I have a customer who recently had condensation on an interior wall of an unheated detached garage.  We replaced her roof last fall.  We installed 6t of ice shield along the eaves, exceeding code of 24 inches past the interior wall.  We covered the entire surface with a superior synthetic underlayment.  We cut the ridge and installed ridge vent.  The structure was up and over.  

We are a contractor in Michigan.  We have experienced extreme weather conditions this season.  It was below zero for several days in a row and then up to 40 degrees.  

The customer has a portable gas heater but says they did not heat it prior to the condensation build up.  They do wood working in this space.  There is no attic space.  It is open.  They have insulation stapled between the rafters.  I believe the garage is built on a slab.

can anyone tell what the cause of this is?  



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My response is based on my assumption that there is fairly free exchange of air between the garage interior and the exterior.  I was not able to follow your description.

For there to be condensation, it means the interior walls of the garage was below the dew point of the air in it.  This could easily happen when there is a prolonged cold spell followed by a sudden increase in exterior temperature. The outside temp goes up faster than the thermal mass of the walls have had a chance to catch up and you end up with this condensing plane.

Adding to what Sean said... although this could happen in any climate, I've never seen it. In addition to a quick transition from prolonged cold weather to mild, the moisture content in the air would also have to spike. A building can withstand short-term periods with minor condensation. Tools, not so much.

Are the walls and O/H door insulated?

Air sealing the garage would help, but it's hard keep outside air out if the overhead doors are operated frequently. I had a shop in my previous garage, but I had it in a separate room in the back, with an exterior door that had a threshold and gasket. That pretty much eliminated infiltration from O/H door operation. It also allowed me to quickly heat and maintain a comfortable temperature in the work area using a low wattage space heater.

I have actually been having this occur regularly this winter on my partially completed house.  I do not have any garage door and the walls are not insulated.  The floor is a 10" suspended concrete slab and I have had several days when the top surface of the slab is condensing because its mass has not yet caught up with the outside air.

One of the by-products of any unvented heat source is water vapor.  A concrete slab without a continuous vapor barrier is no more than a wick for groundwater.

David N, as mentioned this is a suspended concrete slab over a room below.  So no, this is not a ground water issue, this is condensation occurring because the slab temp is below the dew point of the exterior air (the house is not yet heated).

@Sean, David N makes a good point as sub-slab moisture could be the culprit for Michigan garage described in the original post. In that case, moisture would have been obvious on the slab before condensing on the wall.

@Pam, you should check with the owner to see if he noticed any moisture on the slab. Also, did the condensation occur on the north wall? If not, that would imply a highly localized moisture source... e.g., missing wall insulation.

I'm guessing you're involved in this because the owner thought your work may somehow be implicated. Given your description, I can't imagine a scenario where the roof would be involved. The gas heater could easily explain the condensation, but it would have to have been very recent to the event.

We have seen many similar instances this winter in Maine when we had about 1 week of extreme cold (nights well below 0F and daily highs 0 to 10F in Jan) following rather warmer than average late fall early winter . Even subtle differences in moisture from breathing, etc. could have raised the RH inside the building such that the wall(s) in question became condensing surfaces.  When the air temps rose quickly the building materials were slower to react & remained below the condensing point for some period of time not allowing them to dry out and perhaps allowing them to accumulate more moisture. Sublimation is a far slower process than evaporation . . .  We also regularly see marked differences between the northerly sides of roofs and the Southerly ones EVERY year.  Moisture often accumulates on both (especially at night) and is then driven off the S side during the day when the roof deck heats up and re-accumulates on the N side that is still below the dew point.  This is one reason why we have become firm believers that the best roof is fully insulated just like Joe Lstibruck's "perfect wall" only on an angle!

I want to thank everyone for their responses.  

I was finally able to inspect this garage myself.  The insulation running up the rafters was covering the ridge vent cut that we installed.  The garage door did not shut completely due to previous damage.  The cement floor was cracked in various places. The customer told me that she does use a space heater occasionally when working in the garage and in the morning before she leaves for work..

the customer feels that the ridge vent being covered was the source of the problem.  I think it was condensation that was caused by the freeze thaw and spikes in temperature.  She also feels that I need to compensate her for her tools and the replacement of the drywall on the South wall and the insulation that was wet.

Before we installed the new shingles they had two 550 can vents for exhaust that were also covered by insulation running up the rafters. The customer claims that this has never happened in the 30 years she has lived there.  My question is, if they say that it was my fault for not making sure that the ridge vent cut was ventilating properly and caused the condensation to occur, then why, if before we installed the new roof, it had no ventilation, had this not occurred in the last 30 years?  



Pamela, Based on what you posted David Butler and David N have the best sources of drivers of moisture. My bet is 1st source of moisture was slab on the days below zero drawing warm ground moisture upwards, second moisture from propane heater, then wall temp. differentials to cardinal direction of building. As a general contractor of many years.

Did you change color of roof?

Was it in your scope of work to

A) install the ridge vent


B) to install the ridge vent and also cut back the existing insulation

Whether you should have noticed, identified and raised to the customer's attention the problem with the existing insulation blocking the ridge vent is another issue though your customer may not see it this way.

Long story short an unconditioned insulated building will retain its temperature and "lag" the ambient temp.  This can, has and will continue to cause unintended consequences.

It could be that the ridge vents themselves have caused an increase in humidity.  Did you remove standard box vents when you installed the ridge vents?  Do the ridge vents have a fabric bug screen?

I noticed in one of the photos that there had been a recent snow fall.  Snow coverage of the ridge vent renders it ineffective at providing any venting.  Furthermore, it has been show via smoke testing performed by RDH, that many styles of ridge venting, containing fabric bug screens, prevent air flow through the vents at the low pressures that the stack effect places on this interface. 

So if functional box vents were removed and replaced with ridge venting.  There is a good chance that there is no longer any functional upper roof venting on this garage.

Did the builder actually insulate the wall properly? If air is moving through the O/H doors to an exterior ceiling or other penetration it could cause a low pressure in the garage, thereby lowering the temperature within the garage and subsequently the wall causing such a condition to make condensation on the wall.


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