Newly made oak windows and condensation

[CaptainBarnacles]

Hi All,

First off, apologies to mods if this is in the wrong board, I wasn't sure if it should be here or in General Chat.

About a year ago I made a batch of oak sash windows for our old stone-built farm cottage that we are renovating. They were to replace uPVC windows that weren't old and there was nothing wrong with them, they just were utterly out of character for the house. With 13 windows to make I decided to do them in two batches so I made the first batch and installed them just before Christmas last year.

The windows were made using solid English oak with the sashes being side hung and opening outwards. I based my design on one that I saw in the Trend Modular Window System brochure (they call it Traditional Flush, here's a link to the PDF of their brochure http://www.trend-uk.com/en/AT/trend...php?file=bXdzX3RmX2Jyb2NodXJlX3BhZ2VzLnBkZg==) and worked on similar sized profiles.

The windows went together well and after glue up I fitted Aquamac 63 seal where the sash closed against the frame. The double glazing (4,14,6 Pilkington K) was installed using a soft rubber self-adhesive seal that was recommended specifically for wooden windows. The oak was treated with Sikkens Cetol as a base coat then a coat of Osmo UV protection was applied after about a week.

The frames were installed and any gaps sealed with expanding foam then pointed up with mortar and a silicone added where needed to seal the frames where they met oak cladding or lead etc. on the outside facade of the house.

Almost immediately we noticed that in the mornings the windows were streaming with condensation. The rooms are generally kept about 14-16 deg C overnight and we try to avoid creating excess moisture in the house (we have extractor fans in bathrooms and kitchen, wet clothing remains in the porch, laundered clothes are always dried outdoors, we don't have a tumble drier etc.....). We have tried leaving the windows slightly ajar (using the second notch of the handles) to see if the ventilation helps but it had very little effect. We thought at first that perhaps it was the old house drying out as it hadn't been properly heated since..... well, forever really! But after a year of dry and warmth, and having been very well ventilated over the warm summer the condensation is just as bad this winter as it was last year.

We have applied moisture barrier to all floors and walls before insulating them and dry lining all the walls. We don't have any gas heaters, just a small 5kW stove, the rest of the house being heated by an underfloor heating system. The house is usually warm and feels dry.

So one year on and despite trying to mop up the puddles of condensation that form on the window frames as best we can each morning, the oak is now starting to go black and I am feeling deflated that all my hard work appears to be rotting before my eyes. I am just about to complete the second batch of windows and if possible I don't want to make the same mistake again.

I just can't figure out why the condensation is forming as it is. On the remaining uPVC windows left in the house there is a bit of condensation forming but I would say it is only 5-10% of that which forms on the oak windows. I have asked a couple of builder friends but they couldn't come up with a decent explanation as to why it was happening. Additionally they both said that we appear to have done everything possible during the renovation of the house and the construction of the windows to avoid condensation problem - that just makes it more frustrating :?

Anyone got any suggestions?

Thanks,
Paul.

 

[nev]

two completely random and unqualified things spring to mind.
The first is that you have done a better job than the upvc fitters did and you are better sealed than before and its just the house drying out still. I remember reading somewhere it can take a year per 1 inch thickness of stone wall do dry properly. (may be an old wives tale but..)
secondly - is the K glass in the correct way round? Again I dont know if this would make any difference or create the kind of issues you have but could be worth mentioning.

 

[n0legs]

To follow on from what Nev said about a year per inch for the stone to dry out, I was told by a building inspector, for concrete 1 day per mm of thickness. It's on the web somewhere, so I would imagine Nev isn't far wrong.
When we built the previous house we rendered and skimmed inside. My lounge was 26 feet by 18, we took 2 three gallon buckets out on the first 2 days from a heavy duty dehumidifier. the following 3 days there were another bucket and a bit each day. By the 5th day we were down to 3/4s of a bucket.
the dehumidifier ran for a full 2 weeks until we were getting about a cupful of water which we put down to just normal moisture levels in the air.
I can't give you a good reason why you are having the problems but my point is maybe you could hire a good dehumidifier and run it for a few days in the worst affected areas to assist in drying out the building/rooms.
I would then move it to another room or turn it off and see how you fair with the condensation. Run the dehumidifier again if the condensation has returned and check out the amounts of water you are collecting. It might give you an idea if it's the building that's wet , and needs further drying, or you may have a ventilation or other issue.

