Avoid­ing mould grief in base­ment in­su­la­tion

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

Q: I am familiar with many of your ar­ti­cles and have a ques­tion I hope can as­sist me in mak­ing a de­ci­sion, since I know you see the ef­fects of mak­ing wrong de­ci­sions on a daily ba­sis. I am cur­rently de­vel­op­ing my base­ment. My home was built in 1974. The pre­vi­ous owner had al­ready framed the ex­te­rior walls and in­stalled pink R12 fi­bre­glass in­su­la­tion along with a vapour bar­rier, prob­a­bly around seven years ago. The vapour bar­rier was not sealed with acous­tic sealant, only sta­pled. I have also in­stalled a high-ef­fi­ciency fur­nace ap­prox­i­mately five years ago, which may have con­trib­uted to el­e­vated mois­ture in my home. I do not have a fresh-air in­take, ei­ther. Dur­ing the devel­op­ment process I had to re­move some of the vapour bar­rier and in­su­la­tion in or­der to at­tach in­te­rior walls. I im­me­di­ately en­coun­tered mould in the top two feet of the wall, es­pe­cially on the north­ern sides. The mould was on the en­tire wall, and some of the other walls as well, but even more con­cen­trated in the cor­ners where it’s cold­est. Since that point, I have cleaned the walls with bleach and wa­ter. I am cur­rently at the point where I need to in­su­late again. My ini­tial ap­proach was go­ing to be to use pink fi­bre­glass again, with the vapour bar­rier on the warm side with acous­tic sealant. How­ever, since I re­ally don’t want to cre­ate prob­lems again, I am think­ing my ap­proach would be to spray one inch of closed cell spray foam, from Home De­pot, fol­lowed by R12 fi­bre­glass and no vapour bar­rier. I am won­der­ing what your thoughts on this will be for the our cli­mate here in Win­nipeg. Your feed­back on my sit­u­a­tion will be greatly ap­pre­ci­ated.

Thank you, Dino Cate­la­nis and al­most guar­an­tees con­den­sa­tion, frost and mould growth. To solve this is­sue, the wall cav­ity should be al­ways be com­pletely filled with in­su­la­tion, prefer­ably a type that does not al­low air move­ment. Your so­lu­tion of us­ing high-den­sity blown-in foam in min­i­mal thick­ness will ad­dress this is­sue but will not be thick enough to pro­vide a com­plete air/ vapour bar­rier. If you were to in­stall three to four times that thick­ness, it would work, fill the en­tire wall cav­ity and elim­i­nate the need for poly sheath­ing in­side the studs. Your way, 6mm poly would still be needed and you would have a sub­stan­tial ex­tra cost for the foam. A bet­ter op­tion might be to in­stall a thin layer of ex­panded poly­styrene in­su­la­tion on the foun­da­tion wall. This rigid ma­te­rial is mois­tur­ere­sis­tant and al­lows some air and mois­ture move­ment, but not nearly as much as fi­bre­glass. Those prop­er­ties should al­low you to com­plete the rest of the wall as­sem­bly with the typ­i­cal batt and poly method, with­out as much con­cern about con­vec­tive air cur­rents. The key to this method is do­ing a re­ally good job of seal­ing the rigid in­su­la­tion and stud walls along the bot­tom and top to pre­vent air in­tru­sion, as well as the poly on the in­side of the studs. You could com­plete this by blow­ing sev­eral cen­time­tres of foam at the top of the wall, and in be­tween the floor joists, to seal that crit­i­cal area. Your pro­posed method of re-in­su­lat­ing your base­ment walls may be some­what ef­fec­tive in pre­vent­ing mois­ture and mould growth, but you will still need a poly air/vapour bar­rier, and may not be cost-ef­fec­tive. A mod­i­fied ver­sion of your plan, us­ing less costly rigid foam sheath­ing and less blown foam, may be a bet­ter bet. Ari Marantz is the owner of Trained Eye Home In­spec­tion Ltd. and the past pres­i­dent of the Canadian As­so­ci­a­tion of Home & Prop­erty In­spec­tors — Man­i­toba (cahpi.mb.ca). Ques­tions can be emailed to the ad­dress be­low. Ari can be reached at 204-291-5358 or check out

his web­site at trained­eye.ca.


Mould de­vel­ops from base­ment mois­ture in­side the foun­da­tion walls.

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