Theory of condensation in partly insulated room

[CliffordPope]

If you put secondary double glazing on a window it is generally regarded as a good thing. But the moisture that would have condensed on the glass is still there - where does it go?
I just have this sneaking suspicion that it simply condenses somewhere else, such as on a wall. Is the room really drier because of the double glazing, or is the problem simply hidden away? Could it in fact be argued that having a an un-insulated windows is a good way of catching damp somewhere where it can be conveniently mopped up - like a dehumidifier?

 

[Flyfisher]

If the DG results in the room being warmer then the air can retain the moisture until ventilation voids it outside.

 

[paulc]

Moisture will condense on a single glazed window because it is cold. Raise the surface temperature above the dew point, and the moisture remains suspended in the air. If the walls are still cold and subject to the effects of condensation, then they will still absorb the same amount of water. Increasing the levels of insulation (both on the wall and double or triple glazed windows) results in a higher surface temperature assuming the room is heated, thus no condensation.
If you want to capture the moisture in the air, run a dehumidifier and empty out the container daily.

 

[fernicarry]

Hmm, so my suspicion was that air was capable of carrying a lot more moisture than actually condenses onto the window. After all, the air in the room doesn't patiently queue up in turn to touch the glass to deposit all of its moisture. So I googled it.

https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

At 10'C air can carry maximum 9ml of moisture per cubic metre. Lets say the room is 4x4x3 thats 48m3 which would contain about 405ml of water, half a litre-ish. The RH of the air probably doesn't hit 100% so it won't be carrying the maximum, but it could be up around 60 or 70% so lets call it 250ml per room. Iirc from when I used to bother with the window vac I could sometimes collect a couple of pints, but that was over the whole house, a much larger volume. The inside air temp is probably also a bit higher than 10' (not by much in some rooms...) so I've underestimated the moisture in the air. Overall this tends to support the theory that not all of the water vapour condenses on the windows. It also ignores human activities that constantly put moisture back into the air, breathing, cooking, washing. Granted those activiites tend to be highest during the day when the building is a bit warmer and suppressed during the night when its colder.

So I think if you have potential for condensation elsewhere that's going to happen regardless of how the windows perform.

DG may reduce the level of ventilation by improving the air tightness of the window (not to say that SG can't be just as air tight) and that could result in more moisture staying inside the building. I've said this before but I think the trickle extract from my kitchen and bathroom have reduced the amount of condensation I get on the windows. However this winter its noticeably worse due to the big swings in temperature we've had.

 

[Flyfisher]

I can't remember the source (it'll be out there somewhere), but I've read that indoor rooms should have the air changed 4 times per hour and for 'wet' rooms (ie kitchens, bathrooms etc) the figure should be 7 times per hour. So that's a complete change of expensively-heated air every 15 minutes for living rooms or every 8.5 minutes for wet kitchens and bathrooms. I suspect many homes do not have this much ventilation, which is why condensation and the often associated black mould can be a big problem, especially in a busy household with multiple daily showers/baths, cooking and washing/drying clothes all conspiring to saturate the indoor atmosphere with moisture.

I like to sleep with the bedroom SG casement window open by a couple of inches and rarely is there condensation by the morning, even in the winter months. Only if I forget to open the window does condensation form on the window during cold nights.

So, like most things, there's a balance to be found. DG can undoubtedly help, but it's no panacea.

 

[paulc]

I can't remember the source (it'll be out there somewhere), but I've read that indoor rooms should have the air changed 4 times per hour and for 'wet' rooms (ie kitchens, bathrooms etc) the figure should be 7 times per hour. So that's a complete change of expensively-heated air every 15 minutes for living rooms or every 8.5 minutes for wet kitchens and bathrooms.

Those numbers probably come from the Building Regulations (Part F), and applies to "purge ventilation". i.e. The amount of air passing through an open window. Background ventilation from trickle vents or mechanical ventilation systems needs to be a minimum of 0.3l per second per square metre of floor space. But if I'm interpreting the regs correctly, if your house has an as-built air permeability higher than 3m³ /(h·m²), no background ventilation is required (section 1.45 on page 14 https://assets.publishing.service.gov.uk/media/61deba42d3bf7f054fcc243d/ADF1.pdf ).

 

[CliffordPope]

If the walls are still cold and subject to the effects of condensation, then they will still absorb the same amount of water.

That begs the question "What determines the amount of water a cold wall will absorb?"

The walls are 2' thick stone, totally uninsulated. There will only ever be minimal changes in temperature, over a period of months.
Assume air drifts slowly round the house, and round each room. The air passes a window, which yesterday the moisture would have condensed on, but today, because I stuck up the DG, it can't, so the damp air moves on. Why can't it condense on the wall instead?

 

[jamest]

The dew point depends upon the temperature and the relative humidity. If the relative humidity is highish (say 70%) and the room temperature is 20 degrees, then condensation will occur on surfaces that are colder than about 15 degrees, might be too little to notice for surface temperatures above about 10 degrees. If the relative humidity is low-ish (say 20%), then the surface temperature on which condensation starts to occur will be much lower (about -2 degrees). So the bottom line is that it depends, if your room is cool and has low humidity, surfaces will need to be really quite cold in order for significant condensation to occur.

