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Humidity in your home

Humidity in your home

What is Humidity?

Humidity is a term used for the amount of moisture or water vapour in the air.

  • Moisture is produced in your home by you, your family and your pets from evaporation of perspiration, and breathing.
  • Moisture is also produced in your home by your indoor plants as water evaporates from the pores of their leaves and petals.
  • Moisture can enter your home from the surrounding soil through a basement or crawl space.
  • Water vapour is directly added to indoor air through routine household activities such as cooking, showering, bathing, doing laundry, and dish-washing.

We generally deal with two measurements of humidity in home inspections, Relative Humidity and Specific Humidity, and these are based upon the fact that air can hold different amounts of moisture as vapour at different temperatures and pressures.

Rain in the bathroomTo explain this better, have you ever had a bath or a shower on a cold day and the bathroom has filled with steam?

When a room is filled with steam, the air is at 100% saturation, in other words, it can hold no more moisture as vapour.

If the air tried to hold any more water vapour at this point, it would start raining in your bathroom!

The air in your bathroom is warm, but tiles, mirrors and windows are generally much colder that the air, and so is the air that touches them.  This colder air cannot hold as much water vapour as the rest of the air in the bathroom so it does “rain” but locally on the cold surfaces, and this is called “condensation”.

Relative humidity is the amount of water vapour in the air, compared with the total amount of water vapour that same air could hold if it was saturated.

So if you don’t see steam in the bathroom, but the amount of water vapour was 75% of the amount of total water vapour the warm air could hold before the whole room was filled with steam, it would be said to have a relative humidity of 75%.

Specific humidity is a ratio between the water vapour to the completely dry air in any given volume.

There is a third measurement of humidity, and that is absolute humidity but this is difficult to measure because of the fact that temperature and pressure also come into play.

When can Humidity become a Problem?

Humidity affects the well being of many things, people, animals and plants, electronics and buildings.

Humidity and us.

In humans, high levels of humidity, combined with high temperatures can cause a condition called hyperpyrexia, where blood rushes to the surface of the skin but cannot shed the heat through perspiration evaporation, and because so much blood is at the skin level muscles, the brain and other internal organs have their blood supply reduced, which also reduces the oxygen and nutrient flow, which can create fatigue and physical muscular tiredness.  High humidity levels have also been associated with medical conditions such as Asthma, anxiety attacks, Hyperventilation causing numbness and fainting and even heat-stroke and hyperthermia.  Conversely low levels of humidity coupled with low temperatures increase the risk for nosebleeds.

Humidity and our homes.Static electricity is a nuisance

As you can see we need humidity for our comfort and health. But too much or too little humidity can produce a host of difficulties for householders.

Some of the problems are no more than nuisances and occur during the winter season when our windows are closed, and indoor air has little humidity.

These conditions, like static build,-up are familiar to most Canadians.

Other condition can create problems that are far more serious.  These are caused by high humidity and poor indoor air circulation or ventilation are reduced, or insufficient insulation allowing warm moist air to contact cold surfaces allowing the vapour in the air to condense on the surfaces.

Fun with Math:

In a bathroom full of steam, at a temperature of 300C the air can hold approximately 30.4 grams of water vapour per cubic metre.

If the bathroom was 3.25 metres by 3 metres by 2.5 metres high a steamy bathroom would have 741 grams of water vapour suspended in the air (over 1½ pints or ¾ litre).

Drop the temperature by 100
to a more normal 200C degrees and the air would have to lose over ½ pint of water somewhere.  This is why bathrooms should wither have a working exhaust fan, or an opening window, otherwise that ½ pint is going to seep into your home somewhere through condensation!

Problems in homes with humidity that is too high.


  • Condensation on windows, walls or ceilings
  • Wet stains on walls and ceilings
  • Musty smells
  • Mould in the bathroom, washroom or attic spaces.
  • Mould behind washing machines, or dryers
  • Damp surfaces under sinks or around toilet bases. /li>
  • Allergic reactions

 Long-term effects

  • Damage to the house and its contents
  • Ongoing allergies
  • Other health problems

Problems in homes with humidity that is too low.


