Improve Ventilation - design comments

[au8830]

I have a small 2 bedroom bungalow with limited ventilation provision and I'm currently experiencing problems with condensation running down the double glazed windows. The windows do not have trickle vents, and I have been using the "lock open" position 24/7 to mitigate the problems, but don't like this due to the security risk.

I do not dry washing inside, and have an extractor over the shower which is ducted outside and runs during and for at least 30 minutes after a shower with the window wide open. Loft insulation was topped up by previous owners, as well as cavity wall insulation. Some evidence of historic mould was seen on North facing walls, but these don't appear to get damp now - just the window glass.

I have done a quick roughly to scale sketch of the floor plan. The right hand wall (as drawn) is North East facing. The worst rooms for condensation are those labelled Bedroom 2 and the Lounge. Relative humidity hovers around 65-70% with 19-20°C. With the windows locked open for 24 hours and heating on full, temperature was 24°C with 50% RH.

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• au8830
• 7 Nov 2014
• 1

There is currently a double terracotta air brick in the kitchen, about 5 foot from the floor level. (plus another single air brick behind the cupboards) Everything in blue exists already. Red is what I'm proposing as additional.

The front door shown here opens to a double glazed porch area (not drawn). As such I'm not sure that there is any significant air change when coming and going to/from work.

I have considered 3 options:
1. Trickle Vents - very cheap, but have concerns that these don't provide sufficient area, and also about outside noise.
2. Fit 3 additional wall vents in the positions shown in red each providing 8400mm2 unrestricted area. I am considering the Ryton AAC125HP, for reduced noise transmission.
3. Fit a PIV unit in the loft (Nuaire Drimaster) in the position shown in green. But then would this require some trickle vents or wall vents be fitted into the windows anyway, to be effective.

The area provided by the three new wall vents seems to provide sufficient ventilation for the size of property. I've positioned them diagonally opposite the door in each room - is this optimal?

Any ideas welcome. Thanks!

 

[theprinceofdarkness]

As far as I can see, the nuaire drimaster, just blows air in from the loft. So you need vents to let your hot and damp air out.
If you are going to make holes in the ceilings for vents, I would spend the extra on a proper heat exchanger system. this will pre heat the incoming air by the warm air you are venting. It could be a closed system (in theory), with the only outlet is the condensation drain. In reality cold air is taken in, warmed up by the warm air you are venting, so you get warm DRY air as the input to your rooms and you have a drain pipe where your condensation drips from. This is actually problematic as the unit will sit in your freezing loft, so the drain pipe can freeze up, but the condensation keeps on coming so could drip out onto your ceiling. Manufacturers normally include an electric heating circuit actuated by a thermostat to keep the thing "unfrozen".
Frank

 

[Flyboytim]

When your car windscreen is covered with condensation, you blow air over it. Even cold air before the heater kicks in starts to demist it.

You do not suck the air out of the car, or open all the windows and expect the condensation on the windscreen to evaporate any time soon. If you have aircon, you do not put that on its coldest setting to dehumidify the air. You might help it to evaporate a bit faster by blowing heated air to warm up the windscreen, but you soon turn the temperature back down to a comfortable level.

Try the same at home - try moving the air about, point a fan at the wettest windows. Squeegee off the most of the condensation to speed things up and see if the condensation comes back while you have air blowing over it - it will remain dry, even on a cold wet day, I assure you. You don't need to increase the room temperature, and except when you feel the draught of the fan, the room will not feel any colder.

What sort of power fan do you need? About 40 W. It is cheapest to manually remove as much moisture as possible - squeegee, sponge and cloth, before you expend expensive fuel energy on it.

Extract the bathroom and kitchen, and don't steam up the rest of the house by all means, but do not continually let cold air into your heated house by leaving windows open a crack. Post-fitting trickle vents is a good idea - at the top of windows - to provide steady ventilation. Opening bedroom windows for a few minutes on cold mornings will do much to dry the atmosphere indoors.

Consider keeping heating to the minimum comfortable in areas such as living rooms and a little cooler in bedrooms to reduce the vapour density of the air - the humidity available to condense.