 

[MMUK]

Without actually seeing the house I can only offer an educated guess as to the reason you are experiencing this.

Lack of ventilation and a massive difference between internal and external temperatures.

When you made these oak windows, did you allow for a flush mounted trickle vent? Being stone built, I seriously doubt that the property has air bricks so a source of constant ventilation is essential.

 

[bugbear]

I just can't figure out why the condensation is forming as it is. On the remaining uPVC windows left in the house there is a bit of condensation forming but I would say it is only 5-10% of that which forms on the oak windows.

I have no explanation, but can propose an experiment.

The TWO changes you've made are the frame material, and better fitting/sealing.

In order to separate these effects, could you re-fit (or re-seal) some of the remaining uPVC windows, and see if the condensation on these windows changes?

Edit; all the stuff about moisture, drying out, stone, cement etc may (or may not) be correct, but was the same for the old windows and the new replacements. The only changes are new material (uPVC/Oak) and possibly better fitting.

BugBear

 

[MMUK]

Another thing to consider is the change of material. Timber is colder to the touch than uPVC and uPVC retains heat much better. This was a similar issue with aluminium double glazing, hence the later profiles had a composite thermal break extruded in.

 

[Jacob]

Condensation on windows is normal, especially at this time of year. You still get it with DG but usually a bit less.
Puddles of water collect because you have inadequate provision for drainage.
Simplest is to avoid having a seal at the bottom edge and making sure there is a bevel and a permanent gap, so water can drain off to the outside.
Better is a drainage gutter of some sort with pipes off.
It is a design problem. Even if you eliminate all possible sources of water vapour you still will get it from breath and human activity so you have to design for it, or have massive rate of air change.

PS you could see it as a good thing - condensation on the windows means your ventilation is not excessive, and draining it off turns your windows into very effective de-humidifiers.

 

[CHJ]

If the condensation is on the wooden frames it suggests that the frames are not as thermally efficient as the air gap plastic predecessors from an insulation point of view and are providing a colder surface for the air moisture to condense out on.

Remember an individual exhales about 1/2 a litre of water every day, a sealed room can accumulate a lot of moisture especially a bedroom overnight where there is no air movement from opening doors etc.

That 1 kilo loss of weight overnight that gives you a feel good factor on the bathroom scales in the morning is mostly moisture in the air.

 

[bugbear]

Of course, condensation is just a symptom of expensive heat being transferred out of the house through the window.

As the hot air hits the cold glass, it cools (giving up heat to the glass) and loses the ability to hold as much water, hence the condensation.
If this were all that were happening, the glass would warm up to the internal air temperature, and the process would stop; the process is only continuous because the glass is losing the heat to the outside, thus staying cold.

This is practically demonstrated in German/Scandinavian houses with highly effective triple (or more!) glazing, which do not have condensation issues, since the inner pane is at room temperature.

BugBear

 

[Jacob]

I'd also add - oak not particularly suitable for window frames - the everlasting trad material is redwood which is always painted. Oak can be painted but doesn't hold it as well as pine. But they have to be painted - next July if they have dried enough!
Another detail is in fitting - there should be no paint or anything on the backs of frames and there should be an air gap between it and the masonry particularly the cill. This is closed with a fillet of mastic on the outside and usually by plaster on the inside. This gap allows for a bit of "breathability" so that what gets wet has a chance to dry out. Cills primed underneath and set on dense mortar etc can become 100% water logged with little chance of ever drying.

PS and possibly your plastic windows aren't so bad because they have weep holes?

