Engineered timber windows

[Ian Bond]

Please let me know if you have any experiences or opinions regarding the use of engineered timber in sash windows. It's a form of laminate and several companies now appear to be adopting it for making sash windows:

Multi-layer timber and very careful attention to detail, minimise the swelling and warping that causes jamming on
windows of lesser quality during wet weather.

http://www.timberwindows.com/sash-windows.php

I'd have thought that good quality oak, douglas fir, or redwood would be more suited to the job. What about future repairs and maintenance?

All comments welcome...

 

[Andrew Bodenham]

Intersesting website, with some nice design details shown.

Not entirely convinced about the merits of engineered timber for window application though. On the plus side defects can be cut out, and dimensional stability should be good. On the minus side you've introduced a lot of glue joints that don't need to be there. I'd be concerned about long-term performance - think what happens to plywood (even marine ply) when it goes through a few wet / dry cycles.
I'd sooner select defect-free natural timber - after all the components in a sash window are generally not very big, so choosing clean stock isn't that difficult.

Engineered timber has traditonally been used to create large structural members, e.g. beams, where the size of available solid timber sections would otherwise be a limiting factor. Not really necessary for something like a window.

 

[Ian Bond]

Thanks for your comments Andrew.

I have similar concerns myself, as I don't think you can beat well chosen timber of the right species, properly prepared. Its just my gut instinct.

It does seem to be an over-complication of a basic element, and I'd be interested to find out why people are going to all that trouble to laminate bits of timber.

Perhaps it's a way of trying to get improved performance using 'inferior' timber. at a better price than say oak or douglas fir?

Some of the standard redwood sash mouldings that I've seen incorporated into modern windows could have benefited from quality control. Filled splits that re-open, large live knots and so on. Maybe the engineered timber eliminates these problems and makes mass production easier / more economical?

I'm not keen myself, and prefer to keep things as simple and straightforward as possible. And to use the best materials available / affordable. I'd rather spend more on better materials initially, than cut corners only to find that the window becomes a maintenance nightmare.

Perhaps the engineered timber is a false economy in the long run?

 

[MdB]

<cynic mode on>
Engineered wood can be made in a big machine from wood of lots of different sizes so there is little waste. It produces a uniform product which doesn't warp and change size so can be put through a CNC machine by monkeys and you can of course pay monkeys peanuts. This makes the overall product cheaper.
<cynic mode off>

 

[Ian Bond]

Good point MdB, and I can see the logic behind it.

I've not got a clue about what's possible with these machines. I take it you can get machines to make all the mouldings and the joints, and someone then just glues it together like a self assembly kit?

I'm a luddite, using planes, saws and chisels, but I can appreciate that machines could be very useful in saving labour for mass production.

I suppose the machines can replicate any moulding, although you lose the individuality of the craftsman. I'm not sure if the mass producers would bother doing anything other than a standard set of mouldings.

Not good news if you are trying to make an exact copy of an important historic window - I expect that's still a skilled joinery job.

Perhaps a machine produced sash would be OK in theory, but I've got serious doubts about the quality and durability of this engineered timber, backed up by the comments on this thread.

 

[Penners]

For the most part, I share the concerns expressed by others about the material content of an engineered timber window. Even today, there are relatively few adhesives that are truly water/damp-proof, and I would want to be very certain that good quality timber had been used with a truly waterproof glue (polyurethane glue, for example).

However, I do strongly disagree with some of the above comments about machine-made joinery, particularly CNC machining. I defy any handmade joinery, no matter how skilled and experienced the joiner, to match the precision and repeatability of CNC work. And the implication that CNC machines can only work effectively with engineered timber is, frankly, rubbish.

 

[MdB]

However, I do strongly disagree with some of the above comments about machine-made joinery, particularly CNC machining. I defy any handmade joinery, no matter how skilled and experienced the joiner, to match the precision and repeatability of CNC work. And the implication that CNC machines can only work effectively with engineered timber is, frankly, rubbish.

Oh, I agree totally that a CNC machine can do a better job at the precision and repeatability. With a CNC machine, you can tell someone to turn 5 lengths of 3m engineered wood into 15 lengths of 1m all identical with holes in the right place by pressing a couple of buttons. You could do the same with 5 lengths of solid wood, however they are not likely to be exactly straight so will require extra clamping. There is also likely to be a couple of knots or shakes which means that you won't get the full 15 sets out the other end. A good woodworker will also look at the wood and turn it so the best face is the visible face in the finished good. Biff made a good comment on this previously. My point is that there is more skill involved when using solid wood compared to engineered wood even when made on the same CNC machine.

 

[Penners]

My point is that there is more skill involved when using solid wood compared to engineered wood even when made on the same CNC machine.