Relative humidity is a fairly meaningless concept when talking about condensation - the saturation vapour density and dew point are more relevant.

SVD is the number of grams of water in a cubic metre available to condense on colder surfaces than the air temperature, and the dew point is the temperature that the humid air reaches 100% RH, and starts to condense.

Here's a table to get you thinking:

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• Flyboytim
• 8 Nov 2014
• 1

You can see if your window is 5 deg C, and the room air is 20 deg C, there is the potential for every cubic metre of air flowing over the window to condense up to about 10 ml of water.

When we returned to our home after letting it for several years, our tenants had turned what had been a dry house into a rather damp one. They had thankfully used the kitchen extractor, though. They were actually very considerate and clean tenants.

Our large conservatory (5x4 m) was covered in dripping condensation and black mould. Once I had cleared the muck from the trickle vents and cleaned the black on walls and sills with neat thick bleach (not dilute, but allowed to soak and dry on), and we kept fans on during the night and mornings for a few days, then as required on colder nights. The understairs cupboard was actually soaking wet, so removing the carpet, and using a fan, dried that in a few days too.

 

[Nige F]

OP your house is very similar to mine . Have you got wooden suspended floors or concrete ? Is the D/G fairly old ? I had one window renewed complete because the glazing had lost it`s seal ( marks inside the panes ) and the opener was faulty . Also had a new pane in another window . The interesting thing is there is less condensation on the new panes ,I`m thinking the air gap is breaking down slowly on the old ones ( c.20 years) and the inside pane is colder than the new ones .

 

[AronSearle]

Long post ahoy.

You need to think about how much work you are prepared to put into this, and how much money, and what you want to achieve thermally.

Whacking in more air vents is easy, cheap and effective (it's cost depends on if you are paying someone to do the work). PIV systems is the same, running cost is small, you shouldn't need more air vents for air to escape, as there are likely loads of air leakage points in your house, including existing vents. You might want vents in some rooms to control outward airflow in those rooms specifically. Both these solutions mean allowing cold air into the house, so more drafts, more heating costs. Most people don't seem to care, some might, I personally find it silly to do this when money has been spent insulating the place.

Heat recovery systems work well, and have the benefit of allowing you to control air quality. But for them to work effectively you need to pay very special attention to making the place airtight, really special attention, look up guides on air tightness. The point of these systems is you want the airflow going through the system, otherwise you are wasting the full benefits of the system by having air that you just dried and heated going out the walls. Air tightness is a DIY job, cheap and easy, just requires a lot of attention to detail, and air sealing places most people don't even think about (for example light fittings in ceilings, wall sockets). If you have a timber suspended floor, this will need insulation and a membrane. Running a dehumidifier is a similar principle, but without the advantage of fresh air (but a cheaper upfront purchase), as with an air exchange system, you may actually want to reduce air flow depending upon the dehumidifier. How practical and efficient a dehumidifier depends on how much humidity needs to be controlled. Sometimes you only need a marginal reduction, and so this is the best cheap option.

If you are only getting condensation on the windows, and nowhere else, you might just want to focus on improving those. You also may want to consider how much you need to lower the humidity by to stop condensation on the windows.

At 20C, and 70% humidity, a surface needs to be 14c for condensation to occur.
At 20C, and 60% humidity, a surface needs to be 12c for condensation to occur.
At 20C, and 50% humidity, a surface needs to be 10c for condensation to occur.
At 20C, and 40% humidity, a surface needs to be 7c for condensation to occur.

(approx figures). Regarding the last figure, you should appreciate why condensation will occur on poor windows even with good humidity and temperature readings. Older double glazed windows with poor thermal breaks or compromised seals, could see low internal surface temperatures in cold weather that make condensation hard to avoid.

You can assess surface temperatures by hand, or get a surface temperature thermometer for <£10 on fleabay. Then you can more accurately decide how much you need to lower the humidity by (sounds like you already have a hydrometer).

As I said, it really depends on how much effort and work you want to put into this, whacking vents in is a quick cheap fix that satisfies most people.