  • Chapped skin and lips
  • Scratchy
    nose and throat
  • Breathing problems
  • Static and sparks
  • Problems with electronic equipment

Long-term effects

  • Continuing discomfort
  • Damage to furniture and other items

Diagnosing a humidity problem

Dail HygrometerA small, inexpensive and easy-to-use instrument called a hygrometer or relative humidity indicator can measure the humidity level in your house and confirm whether the house has too much or too little humidity.  Once you are aware of the issue, you can decide whether any action is required and, if so, what action.

Two types of hygrometers that are most suitable for household use are mechanical hygrometers and electronic hygrometers.

Mechanical hygrometers cost anywhere from $7 to $30. They are usually made of plastic; often round with a pointer and dial display. Although they are reasonably accurate once calibrated, they may “stick” if humidity does not change for a long time

Electronic HygrometerElectronic hygrometers are more expensive and can cost from about $15 to $60. Again they are usually of plastic construction and have an LCD display so they require batteries.

They can sometimes be slow to respond to changes in humidity, and are usually accurate in the mid to high humidity ranges but can be inaccurate in the lower ranges (below 30 per cent relative humidity). They cannot be physically corrected or calibrated.

Hardware stores, department stores, building supply stores and electronics stores often carry hygrometers. In fact, hygrometers are usually sold wherever you would buy a room thermometer. Hygrometers and room thermometers are often combined into a single piece of equipment.

A hygrometer will show the relative humidity (RH) in your house.  Although the RH will not be exactly the same throughout your home, one hygrometer per house is usually sufficient.  You should place it where the humidity symptoms are most obvious, in the room that you are most concerned about, or where your family spends the most time.  Because hygrometers are small, they can be moved around in your house from time to time.

Don’t place your hygrometer near a radiator, a heat register or a chimney, or in any other location where it could be affected by direct heat or drafts.

Remember that a hygrometer does not produce instant results. It may take up to two hours to provide a stable reading in a new location or to adjust to sudden changes in relative humidity.

Humidity: How Much Is Too Much, How Much is Too Little?

Experts have developed rules of thumb to help homeowners make decisions regarding humidity levels in their house. The limits should be used as guides only. Acceptable or comfortable humidity levels will actually vary from season to season, from house to house, and even between rooms in the same house.

Rules of Thumb

To prevent window condensation during the heating season, the recommended indoor RH is 30 per cent to 50 per cent. When it is below -10°C (14°F) outdoors, recommended indoor RH is 30 per cent.

Taking Action

Humidity can be controlled. If the relative humidity in your home is too high, you can reduce it; if it is too low, you can increase it.

In summer, you can reduce house humidity levels by the use of a dehumidifier or by running an air conditioner.

In winter, a house that is too wet usually has some high moisture sources (for example, a damp basement, roof leaks, many plants).  Deal with these problems first. If high humidity persists, you may need to make simple changes in your family’s habits, such as remembering to open or close doors or windows.  Or, you can install equipment, such as exhaust fans in bathrooms or a heat recovery ventilator (HRV), to remove excess humidity.

Very low indoor RH levels in the winter may result from cold, dry air leaking in from outside. In this case, sealing up the house by weather stripping and caulking will improve humidity conditions indoors and may reduce your heating bills at the same time.

If low humidity problems persist, despite air tightening the house, consider the use of a humidifier.  Humidifiers — both stand-alone humidifiers and humidifiers attached to your furnace — will increase indoor RH levels. But if they are not installed, used and maintained properly, they can also be sources of excess moisture and mould in your home.

Final Analysis

Moisture MeterHumidity levels in your home can be too high or too low. In either case, problems can result.

A hygrometer can provide the information you need to determine whether you have a humidity problem — but it must be accurate to be useful.  What these instruments won’t tell you is if moisture has been present in the past, or still is, in attics or in a crawl space.

For this you would need a moisture meter, and the accurate ones are very expensive costing anywhere up to $2,000, and even a moisture meter won’t tell you what might be “hidden” behind items you don’t have access to.

Flir E40BXInfrared Thermal imaging can at least indicate there might be a problem.  The first problem here is that a quality Thermal imaging camera can cost from $4,000 up to $35,000, and the education to learn how to use it can cost as much as $8,000.

If you think you might have a moisture problem, prices like this make the $350-$600 you would spend on a Certified Home Inspector to come and do a full inspection of your home more than worth it.

The good news is, If you have a humidity problem, it can usually be controlled.