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Hu­mid­ity is the amount of mois­ture or wa­ter vapour in the air. You, your fam­ily and your pets pro­duce mois­ture when you breathe or per­spire. Even your in­door plants pro­duce mois­ture. We add wa­ter vapour to in­door air through rou­tine house­hold ac­tiv­i­ties: cooking, show­er­ing, bathing, do­ing laun­dry and dish­wash­ing. And more mois­ture can en­ter your home from the sur­round­ing soil through the base­ment or crawlspace.

When Is Hu­mid­ity a Prob­lem?

We need hu­mid­ity for our com­fort and health. But too much or too lit­tle hu­mid­ity can pro­duce a host of dif­fi­cul­ties for house­hold­ers. Some prob­lems are no more than nui­sances; oth­ers can be far more se­ri­ous. Many are familiar to Cana­di­ans, of­ten oc­cur­ring dur­ing the heat­ing sea­son, when it is very cold out­side, our win­dows are closed and in­door air cir­cu­la­tion and ven­ti­la­tion are re­duced.

Di­ag­nos­ing Hu­mid­ity Prob­lems

In­stead of guess­ing whether you have a hu­mid­ity prob­lem, why not find out for sure? A small, in­ex­pen­sive and easy-to-use in­stru­ment called a hy­grom­e­ter (some­times re­ferred to as a hu­mid­ity sen­sor or a rel­a­tive hu­mid­ity in­di­ca­tor) can mea­sure the hu­mid­ity level in your house and con­firm whether there’s too much or too lit­tle. Once you know for sure, you can de­cide what, if any, ac­tion is re­quired. The two types most suit­able for house­hold use are me­chan­i­cal hy­grom­e­ters and elec­tronic hy­grom­e­ters. For most house­holds, ei­ther type will per­form sat­is­fac­to­rily if prop­erly used and cal­i­brated. Hard­ware and depart­ment stores, build­ing sup­ply and elec­tron­ics stores of­ten carry hy­grom­e­ters. In fact, they’re usu­ally sold wher­ever you’d buy a room ther­mome­ter, and the two de­vices are of­ten com­bined in a sin­gle piece of equip­ment.

Us­ing Your Hy­grom­e­ter

Your hy­grom­e­ter shows the rel­a­tive hu­mid­ity (RH) in your house. While the RH won’t be ex­actly the same in ev­ery room, one hy­grom­e­ter per house is usu­ally suf­fi­cient. You should place it where the hu­mid­ity symptoms are most ob­vi­ous, in the room you’re most con­cerned about or where your fam­ily spends the most time. Be­cause they’re small, they can be moved around in your house from time to time. Don’t place your hy­grom­e­ter near a ra­di­a­tor, a heat reg­is­ter, a chim­ney or in any other lo­ca­tion where it could be af­fected by di­rect heat. A hy­grom­e­ter doesn’t pro­duce in­stant re­sults. It may take up to two hours to give a sta­ble read­ing in a new lo­ca­tion or ad­just to changes in rel­a­tive hu­mid­ity. Rel­a­tive hu­mid­ity is a per­cent­age that in­di­cates the amount of mois­ture in the air rel­a­tive to the max­i­mum amount the air can hold at that tem­per­a­ture. For in­stance, when air con­tains all the wa­ter vapour it can hold at that tem­per­a­ture, it has a RH of 100 per cent. If the hu­mid­ity ex­ceeds 100 per cent, mois­ture will begin to con­dense from the air. If the air con­tains only half the wa­ter it can hold at that tem­per­a­ture, the RH is 50 per cent. Warm air can hold more mois­ture than cool air, so the RH of a sam­ple of air will change as the tem­per­a­ture changes, even though the ac­tual amount of mois­ture in the sam­ple does not. As a sam­ple of air cools, the RH rises; when it warms, the RH falls.

How Much Is Too Much, Or Too Lit­tle?

Ex­perts have de­vel­oped rules of thumb to help home­own­ers make de­ci­sions about hu­mid­ity lev­els. Th­ese should be used as guides only. Ac­cept­able or com­fort­able hu­mid­ity lev­els will ac­tu­ally vary from sea­son to sea­son, from house to house and be­tween rooms in the same house. Some rules of thumb to pre­vent win­dow con­den­sa­tion dur­ing the heat­ing sea­son:

> Nor­mal rec­om­mended in­door RH: 30 to 50 per­cent

> When the tem­per­a­ture is -10 de­grees C out­doors: 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’s too low, you can in­crease it. This may mean sim­ple changes in your fam­ily’s habits, such as re­mem­ber­ing to open or close doors or win­dows. Or you may need to in­stall ex­haust fans in bath­rooms or kitchens to re­move ex­cess hu­mid­ity. Very low in­door RH lev­els in win­ter may be due to cold, dry air leak­ing in from out­side. Seal­ing up the house by weath­er­strip­ping and caulk­ing will im­prove con­di­tions in­doors and may re­duce your heat­ing bills at the same time. Hu­mid­i­fiers – both stand-alone ap­pli­ances and de­vices at­tached to your fur­nace – can be use­ful for in­creas­ing in­door RH lev­els. But re­mem­ber that if they’re not in­stalled, used and main­tained prop­erly, hu­mid­i­fiers can also be sources of ex­ces­sive mois­ture and mould.

The Fi­nal Anal­y­sis

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

A hy­grom­e­ter will pro­vide the in­for­ma­tion you need to de­ter­mine whether you have a hu­mid­ity prob­lem. If you do have a prob­lem, it can usu­ally be con­trolled.

This ar­ti­cle was sup­plied by CMHC (the Canada Mort­gage & Hous­ing Cor­po­ra­tion). Go to for other ar­ti­cles.

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