Ven­ti­la­tion needed to cure damp smell

Winnipeg Free Press - Section G - - HOMES - ARI MARANTZ

QUES­TION: Fif­teen years ago we added a bed­room and fam­ily room to our 540-square-foot home. The ad­di­tion is set on piles. The crawl space was in­su­lated with Sty­ro­foam and poly and the floor was cov­ered with poly and sand. All of my down­spouts have ex­ten­ders. The only ac­cess to the crawl space is through two old base­ment win­dow open­ings to fa­cil­i­tate air move­ment.

Over time we have no­ticed damp­ness and a musty smell em­a­nat­ing from the crawl space. Last win­ter we closed over the win­dow open­ings and di­verted a heat duct into the area from the fur­nace that has the fan run­ning con­tin­u­ously. The think­ing was this would dry out the sand base and elim­i­nate any musti­ness, but it still pre­vails.

Should we be in­stalling vent­ing in the outer walls for sum­mer air­ing and con­tinue with the heat­ing in win­ter? I as­sume any vent­ing would have to be closed off to elim­i­nate cold air over the win­ter. — Jim Rodgers

AN­SWER: Damp-smelling crawl spa­ces are one of the most com­mon is­sues home­own­ers face, no mat­ter what age or lo­ca­tion of the home. This can range from stand­ing wa­ter to sim­ple odours sim­i­lar to what you are ex­pe­ri­enc­ing. Mea­sures to pre­vent this from oc­cur­ring range from ma­jor ex­ca­va­tion to proper ven­ti­la­tion, but your sit­u­a­tion seems to be more re­lated to the lat­ter.

No mat­ter how well built or sealed a crawl space is, mois­ture in­tru­sion is al­ways a pos­si­bil­ity. You have al­ready ad­dressed sev­eral of the most com­mon causes of the damp­ness, but that may not be a com­plete rem­edy. The rea­son that many crawl spa­ces have mois­ture is­sues is sim­ply the de­sign. These ar­eas are typ­i­cally con­structed with a short perime­ter con­crete grade beam that is the only bar­rier to mois­ture from the sur­round­ing soil en­ter­ing the crawl space be­low grade. As in your case, this grade beam is sup­ported by deep con­crete piers that will re­sist ver­ti­cal move­ment, but they pro­vide lit­tle ad­di­tional pro­tec­tion from the mois­ture in the sur­round­ing soil.

Due to nor­mal ero­sion and soil wet­ting and dry­ing, wa­ter may be able to seep un­der­neath the grade beam and en­ter the crawl space. The poly­eth­yl­ene air/vapour bar­rier you have in­stalled may pre­vent most of this mois­ture from en­ter­ing the liv­ing space of your home, but it is never per­fect. Also, it can trap mois­ture in the soil, al­low­ing it to re­main damp, which may be the ma­jor source of the smells you are ex­pe­ri­enc­ing.

While you have taken mea­sures to min­i­mize mois­ture in­tru­sion by good grad­ing and wa­ter man­age­ment on the ex­te­rior of your home, and have added heat from your forced air fur­nace, you may be missing one im­por­tant com­po­nent from the crawl space. A sump pit and pump, pos­si­bly con­nected to a perime­ter drainage sys­tem, is an im­por­tant part of most dry crawl spa­ces. This pit should be lo­cated near the cen­tre of the crawl space, prefer­ably at a low point in the slop­ing soil. This will col­lect ex­cess mois­ture in the soil and the pump will dis­charge it to the ex­te­rior. If you don’t have a prop­erly in­stalled sump pit and pump in an ad­di­tion of your age, one should be rel­a­tively easy to in­stall. Even with­out a perime­ter drainage sys­tem, this may help to col­lect enough ex­cess mois­ture to pre­vent the damp-smell in your crawl space.

To more di­rectly an­swer your ques­tion, adding sum­mer vents in the perime­ter grade beam is a very good idea. The warm, fresh air that will en­ter the crawl space through these vents will help elim­i­nate the mois­ture and the of­fen­sive odour. These vents may be dif­fi­cult to in­stall, re­quir­ing cut­ting or drilling through the con­crete grade beam, but may be well worth the ef­fort and cost. You should en­sure that you have at least one vent on op­po­site sides of the foun­da­tion to al­low a good cross draft to de­velop. Also, these vents must be screened to pre­vent in­spects and other pests from en­ter­ing the crawl space. Fi­nally, win­ter cov­ers will be crit­i­cal, but these may be in­su­lated with rigid foam, sim­i­lar to the other in­su­la­tion al­ready in place.

The last is­sue to ad­dress is the heat­ing and ven­ti­la­tion of the crawl space in the heat­ing sea­son. You have taken ex­actly the right steps by in­stalling a warm-air duct from the fur­nace and run­ning the fan con­tin­u­ously, but you have erred by clos­ing the ac­cess hatches. The two open­ings into your base­ment are the only means of ven­ti­la­tion of the damp crawl space in the win­ter, so they should re­main open. Pump­ing warm air into this area will help dry out the mois­ture by sim­ple cir­cu­la­tion, but much of this damp air may re­main trapped with­out any means to es­cape. Clos­ing these ac­cess hatches may pre­vent the musty smell from en­ter­ing the base­ment, but will only make this is­sue worse when they are opened.

To sum up this dis­cus­sion, mois­ture pre­ven­tion for crawl spa­ces has two main ar­eas to ad­dress. Firstly, pre­ven­tion of wa­ter en­ter­ing this area by good ex­te­rior grad­ing and wa­ter man­age­ment is very im­por­tant. Se­condly, when wa­ter does pen­e­trate this area, de­spite your best at­tempts to min­i­mize it, it must be elim­i­nated. This can best be achieved by a com­bi­na­tion of a prop­erly in­stalled and main­tained sump pit and pump and good ven­ti­la­tion.

If all of these com­po­nents are in place, the mois­ture, along with the of­fen­sive smell, should dis­ap­pear.

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