How to prevent moisture behind batt insulation?

[PatFlemming]

Good day all,

I know that in a ideal world, concrete basement walls should be spray foamed or insulated with ridgid foam to create a thermal break.

Well, I'm getting a home build, and typical of minimum code, the basement walls will have pink fibreglass batt insulation with vapour barrier, coming about halfway down the wall.

Based on everything I hear and read, it seems like moisture saturation will be inevitable. Can anyone recommend any best practices or preventative measures I can use to minimize the risk of future moisture or mould problems? My basement is going to be dry walled upon delivery so I dont want to have to rip it all up a few years later.

Thanks

[good2know]

Only one way to prevent it. Don't do it in the first place.

The build itself will trap all sorts of moisture in the wall. The concrete will be wet from condensation. A winter build is the worst.

If they seal the rim joist area all up the moisture has no where to go.

The question becomes how long will it last before it smells so bad I must do something. I have a town built in 93 with insulation like you describe. It's a just a tiny bit musty smell. All carpet is gone which helps.

[PatFlemming]

Thanks for the reply. Good to know what awaits

How do you figure winter builds are worse than summer? Isn't there less condensation in the air in cold temps, and snow doesn't saturate the lumber as much as rain and humidity does?

[good2know]

A common practice is to run propane heaters in the house. One or more commercial heaters that convert the propane into water and co2/co increasing the interior humidity way above recommended levels.

In the basement a lot of this moisture gets absorbed into the concrete. The builder will then cover up the damp concrete with wood, absorbent material and a lovely plastic sheet to seal it all in. All the wood down there has also been absorbing moisture as well as the joists and subfloor. All that moisture can cause problems with the hardwood too.

Air infiltration around the sill plate can allow warm moist air infiltration in summer (bad) which will condense in side the wall and freezing cold air in the winter which freezes as frost or ice inside the walls and joist ends.

Building Sciences has an excellent collection of articles explaining the problems with these minimum code methods.

[PatFlemming]

Thanks for the information good2know!

I believe my builder uses natural gas rather than propane for the drying process. Does natural gas also convert into water?

[good2know]

Yes its pretty much the same.

[JMurphy83]

I am currently researching this same thing which is what brought me here.

I have read in both a book published by Mike Holmes, and on the National Resources Canada website (http://www.nrcan.gc.ca/energy/effici...ion/15639#a6-2)
that if you put 2 inches of foam insulation against the concrete, followed by your stud wall filled with batt insulation, you will not need vapour barrier. Refer to section 6.2.4 on that website.

I haven't completed my basement yet, but I am leaning towards not putting up a vapour barrier and following this method. The only question I have now, is that I already have 1 inch of foam against the concrete and the stud wall is complete. Will I run into problems where I don't have 2 inches of foam?

[good2know]

The closed cell foam must be 2 inch to qualify as the vapour barrier. Always best to consult with building inspectors dept as to what they want to see.