How to control indoor air quality

I recently blogged about how I solved an indoor air quality problem in my house by fitting a single room MVHR unit, but thought it might be useful to look into the different ways to tackle this common problem.

A family seated at the dinner table wearing gas masks

There must be a better way

What causes mould, condensation and humidity indoors?

The amount of moisture that the air can hold varies with temperature. Warm air can hold more water than cold air. This is known as the Relative Humidity. An RH of 80% means that the air contains 80% of the maximum for that temperature. Cooling the air would cause the RH to go up, until at 100% the air could no longer contain that water, and it would condense out. So we need to avoid very moist air from touching anything cold and causing condensation.

The main sources of moisture in your home are wet rooms (ie: showers), cooking, and the presence of people. These all release moisture which if not ventilated can build up. Besides humidity the other problems for indoor air quality include CO2, volatile gases, radon, outside pollutants, etc. However monitoring humidity is cheap and easy, so can serve as a useful proxy for indoor air quality in general.

How can we tackle a dampness problem?

The first thing to do is buy a cheap humidity monitor and see what your RH is. It’s very difficult to tell without one, as people are pretty insensitive to humidity. If you’re seeing above about 70% RH in winter, you should take some action.

Eliminate sources of moisture


This means doing simple things like putting lids on pots on the stove, or opening the windows while showering (assuming it’s warm enough!).

  • Pros: Simple, costs nothing
  • Cons: Limited effectiveness, especially in winter.

Mechanical Extract Ventilation (MEV)

This includes simple extractor fans in wet places like the bathroom or kitchen. These vary widely in price, with the more effective humidistat controlled versions costing several times that of the cheapest. The range of fitting options are wide, you can fit them through walls or windows or you can mount them in the ceiling and run a duct through your loft to the eaves.

The main drawback is that by simply chucking all your nice warm internal air out they create negative pressure which sucks cold outside air  in elsewhere. So while your indoor air quality will improve, your house will be colder. At current energy prices it is still cheaper to burn more fuel to replace the lost heat than it is to pay for a more expensive MVHR option (see below), but there is the comfort and carbon footprint angle to consider. Most extractor fans are also very draughty when not running, although ones with shutters are available.

  • Pros: Wide range available, can be very cheap.
  • Cons: Substantial heat loss

Positive Input Ventilation (PIV)


Essentially the opposite of MEV, PIV continually forces fresh air into the house, creating a slight positive pressure within. That air needs to have a defined exit path, so you’ll need continually open airbricks, trickle vents or MEV as well.

PIV units are often mounted in the loft where they draw air that is not quite as cold as external air, but if you’ve insulated your loft as well as you should have the difference will be pretty minimal. Effectively the have the same drawback as MEV. Since they run continuously the temperature swings will be less uncomfortable, but overall your heating bills will be higher. Some are available with heating elements to warm incoming air, but since this is straight resistive heating it’s a very expensive and dirty way to heat your home.

  • Pros: Effective, and don’t cause sudden temperature drops.
  • Cons: Same heat loss as MEV, or very expensive if pre-heating the air supply.



Dehumidifiers can get the moisture out of the air without venting it to the outside, and have the bonus that the dried air releases some heat, and is easier to heat than moist air. This would seem like a good option for a green home, if not for one major risk.

Excess humidity means that the home is not well ventilated enough. Besides moisture other pollutants such as CO2 can build up. While a dehumidifier can remove the moisture it doesn’t remove anything else, and excessive CO2 levels indoors can cause fatigue, headaches, etc. I would advise people to use dehumidifiers sparingly, and only to deal with point sources of humidity such as drying clothes indoors. Don’t leave them running continuously unless you are also monitoring CO2 levels. Unfortunately CO2 monitors can be quite expensive.

The better solution IMO is to ventilate, not dehumidify.

  • Pros: Tackles the problem directly
  • Cons: Not particularly cheap, risks masking air quality problems.

