Roof leaks, shallow groundwater, driving rain on leaking walls/windows - there are many sources of visible liquid water into a structure or home, but I want to talk about the ones we can't see. These issues can apply to any structure new or old.
First a few things to get out of the way.
I am not a professional. I hold no relevant certifications. I like healthy homes and I read a lot. I'm passionate about about alternative building as well as saving old structures. I'm also passionate about moisture management and mold abatement, because mold is a common health concern in alternative and old structures, and moisture related decay is often the primary driver of structural failures. I hope I can help some folks identify or prevent causes of structural moisture accumulation.
Wet air rises faster than hot air. Water vapor on it's own is always trying to rise.
Vapor barrier and vapor retarder are terms often used interchangeably. For conversational purposes a vapor barrier stops nearly all water vapor, a vapor retarder just stops some or a lot of it.
Water vapor from occupants is real, because we all (and our animals/friends) exhale a significant amount of water with every breath - but we can't change that input, so I'm going to ignore it here.
RISING DAMP The ground is always breathing, and in most places, that means moisture is always coming up from the ground. This moisture is sometimes called rising damp. Modern structures have a vapor barrier underneath their basements/crawlspaces and on the outside of their foundation walls. This prevents airborne water vapor from entering the structure from the ground. If you have an old home, you likely do not have a ground source vapor barrier (unless it's been added $$$) and thus have a source of water vapor coming up from the ground. Rising damp. Some places take rising damp into account in building code, but in the US most trade professionals seem to be unfamiliar with the concept. Rising damp is a source of continual moisture in a lot of old homes. If you don't have a vapor barrier on the bottom, and you add one elsewhere you can trap moisture that would otherwise dissipate up and out of the structure. Even adding insulation, with no added vapor barriers, can slow down vapor movement enough to induce mold conditions in some structures.
THERMAL PUSHING is the movement of moisture in solids as a function of temperature - This is an oversimplification, but in terms of a structure - when a vapor permeable solid item, like drywall or wood, encounters water vapor, it will enter from the warm side and move towards the cool side of the material (sometimes holding on to some of it along the way, but not always.) In warm wet climates, this means moisture is always trying to enter your home's materials through the walls, coming from the outside in. In cold climates, when the interior of a home is heated, this pushes moisture into the walls from the inside moving out. When the cold side of the wall/material is below the condensation point of the water vapor that is diffusing through it, nearly all airborne water vapor can become trapped in the material. Absent of a temperature gradient moisture in solids moved primarily via diffusion.
AIR LEAKS Wind is an awesome tool - but even on a perfectly calm day air leaks can be responsible for a huge amount of moisture making it into places it shouldn't be. When there is a temperature difference between inside and out there is always going to be a pressure difference to drive air movement. Wood stoves (without their own dedicated and sealed air inlet) add tons of negative pressure. Any appliance that has an exhaust, but not an inlet, adds to negative pressure. The higher the pressure differential, the higher the velocity of the air leaking. Air exchange is good, and should always be there, but where the air leaks through matters. Air goes out, and air has to come in somewhere to make up for it.
Makeup air in hot wet climates, when parts of your structure are cooler than the incoming makeup air. Hot wet air pulled through cool walls means added moisture in the walls.
In cold climates - where does your warm wet air go? Where does IT leak to? Warm wet air leaks into cold attic spaces can put all the moisture leaving a structure into one place.
In both circumstances, air leaks lead to materials with a locally high moisture content because of the thermal pushing that takes place along the way, but this is a slow process and only removes a tiny amount of the moisture that's in the air. In both circumstances, if the dew point is reached, condensation can strip much of the moisture from the moving air, all in one spot, in a short amount of time.
CAPILLARY ACTION is when a liquid is pulled through a tiny space via cohesion. Capillary rise can pull water higher than any structure any of us will ever build, it just needs enough time. Saturated soil outside can become a concern for water intrusion into a wall via capillary rise. A large air leak that causes a condensation point can saturate a portion of a structure in a surprisingly short amount of time, because capillary action will pull liquid water through drywall/plaster/woods and disperse it as fast as it can condense. A spill on a floor and get pulled up an adjoining wall where a few seconds of water exposure takes weeks of drying time.
All mold problems are moisture problems, but not all moisture problems are mold problems.
For every moisture problem, there are probably several options for solutions.
Solutions can sometimes be complicated. Old homes that aren't insulated generally weren't built with permanently vented attics/roofs. insulating them sometimes slows moisture and heat down while also making the attic a condensation zone. Insulating an old house without ventilating the attic is a common good idea that can lead to moisture accumulation.
Solutions can sometimes be simple, like adding a passive vent from a damp crawlspace or basement to allow wet air to leave without having to pass through living spaces.
Many hard to beat mold problems are the result of one or more of these invisible sources acting over a long period of time. These kind of mold infestations can particularly hard to beat - because the moisture that causes them isn't concentrated like a mold event after a water spill. Mold from these invisible sources is often an indication that much of the materials that make up a room or structure have accumulated enough moisture to be bio available for molds as well as some insects/bacteria. Attempting to solve a mold problem with passive ventilation, once structural accumulation of moisture has occurred, can be nearly impossible. It looks like it's working but as soon as ventilation drops or stops, mold is a few hours or days away as the residual moisture makes it's way out. It can take years to re cure a structure back to a healthy moisture content.
