What do you think caused this

Hi Folks

Basement walls, mold on surface, material moisture content 19%.

One section of the wall was opened :
1-Vapor barrier, there (hot side of course),
2-Bat insulation
3-Framing is right against the foundation wall, no gap between them
4-Framing is not wet, but also have 19% humidity
5-Foundation is not wet (no water)

do you think number 3 could be the cause of the mould?


No, the problem is excessive moisture getting to the foundation wall. How water is getting to the foundation is something that needs addressed. Possibly poor grading or poor exterior water management in general. You have just uncovered the first piece of the puzzle I am sure there are a few more pieces that needs to be found before identifying the true cause(s).

I disagree with the above based on the limited information given.

The 19% moisture is what I would expect given the location of the studs.

It appears the mould was only found on the interior surface of the painted drywall.

It appears that the house has electric baseboard heat. If the owner, in an attempt to save heating $$$, shut the temp down too low or off, a microclimate would form in the lower corners** (was this a north facing location?) that would cool the air down significantly due to low/no air /heat flow. When 40% house air at 20C (68F), is cooled down to 10C (50F) without removing any moisture, its RH rises to about 80% (from memory only)…at this RH, mould growth is supported by ambient air moisture…no wet materials needed!

** I have also seen this condition when beds/couches are pushed up against outside walls…they block the heat from keeping the surfaces/air warm and also create a microclimate.

PS: #3- a gap between framing/insulation and the wall is a bad practice!!

It makes sense, how can we control that?

In New home Construction , Canada ,wood framed walls and concrete walls need an air space between them , also the bat insulation can not touch concrete , to prevent a mold situation .

You can`t have bat insulation touch concrete , which is why their is almost always a 1 inch space between 2x4 walls and concrete walls .

that,s what i though, thx

How old is the house? Could be from concrete “sweating” after it was poured. Of course the apparent mold growth on the drywall is from the concrete wall. Any cracks? If not, I would say the concrete was sweating or the vapor barrier install was crap. If there was no gap between the concrete and the batt insulation, mold will grow if concrete is sweating…(still in process of drying after the pour). Poor or no airflow between the batt and concrete wall will also cause condensation if the foundation wall is older…

No, it’s # 1.

You are not making some outrageous claim that the vapor barrier traps moisture and since the outside is dirt prevents the wall moisture from evaporating are you ?
Next you will be telling us the wood framing absorbs moisture off the foundation or some other **rot **like that.

The vapor barrier is in the wrong place. Batt insulation should not be exposed to the water vapor transmition from the concrete wall.
Will cause problems everytime when the moisture is trapped between the vapor barrier and the concrete wall.


may i note that no mould was present inside the walls only on the out side surface

the black stuff you see on the concrete is tar paper. so it’ built like this:
-tar paper
-framing + bat insulation
-Vapor barrier

the apartment complex is about 20 y.o…no cracks in the foundation

and i forgot to mention. in the picture above, one wall is common to the foundation, and the other wall is common a concrete seperating wall (other apartment on the other side of it)…built exaclty like the foundation wall

Mold spores can’t dig through plastic can they?
Air sealed wall will not have mold if it can’t get in.

With more information, we can now move up to #2. (Thermal Bypass)

(Still don’t have all the info needed however).

A little more info and we can move to #3. (Thermal Bridging)
Bottom plate and framing conducting heat away at the floor and corners.

Got a reference, either code or other research organization?

Their are situations where the 1" air gap cause problems!!! The first cases that I heard of from the early 1980’s were actually colder areas of Quebec.

How so? Research??

*I see you confirmed this in a later post!

OK lets start with the NBC. It calls for a bitumen coating on the inside concrete from the bottom of the wall up to the level of the exterior grade.
NBC also calls for an air space between the framing and insulation and the exterior concrete wall. This is for the moisture to escape up the space and then out side.
NBC also calls for 6 mil polyethylene on the warn in winter side of the framing.
NBC also calls for weeping tile around the perimeter of the foundation and damp proofing or water proofing of the exterior wall. This is dictated by local conditions.

You say there is tar paper. Sometimes the bitumen comes off the wall in a sheet that looks like tar paper. Did you check it to make sure what it was.
When you refer to sweating do you mean condensation?
Mac was close but not right, if it is condensation. At 20 C 50% RH rises to 100% when the temperature drops down to 10-11 C. At 100% you get precipitation or condensation on surfaces that are colder then the dew point. Dew point is 100% RH. Out side it called rain.
Mould will start growing on Moist organic material within 48 hours. Within two weeks you will see patches of it. A quick field test is to collect a sample on some sticky tape and put a little bit of chlorine bleach on it. If it disappears it is mould if it stays it’s dirt. If it disappears recommend mould testing by an expert.
The problem in the photos looks to me like the bottom of the wall has mould from moisture but the source of the moisture is most likely condensation. Also moisture could be wicking up from under the wall or from the concrete exterior wall.
Water coming through the wall is wet conditions on the other side. Do you have weeping tile in place and what are your local conditions and local practise.
No drainage via the weeping tile could cause water to build up and seep in.
I can not tell you what the problem is because I’m not there. Hope the above helps you figure it out.

Basement Insulated Walls:slight_smile:

thx Vern appreciate it:mrgreen:

So the best way to do it is with foam bords, between wooden structure and the concrete.

Marcel, in the pictures it shows a corner (2 walls) , one is common to the foundation, the other is separating wall (concrete)…apartment on the other side :shock:… this is what’s bugging me…same condition, same wall construction different ‘‘exterior’’ condition (grade vs apartment)

Not being there, could be a humidity problem from the first problem. :slight_smile: