cleaning it with an ordinary wet cloth and not noticing grit had fallen from the limewash wall above. I found tiny scratches, luckily much less obvious than the distortions and scratches on the old Victorian glass of the window behind. Your supplier should have advice sheets on cleaning: stick to what they say. Some everyday cleaning products and cloths can damage polycarbonate and it should not be wiped and dusted with dry cloths.

Apart from the sill, none of the sides of this window are straight, so trimming was needed.
Detail – the sheet and foam fitted neatly upwards into the slot in the beam above this mullion window, along which the old wooden shutters used to slide. We have one complete working example of a traditional shutter, so that window does not need polycarbonate.

We had planned to remove the polycarbonate each spring and reinstall in autumn but, on reflection, there’s not much point on the mullion windows as they cannot be opened. So these will only come off for cleaning. Polycarbonate on opening windows will go in the garage for summer storage.

Keep any cardboard sheets the polycarbonate is packed in. It will be useful to protect them during summer storage. I also kept some of the protective film, in case any sheets turned out to need more work.

And finally, there is a great deal of information on line about secondary and double glazing. For modern double glazing, optimum air gaps are no more than 20mm, beyond which convection begins to remove some of the benefits.

For a complicated listed building like ours, secondary glazing can only be fitted in practice with much larger air gaps, of 50 to 150mm, so thermal performance cannot be anything like as good (though noise insulation is improved, if that is what you need – but we don’t).

I can’t quantify the heat performance of our secondary glazing, and anyway it would not tell us much if we knew.

That’s because an unquantifiable part of the heat loss is through the leakiness of our old windows, especially our 7 leaded windows, which have draughts around many of the small panes, and I cannot see any other simple way of stopping that.

All I can say is that there was an immediate improvement in thermal comfort once secondary glazing was installed, from a combination of reduced draughts and improvement in thermal efficiency.

PS One way to judge which windows in an old house are leakiest is to see how many spiders webs are on and around them. Where there are draughts, spiders put webs, because draughts help them catch prey more effectively. Till installing the secondary glazing, our leaded windows have had to be cleared of webs every couple of weeks. Spiders are ineradicable from a 16th century house, though thankfully ours are mostly the tiny bodied, long-legged but innocuous looking Daddy Longlegs.

Long-lasting lime plaster for repairs

I’ve just done a few pre-winter repairs of the outside walls in spots where surface render has lost its grip on the clay daub beneath. To my surprise, chalk lime plaster bought in 2014 is still usable.

It appeared to have solidified under an inch or so of water. But after scraping at the surface it quite quickly began to recover its original consistency. Lime putty when stored properly lasts many years – I still have an old tub of it in good condition – and this plaster is simply putty, chalk and fibre.

It is not entirely traditional: it uses synthetic fibres and they last indefinitely in the tub, unlike haired plaster, where the hair begins to disintegrate after a month or two.

Chalk-lime plaster – quick repair ahead of the winter

The old plaster is not ideal, because it takes hard work with a trowel to get back to the right consistency. But having forgotten to order a new tub, it’s good enough to protect small areas of exposed clay from rain and woodpeckers for a while. Next, I must order a new tub.

Chalk-lime plaster used externally has proved extremely effective over the years in temporarily patching areas of old wall that we haven’t fully repaired yet. A thin layer behaves almost like a sticking plaster over the cracks where old render is beginning to come away from the wall. Once cured, it has considerable strength and resists tearing.

There’s one quite large patch where the old render has almost entirely lost its grip on the clay – tapping reveals a hollow behind – but plaster over the cracks has kept it in place for the last few years.

It’s admittedly a bodge, but we wanted to delay full repairs as long as possible while effort and money went into other priorities. In due course the whole of one gable and about half of one wall still await a full overhaul.

Both will be a challenge. In the days when our house was part of a farm, a lot of running repairs on the clay were done with cement render. A bit less than a third of the walls is still cement render. The rest is our past repairs and some areas of wall that have been untouched for many years.

The latter are not even rendered, but are clay daub covered with many coats of limewash over a long period. The limewash has built into a thick layer that, until we examined it closely, we thought was in fact a render. So our farm building’s exterior walls may once have been rough clay daub given a protective limewash.

Beautiful boulder clay

This unprepossessing grey mess I collected in a bucket is boulder clay. If you’ve got a house with clay walls, it’s like finding a seam of valuable minerals. There’s no clay to beat it.

Our part of Suffolk mostly sits on thick layers of the pale grey, white-flecked natural building material, laid down by glaciers grinding their way over chalk. But it’s usually deep down and hard to find.

It comes near the surface here, because that’s why our pond exists, sitting on an impermeable bed of clay. But it’s not so easy to dig out without draining the pond, which we think was once a clay pit.

Digging a trench to plant a hedge yesterday I kept finding lumps of it, deposited there after earlier work on the pond. It has got muddy, but that doesn’t matter – a bit of earth won’t spoil its performance.

Not only does it make a good, plastic clay for wall repairs when mixed with straw, it is also less prone to shrinkage cracks than ordinary yellow clay, and dries as hard as a lump of chalk.

If you have clay daub walls and come across any, store it till you need it. Wonderful stuff.

Updating an old door without replacing it

This nice door was made for The Old Brewhouse when it was a farm service building where nobody lived.

It has thin planks with cracks between, and is a heat sink in the house during winter, no matter how much draft proofing is stuffed in and around it.

As a listed building, we’re supposed to make a formal application to the council heritage department and pay a fee if we wish to replace it. Our solution was to leave the old door untouched, apart from a few new screw holes, and build an identical door on the inside of it to double the thickness, cover the cracks and improve its thermal performance. It should make a noticeable difference to the warmth of the room in midwinter.

Continue reading “Updating an old door without replacing it”

Ancient drains

I’ve just laid 35 feet of new drain from the house down to the pond, and discovered a layer of what seem to be 18th or early 19th century drains, of a type that a little internet research discovers were named horseshoe drains. I have often noticed, whether at Greek ruins or at Norman Castles, that the remains of old drain and sewage systems fascinate visitors, myself included, so the annoying fact that one of our key drains was totally clogged with tree roots at least produced something nerdishly interesting. Continue reading “Ancient drains”

Repairing the floor – 3

I go into a great deal of detail in these floor posts, on the grounds that if you’re interested in repairing a very old floor you’ll need it; for the rest, read no further! Over the years it’s the blog posts like this that seem to be read most, I assume by people doing similar jobs.

Rotten sections, weakened sections, blackened, stained and crack and holed sections – the whole messy old floor came together into one attractive, if battered, whole, once beeswax polish was applied.

It’s great what a coat of beeswax can do for a damaged floor.

Continue reading “Repairing the floor – 3”

Repairing the floor – 2

The first thing to do was to protect the room underneath from debris falling through the many cracks as we worked on the floor above. The solution was a plastic tarpaulin tied to the joists:


The second thing to do was to make sure the boards were firmly on their joists so not springy when walked on. There were two bad patches, where the boards had to be screwed down hard onto the joists to stop movement. Since the undersides were exposed, we were also able to push in wood spacers from below. Continue reading “Repairing the floor – 2”

Repairing the floor – 1

The floor we want to repair is of unknown age but must be pretty old, not just because of its battered state but because the boards are of uneven widths of up to a foot or so, presumably because they were cut from the same tree. Continue reading “Repairing the floor – 1”

Old floorboards

And now for the next challenge, but I need to get fit first: does everyone get quite as stiff as I do when trying to work for long hours on a floor? It is the most uncomfortable position to work in, apart perhaps from plastering a ceiling, but we are about to start on repairing a lovely old floor that until now has been covered to protect it, so the pain will be worthwhile (I hope). Not entirely sure what it is, but think that it is oak or elm. More later, as we get going on the job next week.


The hot, dry weather has produced many more cracks than usual in our front wall.


Much of it is plain clay daub with thick layers of limewash and here and there a smear of recent cement where previous owners had tried to stop the clay falling out. There is no evidence of it ever having been rendered or plastered on laths. Continue reading “Cracks”

Limewash over emulsion paint 

I’ve been wondering how best to  redecorate a room whose surfaces are finished in old materials – clay and plaster – but which have been painted with modern emulsion. That may be creating a breathability problem.

Very modern emulsions are impermeable to moisture. Cruder basic emulsions have a certain amount of breathability, but it’s hard to tell which it is, looking at the result at least 30 years after it was last painted. So should the emulsion be removed? Continue reading “Limewash over emulsion paint “

Rat assault 

Just discovered a disadvantage of shallow brick footings: a large rat managed to tunnel right under them and up inside where the water and waste pipes enter and leave the building.

Caught the rat with an electronic trap and filled the hole down to the bottom of the footings with a couple of buckets of limecrete, using lime, sand and gravel.

Had to do it twice more because another rat twice bypassed the limecrete by digging a longer tunnel, partially removing the still soft limecrete as it went. Continue reading “Rat assault “

Old fashioned paint

It has been a positive pleasure painting the windows in the last few days using custom-mixed linseed oil paint from Ingilby’s of Glemsford. The window frames were stripped and repaired, and much of the putty renewed, some time ago, and they have since been given several coats of pure linseed oil to prepare and waterproof them. But for various reasons, I didn’t get round to completing the job.

Now I’m painting, I can immediately see the benefits of using linseed oil on old wood. Where I’ve left a leading edge of paint on bare wood, the oil spreads out of the paint into the wood, which seems to lap it up, almost pickling the timber in oil. That can only be good for long term preservation, especially as absorption is helped by the slowness with which the paint dries. Continue reading “Old fashioned paint”