I had a curious experience upon fitting secondary glazing. I have single glazed window on the street side of a room and slimline double glazed units on the garden side. I used to get dreadful condensation on the single glazed windows and only a little bit on the double glazed ones. After fitting secondary glazing to the single glazed windows I got no condensation on any of the windows, despite doing nothing to the double glazed units! I can't really explain that.

 

[Flyfisher]

That begs the question "What determines the amount of water a cold wall will absorb?"

The walls are 2' thick stone, totally uninsulated. There will only ever be minimal changes in temperature, over a period of months.
Assume air drifts slowly round the house, and round each room. The air passes a window, which yesterday the moisture would have condensed on, but today, because I stuck up the DG, it can't, so the damp air moves on. Why can't it condense on the wall instead?

I'd bet the surface temperature of the wall is different on the outside to the inside. Yes, the mass of the wall will slow down the rate of change of its temperature (a bit like the thermal lag experienced with UFH) but the inside surface will roughly follow the room temperature while the outside surface will roughly follow the outdoor temperature, resulting in a temperature gradient through the wall. As, long as the inside surface remains above the dewpoint of the indoor air, which will depend on its relative humidity, there will be no condensation. And, as previously mentioned, the warmer the room, the warmer all its surfaces and the more moisture the air can carry, thus reducing the likelihood of condensation. If DG contributes to this warming then it will reduce condensation.

It's important not to judge the temperature of something by touch alone because our fingers are far warmer than our homes (usually!) so when we touch something we're really sensing the item's thermal conductivity rather than its actual temperature. Thus, items with a relatively high thermal conductivity such as stone or metal will generally feel colder than an item with a relatively low thermal condutivity such as wood or cloth, for example.

Our kitchen has granite worktops and these feel 'cold' to the touch yet they never attract condensation no matter how steamy the kitchen becomes when cooking. Conversely, The breadboard that sits on top of the granite worktop feels 'warm' to the touch, as does the tea towel also sitting on the granite. In fact, each of these items will actually be at exactly the same temperature (assuming they haven't just been brought into the room), even though they 'feel' as if they are different temperatures because of how our warm fingers sense them, ie the highly conductive granite will draw heat away from our fingers more quickly than the wooden breadboard or teal towel and we sense that as the granite being 'colder' than the other items.

'Wind Chill' is a similar phenomenon in that it's a perception thing rather than an actual temperature thing. Place a thermometer outside on a cold but still day and it will read a certain temperature and anyone standing nearby would 'feel' that temperature. If the wind started to blow, the thermometer would not register a change in temperature (why would it when the air is the same temperature as before but is now simply in motion?) but the person nearby would 'feel' colder because their skin is warmer than the air and the effect of the wind is to cool them down faster than still air - an effect akin to increasing the thermal conductivity of the air. But fundamentally, if the air is at, say, 10C then no amount of wind can cool something to below 10C. It can only cool things down faster than still air.

 

[Stevers]

Black mould tends to appear in the corners of a room, so is this still air as I had thought, or are the corners also colder being effectively a block of masonry with two outside walls and no inside walls to warm it?

 

[Flyfisher]

Could be still air, ie non circulating air that isn't being heated by the heating system (whatever that may be) and is therefore able to cool down to below the dewpoint of the humidity being carried in the air (a sort of micro-climate effect). Mould can also occur in other areas of still air, eg behind wardrobes or other furniture, but the basic mechanism will be the same and, of course, outside walls will be more likely to be cooler than internal walls so that will also have a bearing on any 'micro-climate' effects.

 

[CliffordPope]

But fundamentally, if the air is at, say, 10C then no amount of wind can cool something to below 10C.

Then how can carburetters freeze up even if the air temperature is above freezing? Or how do people still get frostbite in cold winds greater than freezing?

 

[a twig]

You can’t get frostbite unless the actual temperature is below 0. You can get hypothermia - but not frostbite.

Source: https://www.weather.gov/safety/cold-faqs

 

[a twig]

Your carb can freeze because the petrol in it is evaporating which has a cooling effect which can lower the temperature of the carb below 0 - if the humidity in the air is high enough, ice then forms.

This paper demonstrates how you can use passive evaporative cooling to achieve temperatures of -15’C even when the ambient temperature is 26’C

https://arxiv.org/pdf/2107.04151.pdf

Camping in ye olden days before 12V fridges were a thing I used to keep my milk cool by rigging up a bush fridge, a small frame with a wet towel over it, and put the bottle of milk inside. The evaporation from the towel lowered the air temp inside the frame and helped keep the milk at least a few degrees below ambient.

 

[CliffordPope]

There are numerous official websites saying windchill can cause frost bite even if the air temperature is above freezing, just as many as say the opposite. I wonder if this is a matter of definition, and "frost bite" is not quite as clear cut as it might seem. Perhaps extreme localised hypothermia is really a kind of frostbite/3

Carburettors freeze up partly from fuel evaporation, but also from the pressure drop from the venturi effect.

