I got a call yesterday from a builder who has this problem: Homes are brick veneer,(with weeps)tyvek was installed properly,extra precautions/methods were used to limit build up of mortar behind brick during construction. Paperfaced fiberglass batt insulation was installed (not stapled to studs) Paper is facing towards the living area and a layer of foil paper is installed over the insullation in the basement (unfinished) We had heavy rainfall and unusually warm temps a few weeks ago followed by a deep freeze. Relative humidity was 45-47%. These are unoccupied and kept at 65 F. The occupied home has moisture readings of 25%+ behind the drywalled walls. Could it be there is air movement around the insulation because it was not secured to the studs thus causing warm and cold air to clash and create frosting?
Have been called on this issue in unfinished but insulated homes many times in my career. Have had people (owner-builder) want to throw all the insulation out because there was moisture in the fiberglass batt surface touching the sheathing!!! “We were told “It’s wet and useless, get rid of it””. I told them to take the batts out, let’em dry and let the stud cavities sheathing dry for a week or so, put it all back together with a proper air/vapour barrier. Move on!!
What’s happening? Air is leaking around the edges and through the batts (no continuous interior air barrier system installed) and then hitting the cold sheathing where it condenses and may eventually go to frost.
See: http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd175_e.html
A quote from this 1976 digest (quoted this in a post yesterday or Saturday):
"It would be more precise to define a vapour barrier as a vapour impermeable layer that resists the diffusion of water vapour under the action of a difference in vapour pressure. By defining it with reference to its resistance to the diffusion of water vapour it has been **implied **that diffusion is the prime cause of condensation problems. It is probably more accurate to say that vapour diffusion by itself never initiates a problem.
Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance -foil) that does not seal off the openings will be useless."
A much lesser amount of condensation may occur with winter building even if there is a full interior air barrier and vapour barrier. The studs maybe were not fully dry or picked up some moisture during the site building process. When the insulation is installed and the heat turned on, the studs begin to dry and the moisture moves to the cold sheathing, again condensing. If the exterior sheathing/cladding system is much more permeable to vapour diffusion (we say its overall permeance should be at least 10 times more/higher than the interior vapour barrier), the moisture will fairly quickly dry outwards when temps warm up. No mold will grow at this time as the sheathing is cold/frozen.
I hear the phrase “when the heat meets the cold” or “when the cold and warm air meet” a lot. These and other “street level” such phrases as “The frost is coming through the roof boards” or “The ice/frost on the inside surface of the drywall must mean there’s no vapour barrier installed” indicate that the physics of these phenomena is not well understood and thus recommendations for corrective actions may be way off base and cost customers unwarranted expenses.
In a brick veneer wall with a standard 1" drainage space (rainscreen), it shouldn’t. If the masons don’t properly clean and prepare to catch excess mortar from the rear side of the brick, the bottom of the space fills with mortar and the drainage function mighjt be impaired. In this case, mortar would be against the exterior TYVEK most likely impairing its water shedding properties and possibly holding moisture against it. With certain conditions, the moisture may then diffuse inwards into the sheathing.