No argument there.

 

[Andrew Bodenham]

Having rebuilt a number of customers sash windows over the past few years I've found that there are 4 key factors to success in the sash window world:

1. Design detailing. One of the commonest problems I see is rot starting in the corners of sills because the stiles are rebated into them, creating reservoirs for water and plenty of end grain waiting to soak it up. This is just one of many design weaknesses which can be dealt with on the drawing board ( or preferrably CAD package these days ).

2. Material selection. As others have said above, there's no substitute for good timber selected with care and understanding. Some old windows I've worked on have been made from fantastic quality wood, and that shows in how well they survive. Is engineered wood beter than 5th quality redwood? Probably, but neither will outperform selected oak or Doug fir.

3. Assembly detailing. Loads of problems start because of poor detailing - a classic example is the use of ferrous screws / nails to "reinforce" joints. As the fastenings rust and expand so the inevitable failure begins. Joiners 100 years ago were just as guilty as modern ones of banging in a few nails, but the modern ones have so many alternatives that there is really no excuse for poor installation and assembly today, and that include repair work.

4. Maintenance. The best made winows are not going to last more than a few years if they are not maintained. That means intelligent selection of the initial paint system, followed by frequent inspection and timely intervention with more paint, putty, or repairs. So many of us are picking up years of neglect or bodged work by previous owners or their decorators.

Those old windows we love worked well when they were installed; it's interesting to look at the list above and work out which factors contributed to the problems which get flagged up on PPUK every week.

 

[Ian Bond]

The lack of precision in handmade stuff is part of its charm.

Having said that, machines are what we've got in place of lots of craftsmen, and we'll have to put it down to progress for now, like so many other things. I can live with that.

And precise mouldings and joints can't really be seen as a disadvantage or regressive step.

So that still leaves some doubt as to the benefit of engineered timber - I don't think that the window is improved much by it, and the extra stability argument doesn't hold up if the glue fails.

Is PU glue a permanent bond, as strong or stronger than the original timber? So far I've 'stuck' with either PVA wood glue, or expoxy for stuff that I seriously want to stick.

Just wondering if PU would be any good for scarfing repairs and that sort of thing. You need a thicker, gap filling adhesive to get a good bond - I've used 2 part fillers successfully.

I'm all for new technology and materials if they deliver better performance and increased durability. But not if they save costs in manufacture but fail early and need replacing, conveniently for the manufacturers' future business!

 

[Andrew Bodenham]

Polurethane adhesives are very useful for repair work due to their tolerance of dampness in the timber being glued, and their joint-filling properties. One always aims for tightly mating joints, of course, but with the best will in the world this is not always possible. The foaming nature of PU glues helps to eliminate voids within joints which would otherwise be water traps.

Of course, not all PU glues are the same, and for serious structural repairs, or in high exposure areas I tend to use the more expensive Balcotan range of adhesives. These really are excellent, and the wide range of open joint times and curing times available is valuable - no good trying to glue up a complex assembly like an external door with 5 minute 'builders' PU!
The fast cure versions are great for small repairs and splices though, and allow work to proceed at a good rate.

I steer clear of epoxy for on-site work, as it is rare to get the timber into a suitably dry condition for the expoxy to bond reliably. Good for workshop stuff though.

My PU glue testpiece is a post in a trellis I made about 10 years ago. Cutting the posts to length I managed to take too much off one of them, so stuck the off-cut back on with a simple lap joint and Evo-Stick PU. No mechanical fastenings. A decade later it's still holding together, depsite the best Lancashire coastal weather. I'll let you know when if it fails.....

 

[Penners]

Is engineered wood beter than 5th quality redwood? Probably

Well, that's probably true, Andrew. But 5ths are not the only redwood. A good clear, unsorted Baltic pine would - I suggest - be less of a risk that an engineered timber of dubious provenance in terms of its materials.

But you're absolutely right about maintenance. No matter what a timber a window is made of, if it's neglected it will fail prematurely, and if it's well looked after it will last and last.

 

[Penners]

I certainly echo what Andrew says about PU adhesives. They are immensely strong and weather-resistant, and have good gap-filling quality, because they have a tendency to foam somewhat while curing. This does mean, however, that effecting cramping is necessary during curing if joints are to be prevented from being pushed apart by the foaming effect.

Thanks for the info about Balcotan, Andrew. I'd never heard of it, but will keep it in mind.

 

[Ian Bond]

Ta for the info on the PU glue. Good point about open time.

