In the seventies and eighties, as homes became faster to build and tighter, many problems arose such as, window condensation, mold, truss uplift and an increase in respiratory diseases. Comfortable housing in cold climates does not happen without a bit of effort.
Cold weather construction is all about five critical details: air-leak sealing, rigid foam insulation, mechanical ventilation, avoiding or preventing truss uplift and, a cold roof is a great roof. I refer you back to attic ventilation as part of a great roof.
Air sealing is all about preventing moist warm air migration into cold air cavities in the exterior wall and unheated attic. This means, checking all areas where any polyethylene vapor barrier meets drywall/sheetrock. In other words, gaps around doors, windows, attic stairs or hatch and electrical boxes.
Rigid foam is worth the expense in cold climate walls, under the siding, masonry, or stucco. This breaks the thermal bridge caused by the studs and will increase energy performance from the uninterrupted layer of insulation. Rigid foam is not prone to condensation and helps tighten the envelope.
Mechanical ventilation is needed once the home becomes tighter. Condensation or water will pool on windowsills once the cold settles in and this causes black mold growth. An old home that used to have leaky doors and windows, provided fresh air, air exchange and moisture exchange by defect.
The best solution is a heat-recovery-ventilator. HRVs furnish a steady supply of fresh air while grabbing and recycling 90% of heat in outgoing stale air.
A 20th century problem in homes is that we produce a lot of water vapor from bathing, showering, cooking and laundry. This vapor turns back into water when it hits a cold surface such as a windowpane. If there is a cold surface deep inside a wall, water will deposit there and create decay.
Therefore, rooms must be very well ventilated and surfaces just warm enough to prevent vapor condensing. Most old homes had solid fuel fires burning and these needed lots of air to help burn the fuel. Air was pulled from door and window gaps and this air circulation helped draw out all the dampness in the structure.
Today, that is no longer the case, central heat has been installed providing heat but zero ventilation. Door and window gaps are sealed, chimneys are blocked to prevent cold draughts. Kitchens and bathrooms generate water vapor in the heart of the house and therefore, there is a lot of water vapor with nowhere to go to go back to liquid form.
This vapor will work its way into bricks, mortar and stone and wood to destroy them with repeated freezing, expansion and rot. Water in the wood rafters and wood anything of an old home means decay and dry rot making a perfect home for fungi and beetles. 20th century paints, flooring and wall finishes were developed with modern buildings in mind. Glossy and washable = low vapor permeability. New damp-proofing techniques applied to old homes trying to get them to behave and function like new ones is not working out very well. Tailor made retrofitting, insulation and problem area diagnosis is worth the time and effort, never be in a rush to just get it done.