 

[n0legs]

If the condensation is on the wooden frames it suggests that the frames are not as thermally efficient as the air gap plastic predecessors from an insulation point of view and are providing a colder surface for the air moisture to condense out on.

I agree with you on this.
A hell of a lot was made of the "Thermal Break" features of my aluminium windows.

 

[dickm]

If I read the original post right, most of the condensation is on the glass, rather than the frame. So the glass must be at a temperature below the dew point for the air in the room, which if the Relative Humidity in the room is high, will also be a relatively high temperature. Suggests that reducing the RH in the room through better ventilation is the answer but, of course, this will lose heat as well. So one solution would be forced ventilation with heat recovery. We have a Genvec unit in a house that is otherwise hermetically sealed, and get no condensation even with Scottish outside temperatures. (But also have triple glazing). It's a real upheaval fitting such a system, but in my opinion, well worthwhile.

 

[Jacob]

The windows are already reducing relative humidity, rather too well! All you need to do is drain the water away to the outside.
Better to have it on the window and draining away, rather than on the walls somewhere else, or removing it mechanically.
Condensation collection channels were quite normal in a lot of Victorian buildings and later. You often find they have been filled or blocked, as people seem to have forgotten what they are for.
Plastic windows have weep holes to reduce the problem.
Modern houses have higher temperatures and reduced ventilation. If you don't provide for condensation removal you get pools of water or damp walls etc. It's a simple design problem.
We are looking at "Passivent" for our building. Have been talked out of heat exchangers as uneconomical.

 

[bugbear]

If I read the original post right, most of the condensation is on the glass, rather than the frame. So the glass must be at a temperature below the dew point for the air in the room, which if the Relative Humidity in the room is high, will also be a relatively high temperature. Suggests that reducing the RH in the room through better ventilation is the answer but, of course, this will lose heat as well. So one solution would be forced ventilation with heat recovery. We have a Genvec unit in a house that is otherwise hermetically sealed, and get no condensation even with Scottish outside temperatures. (But also have triple glazing). It's a real upheaval fitting such a system, but in my opinion, well worthwhile.

Sounds impressive!

Edit; it occurred to me that a heat recovery/ventilation unit is doing exactly what the cold glass does - introducing warm moist air to a cold surface, which must surely provoke condensation. Obviously, in the case of a heat recovery unit the warmth is then transferred to the incoming fresh air, not wasted.

A quick search confirms this - heat recovery units are also, unavoidably, de-humidifiers, but rather less objectionable ones than an opaque, dripping window.

BugBear

 

[blackrodd]

You're problem normally is seen the other way around, timber windows running with condensation changed for upvc double glazed units and just a hint of condensation with the new units.
I would go with double glazed units faulty or the wrong way round/labelled.
It's a pity that a fensa registered bod has not been involved in the instalation to fall back on.
check with the glazing supplier and see if there may be a story regarding a duff batch. HTH
Regards Rodders

 

[pmagowan]

There are a number of issues here and really you require more information to determine where the problem lies. Condensation occurs when air is cooled such that it's relative humidity reaches 100%. In a house this is usually when warm air (which holds a lot of water) is cooled by poorly insulated components of the house (such as windows) and as such has a lesser capacity to hold water, reaches dew point (100% RH) and thus condenses onto the surface. If you have carried out insulation work to a house then it is likely that you have prevented condensation on walls and thus it gets concentrated on cooler elements such as windows. How cold something 'feels' is (more or less irrelevant) as this is due to thermal conductivity rather than temperature (metal feels cold when plastic does not even at the same temperature).
Solutions; reduce the moisture content in the air, dehumidifiers etc. improve ventilation so that condensed water is reabsorbed by fresh air, improve insulation of windows, reduce humidifiers such as plants, cooking, drying. Get more information using cheap IR temperature scanners and RH meters. You should then be able to work out where the problem lies.

