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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I agree with previous comments that the primary sources of moisture are the un-vented space heater and the slab.  But there could be additional sources.  It is not clear if this space is primarily a wood shop or a garage.  A wood shop could have a lot of wood that is not dried down yet and a garage could have piles of snirt.  Snirt is a Minnesota term for the combination of snow, dirt and salt that collects around the tires.  Whenever you get a warm spell you get puddles of water on the floor.  Also, extended cold spells create lots of frost on roof decks and sudden warm spells melt all of the frost and ice buildup, wetting other surfaces.  I would suggest moving the thermal boundary to the flat ceiling, sheet rocking the ceiling, add soffit vents and install a vented heater.

For condensation you need one simple condition. A surface below the dew point. There are lots of reasons, this isn't the exterior surfaces. They mostly boil down to paper surfaces and sun heating the exterior.

Increasing the ventilation will reduce this situation. HVAC engineer fix that problem with sensible reheating, which means heat with no moisture.  The gas heater is unvented combustion, which means the gas and O2, becomes CO2 and water vapor. That water vapor could be condensing on a cold surface.This building lacks a reasonable moisture barrier due to sloppy, unfinished construction and installation of insulation.

If the heater isn't the source then the slab is the most likely source.  If it is the Slab, there are a few means to reduce it. Keep the water away by gravity. Gutters, slop grading away from structure, drains. Now that moisture is away from the slab, stop it coming through the slab with an epoxy coating.  You want some permeability, just less than the raw concrete.  

Ridge vent is 4 steps beyond the construction you have pictures. If there is no propervent / ventchutes allowing air to move under the roof decking, then you shouldn't have a ridge vent. They have a HotRoof. This means that the shingles are installed on an insulated surface.  This robs a few years off the life of the shingles. If I have to take care of this building, I install an exhaust fan, at least 0.5 cfm per square foot of garage floor. Then control it with a line voltage humidistat. 

Do not be fooled it is very common problem for cast iron wood working equipment to sweat and rust if keep in  a unheated garages when the weather changes quickly from cold to warm and damp.

Now wet drywall is very strange. To me drywall does not have enough thermal mass to stay below the dew point long enough to get very wet.

I would like to see photos of the wall damage.

I guess the lesson to be learned is if it works don’t change the can vents.



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