The Timber Research and Development Association (TRADA) have the following to say on the subject of engineered timber:

Engineered timber is a generic term used to describe a wide range of wood-based products that have been engineered to provide enhanced performance characteristics. This improves their suitability for particular end uses, especially in demanding structural situations, but may also be designed to improve other characteristics such as durability. Usually these products are composites that combine wood or wood fibre with adhesives and possibly other materials. Some reasons for engineering solid timber into such composites include:

* to overcome the dimensional limitations of sawn timber
* to improve performance; structural properties and stability
* to transform the natural orthotropic product into one with more homogenous properties
* to optimise the use of a valuable resource and minimise waste.

I think the last 2 point are particularly relevant to mass production, which is fair enough.

I strongly agree with the 4 points you made that are key to getting a long lasting window. This should be of great use to anyone contemplating new windows, in conjunction with the aesthetics and energy efficiency considerations.

The introduction of engineered timber into window manufacture was news to me, hence this thread, as I want to avoid repeating costly mistakes, and help others do the same.

Having rebuilt a number of customers sash windows over the past few years I've found that there are 4 key factors to success in the sash window world:

1. Design detailing. One of the commonest problems I see is rot starting in the corners of sills because the stiles are rebated into them, creating reservoirs for water and plenty of end grain waiting to soak it up. This is just one of many design weaknesses which can be dealt with on the drawing board ( or preferrably CAD package these days ).

2. Material selection. As others have said above, there's no substitute for good timber selected with care and understanding. Some old windows I've worked on have been made from fantastic quality wood, and that shows in how well they survive. Is engineered wood beter than 5th quality redwood? Probably, but neither will outperform selected oak or Doug fir.

3. Assembly detailing. Loads of problems start because of poor detailing - a classic example is the use of ferrous screws / nails to "reinforce" joints. As the fastenings rust and expand so the inevitable failure begins. Joiners 100 years ago were just as guilty as modern ones of banging in a few nails, but the modern ones have so many alternatives that there is really no excuse for poor installation and assembly today, and that include repair work.

4. Maintenance. The best made winows are not going to last more than a few years if they are not maintained. That means intelligent selection of the initial paint system, followed by frequent inspection and timely intervention with more paint, putty, or repairs. So many of us are picking up years of neglect or bodged work by previous owners or their decorators.

Those old windows we love worked well when they were installed; it's interesting to look at the list above and work out which factors contributed to the problems which get flagged up on PPUK every week.

1. Design Detailing

Had this discussion with gsport george recently, concerning the outer bead trapping water on the sill. It's another design defect eliminated with a chisel quite easily. Of course, it should be designed properly in the first place.

http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=9484&p=108189&hilit=water+trap#p108189

I've also noticed that rain tends to get driven under the bottom sash rail and pool there, despite the slight camber of the sill. Perhaps that's insufficient?

I thought that the frame stiles are usually tenoned into the sill - I've not seen any rebated, but am no expert. As I'm going replace a sill shortly, I was going to have the bottom ends of the stiles just butted up as closely as possible to the top of the sill - no joints, perhaps some glue - and just make sure there are no gaps and it's painted well.

2. Material selection

Agreed.

3. Assembly Detailing

Wedged and pegged joints seem to work well, and I've even done this without glue. It's also possible to take the sashes to bits (with care) to replace components. Perhaps not as easy (or even possible) with an indestructable PU glue?

No need for screws or nails - or No More Nails!!!!

4. Maintenance

Couldn't agree more, a recurring theme. Good design, timber and workmanship helped some of my windows survive decades of neglect with only minor rot, but it's still a lot of effort to put right now.

To my mind, it's important that modern timber windows retain the 'repair-ability' advantage over uPVC, through good design.
Which leads back to some of my doubts about the durability of the engineered stuff.

We'll have to review this in another 10 years, along with your trellis!!!

 

[Andrew Bodenham]

I've also noticed that rain tends to get driven under the bottom sash rail and pool there, despite the slight camber of the sill. Perhaps that's insufficient?

This is another common problem. If the bottom sash rail is a nice fit down onto the sill then a perfect capillary gap is created which wicks in water and hangs onto it long after the sun has dried everything else up. That's water just where you don't need it. And I do find that the degree of slope on sills is often rather mean - I've seen cases where rain creeping under the sill has expanded the bottom face, cupping the almost level top face and effectively stopping any run-off, with rot the inevitable result.

There are various solutions which rely on providing a larger gap to eliminate the capillary effect. Seems a bit counter-intuitive, I know, to be building in a 'poor fit', but that's what it needs. For refurbs I tend to stick a length of EDPM 'P' section draught strip along the back of the underneath of the bottom rail - this lifts the sash a couple of mm when closed and allows that water to drain away whilst maintaining draughtproofing. Sometimes a bit of a tweak is then needed to get the meeting rails back into register when closed.
I'm keen to hear if others have more sophisticated solutions to this particular problem, especially as I need to make a complete set of windows for my own house in the
near (that'll be 5 years+) future.