 

[Jacob]

You should then be able to work out where the problem lies.
Not trying to be facetious but your problem lies in pools on your window cills. The answer is to design in a bit of collection/drainage in the old fashioned way. You then also have a passive process of de-humidification which costs nothing to run.
I've done much the same here - chapel conversion retaining single glazed windows with drainage channels added. Works really well, water drips steadily out. The only prob is heavy frost when the weep holes freeze up, but this is never for long.

And you will have to paint your window frames.

 

[Graham Orm]

The units with a 14mm air gap are undersized by modern standards. 20mm Is the optimum gap, anything bigger and the air will start to circulate and transfer the heat out, that's why the Scandinavians have several pieces of glass. As already mentioned the problem very likely stems from the people sleeping in the room. Do you find the same problem in empty rooms?

Heat and ventilation are the only answer.

I have fitted more than my share of uPVC windows and often had complaints that the condensation is worse. This is because we removed the natural ventilation created by the draughty old wooden windows.

I actually had the problem myself. I installed an extractor fan and set it on a timer to come on for 3 hours every morning after we had left for the day, we then made sure we left the bedroom door open to allow maximum air flow. It cured the problem.

Jacob.
The drains that you describe are on the outside of the window and pass through a chamber that makes up the outer wall of the frame. It's designed to allow condensation from within the frame (not within the building) to escape, but more importantly any rainwater that gets into the rebate where the sealed units sit. If they were allowed to sit in frozen water (or any water for that matter) they would fail straight away. There is no passage of water from inside to outside on any extrusion I have ever used in 20 years of fitting them. Nor have I ever seen such a thing in wooden windows. Obviously this could be a regional thing, I'd be interested in seeing a pic if you can find one? :wink:

This is the best diagram I could find.

 

[wellywood]

I think you would be better off trying to tackle the source of the problem which is inadequate ventilation/heating. Adding some sort of drainage system to your windows while it would help remove the water is really only the ambulance at the bottom of the cliff.
Reducing any moisture output will certainly help but, as two adults will exhale up to three litres of water every 24 hours (that's a litre overnight while you're both asleep), there are limits to what you can achieve.
Here in NZ damp is a big problem given our environment and common home construction methods and there is a higher than average incidence of respiratory problems.
We solved our severe damp issues (weeping windows. mould in the cupboards etc) by installing a positive pressure ventilation system. Air is drawn from the roofspace by a small fan and distributed around the house via flexi ducts to vents in each room. The damp problem disapeared virtually overnight with the bonus that the house even seemed warmer as well as drier as a result. It is cheap to run (the fan draws about as much power as a light bulb).
I realise this might not be practical solution in some cases given the difference in house construction (most NZ homes are single story dwellings) but it's worth considering where it can be installed.

 

[Jacob]

.....
Jacob.
The drains that you describe are on the outside of the window and pass through a chamber that makes up the outer wall of the frame. It's designed to allow condensation from within the frame (not within the building) to escape, but more importantly any rainwater that gets into the rebate where the sealed units sit. If they were allowed to sit in frozen water they would fail straight away. There is no passage of water from inside to outside on any extrusion I have ever used in 20 years of fitting them. Nor have I ever seen such a thing in wooden windows. Obviously this could be a regional thing, I'd be interested in seeing a pic if you can find one? :wink:

OK I'm not sure about plastic windows I've hardly been near one. I knew they had weep holes though.

With trad windows drainage is a given - sashes drain via the meeting rail gap and by the bottom sash to staff bead gap. These should never be draft proofed. Casements drain via the bottom rail similarly. French windows (i.e. yer actual trad ones in France) often have a drainage detail much as I've described. Many larger buildings have drainage collection channels and means of taking the water outside. Here frinstance the windows sat on a stone cill with a channel cut just inside, (mirroring the drip channel you'd find in a stone lintel outside) taking water to a short length of 1/2" lead pipe sticking out just under the centre front of each window. In other buildings I've seen timber drainage channels integral with the frames.This sort of detail was universal, commonplace and is easily overlooked - there isn't much to look at!