 

[CliffordPope]

When you compress air it gets warmer - when you lower the pressure it gets colder. Similar to twist-effect cooling, cf elastic bands. Experimental refrigerators have been built using this principle.

Anyway, back to condensation in my house. Obviously warmer air can carry more moisture, but isn't it simply absorbing more of the moisture that would otherwise have condensed? Once that reserve has been used up, surely it goes on condensing again? Unless the air has been blown out by ventilation, which lets in some more cooler air.

 

[Flyfisher]

There are numerous official websites saying windchill can cause frost bite even if the air temperature is above freezing, just as many as say the opposite. I wonder if this is a matter of definition, and "frost bite" is not quite as clear cut as it might seem. Perhaps extreme localised hypothermia is really a kind of frostbite/3

Carburettors freeze up partly from fuel evaporation, but also from the pressure drop from the venturi effect.

I've always thought that frostbite was the physical freezing of body tissue, which is obviously very damaging. The only way I can understand above-zero wind temperatures causing frostbite would be if the affected body part was wet, in which case the wind would speed up the evaporation of water and draw out the latent heat of evaporation from the body, thus reducing its temperature. But again, once the water had all evaporated and the body part was dry then the body part would stabilise at the above-zero temperature of the wind.

The venturi effect in a carburettor is a fair point, but is not really relevant to why a person can feel colder in the wind (unless they have their mouth open perhaps? )

 

[Flyfisher]

When you compress air it gets warmer - when you lower the pressure it gets colder. Similar to twist-effect cooling, cf elastic bands. Experimental refrigerators have been built using this principle.

Anyway, back to condensation in my house. Obviously warmer air can carry more moisture, but isn't it simply absorbing more of the moisture that would otherwise have condensed? Once that reserve has been used up, surely it goes on condensing again? Unless the air has been blown out by ventilation, which lets in some more cooler air.

I've not heard of 'twist-effect cooling' but I assume there is an input of energy to cause the 'twisting' in much the same way that a fridge working on the compression principle requires an input of energy (in this case to drive the motor/compressor). And that's exactly the point. It requires an input of energy to create and maintain a temperature differential, which is why my granite worktop, breadboard and tea cloth will all be at the same temperature, even though they will 'feel' different only because of how our fingers perceive temperature, as previously mentioned.

Back to your house, yes the warmer air is now carrying the moisture that would have otherwise condensed. The 'reserve' you mention is the effect of raising the dewpoint. Yes, there could still be condensation but it's now more likely that the warmer air carrying more moisture will be expelled from the house via the ventilation before that happens. And the cooler air coming in will (most likely) be drier than the warmer air going out, thus creating a cycle of air carrying moisture from the inside to the outside. The actual effectiveness of this cycle will depend on the relative temperatures, moisture load and rate of ventilation and these are the things we have to balance for a comfortable and healthy home environment.

 

[a twig]

Sorry - long post alert but hopefully going from first principles should help!

Based on ambient air temp, the air in the room can “carry” a certain amount of moisture (up to 100%) humidity.

Condensation happens when that air comes into contact with something that is relatively cold like a window pane, this drops the local temperature of that air which reduces the “moisture carrying capacity” so the moisture that there is no longer capacity for condenses at that temperature interface.

As an analogy you can think of it like having a cupful of water on a table then suddenly the cup holding it halves in size, and half the water ends up on the table.

So in a closed system - if you want to minimise condensation there are two main routes:
1) Reduce the temperature differentials, which is why Triple Glazing gets less condensation that Double Glazing which in turn gets less than Single, due to insulation meaning the interior surface temp is closer to the rest of the room.

2) Minimise the chance that the cold surface has to reduce the temperature of the air at the interface - this can be done by circulating the air, if the air is passing the cold surface rather than static above it, the cooling effect will be reduced. This is often why leaving your curtains open at night results in less condensation on the windows in the morning.

The complicated bit comes as your house is not a closed system and there are “moisture generators” - humans breathing and sweating, washing themselves, washing clothes etc. Now no matter how warm you make your house, if it is fully sealed and there is no air exchange you will eventually get condensation on surfaces no matter how good your insulation and air movement is.

That is solved through:
1) ventilation - exhausting the damp air outside and replacing it with air from outside which is more dry - you are right this can be less effective if you don’t have an MVHR system and the difference between the internal and external air temperatures is significant, as the cold air you are bringing in to replace the warm damp air will reduce the air temp and so will have less moisture carrying capacity until it is warmed up. Obviously the rate of air exchange and external weather conditions plays a huge part here as well, which is why MVHRs are brilliant and I wish I could install one here.

2) controlled condensation / absorption - using various dehumidification technologies to pull moisture out the air in a location that you want and use some form of management to discharge that moisture from the home as water rather than damp air.

 

[a twig]

Ha looks like FF just said the same but in far fewer words