 

[yamin]

At risk of looking a bit simple and lightweight, after the meaty and expert discussions above, I would raise one point/question. Are these engineered wood windows cheaper? I ask because (and I know this opens up a whole other debate) it is proving prohibitive, cost wise, for us EVER, and I mean EVER to replace our hideous UPVC windows, put in during the 80s. At 20 something years, they are no where near failing incidently, but they DO look completely hideous. If there was a cheaper way of replacing them with a wood alternative, then that might be something to consider, and a step, even if not totally traditional, to restoring our house frontage to how it ought to look. I'm thinking, would this move be like putting a good quality reproduction sideboard in your dining room, or like popping down IKEA and trying to pass it off as one? Just my meanderings........

 

[Mark]

I notice that a lot of the high performance (U value wise) windows being touted in the green building world are made from laminates including a layer of cork for insulation. Could this also apply to the engineered sections being used in these windows?

 

[Ian Bond]

Very possibly, Mark. It would be a useful marketing claim to make, although I think timber window frames are pretty good anyway on that score.

Just watching that young fella on TV telling me about the new Ev*r*st windows being fitted at Tan Hill. You know, replacing the ones that Ted Moult fitted 20 years ago....

"You only fit double glazing once (every 20 years), so fit the best .."

Doh!

Yamin - I don't know the answer to that, but I guess it would be cheaper than Oak or douglas fir. There's a fair amount of work in building a good window, even with machines and engineered timber, so its never going to be cheap. It should be a once in a lifetime purchase though (see Ev*r*st comment above) :mrgreen:

I think it's a false economy to just go for the cheapest - you'd certainly want to make sure that the windows were well designed, and made from a durable timber as per Andrew's points above. I've made the mistake once of fitting poor quality windows - total waste of money.

 

[Ian Bond]

I've also noticed that rain tends to get driven under the bottom sash rail and pool there, despite the slight camber of the sill. Perhaps that's insufficient?

This is another common problem. If the bottom sash rail is a nice fit down onto the sill then a perfect capillary gap is created which wicks in water and hangs onto it long after the sun has dried everything else up. That's water just where you don't need it. And I do find that the degree of slope on sills is often rather mean - I've seen cases where rain creeping under the sill has expanded the bottom face, cupping the almost level top face and effectively stopping any run-off, with rot the inevitable result.

There are various solutions which rely on providing a larger gap to eliminate the capillary effect. Seems a bit counter-intuitive, I know, to be building in a 'poor fit', but that's what it needs. For refurbs I tend to stick a length of EDPM 'P' section draught strip along the back of the underneath of the bottom rail - this lifts the sash a couple of mm when closed and allows that water to drain away whilst maintaining draughtproofing. Sometimes a bit of a tweak is then needed to get the meeting rails back into register when closed.
I'm keen to hear if others have more sophisticated solutions to this particular problem, especially as I need to make a complete set of windows for my own house in the
near (that'll be 5 years+) future.

Perhaps a better solution hasn't been found in the last 400 years because there isn't one :?:

My later Victorian windows seem to have incorporated all the good design aspects you mentioned - one or two minor weaknesses had crept back into the modern version I had made.

I've removed the draught strips from the underside of my lower sashes. However, they had been stuck into a routed groove near the front edge, and I was concerned about water being drawn into the rail and starting rot, as the sash was still a pretty tight fit. Maybe a stick-on strip would be better here, as you've done?

Another method I used on a cellar casement was to have a steeper angle to the sill for better drainage. I'd made a new frame but re-fitted original sashes, whose bottom rails were square, so there's quite a gap between the bottom rails and sill. In this case it doesn't matter visually as the window's in a light well. A large gap like that would look ugly above ground.

Every single old window I have has rot at the lower sash joints, so I suppose it is inevitable if maintenance slips. The design of the joints means the stile end grain is in contact with the sill, but I can't see how you'd avoid that.

As you've already said: Have a proper fall to the sill, weatherstrip to raise the lower sash slightly, and keep everything well painted.

My Victorian Architectural text books don't go into this level of detail with the windows!

 

[biffvernon]

Gosh, what a lot of sensible things folk are coming up with these days.

Nobody's mentioned paint yet. Covering any wood in a watertight paint, which eventually cracks, lets water in and prevents drying is a sure way to promote rot. Using water vapour permeable paint or no paint may be more important than all the design details and timber quality put together.

Remember most paint manufacturers aim to sell you another can of paint as soon as possible.