Insulate cold spots


Condensation will only form when air is allowed to cool so much that the water condenses. By eliminating cold spots you prevent this dampness from forming. Old windows and doors can be replaced or secondary glazing fitted, air leaks can be plugged, and cold spots on walls can be insulated (watch out for voids in cavity insulation, or bits that people forget to insulate like dormer windows and the ceiling of bay windows).

  • Pros: Often an easy DIY job, reduces fuel bills.
  • Cons: Not always easy to do, cures condensation but doesn’t actually improve air quality.

Whole house MVHR (Mechanical Ventilation with Heat Recovery)


This is the gold-standard solution. Air is extracted through ducts, passed over a heat exchanger and vented. Fresh air is drawn in and picks up heat from the heat exchanger to supply warmed air.

Generally it is impractical to retrofit a whole-house MVHR unless you’re doing a major refurb, due to the need to run ducts. MVHR also requires a high standard of air tightness in the house, which has to be designed into the structure of the building. MVHR is popular in high-tech Passivhaus eco-homes, but is very rare otherwise due to the four-figure pricetag.

  • Pros: Effective, quiet, comfortable, green.
  • Cons: Expensive, requires highly air tight house, requires duct runs.

Single room MVHR


This is a hybrid of conventional MEV and MVHR. A continuously running fan is fitted through the wall of wet rooms such as the kitchen and bathroom, and includes a heat exchanger to warm the supply air. Often an old bathroom extractor fan can be directly replaced with a single room MVHR, making them ideal for retrofit.

In my case I’ve found a single MVHR in the bathroom has been sufficient to control humidity for my whole house.

  • Pros: Effective, retrofittable, reduces heat loss.
  • Cons: More expensive than MEV while doing basically the same job.
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4 Responses to “How to control indoor air quality”

  1. Mohan Says:

    Wow – thanks so much for this! Have been googling this subject for a while now and nothing out there is as comprehensive and clear as this article. I now finally feel I have enough info to make a good decision. Thanks Andy!

  2. Martin stoker Says:

    I’m the same as Mohan, just spent a fortune redecorating my bathroom, included underfloor heating as well as an electric radiator so that I could dry towels without turning the whole house heating on, still I have major condensation after my daughter spends 20 minutes in the shower each morning, I thought I could heat the problem away. If only I’d seen this article 2 months ago, I tiled over my old wall mounted extractor fan 100mm hole & put an extractor above the shower, this just expels the warm air & draws in cold, after reading your post I’ve decided to retro fit a heat sava 100, all I need to do is core a 100mm whole through my travatine tile, the rest of the whole & electrics are still there. Many thanks

  3. Annemarie Sweeney Says:

    As mentioned above, this is a very comprehensive account! As heat loss is a ‘con’ for both MEV and PIV system solutions – could you advise whether installing BOTH would be a disaster? I prefer the MEV solution to localised fans in wet areas ( as I know they will not be used manually as they should) but if I don’t have trickle vents in my windows ( and I don’t) do I need to combine PIV and MEV together…? My house permeability rating is 4-5 and so MHRV is not essential ( and therefore cost has ruled it out). Any guidance on this would be greatly appreciated!

  4. Andy Says:

    For PIV you will need to have a defined exit path. If the fabric of your house doesn’t provide for that (either by vents or just being leaky) then you’ll need to add some.
    Personally I think it might be overkill. PIV is usually used as a remedy for leaky old houses with poor insulation and problems with damp. If you’ve got a reasonably well-insulated place that’s hitting the air change targets in the building regs already then I suspect targeting wet rooms with MEV might do the job. I’m a big believer in running them on a humidistat, that way they are always running when they need to be, and never when they aren’t. You might want to consider one of the single-room MVHR units, they aren’t super expensive and will reduce the amount an extractor fan will cool a bathroom or kitchen.

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This page last updated 18 May 2013