Roman philosopher and poet Lucretius (96-55 BC) is attributed to first penning “One man’s meat is another man’s poison”. While this has over the years morphed to mean that what is right for one, is not necessarily right for another, and is used to indicate a difference of opinion, when it comes to keeping your home (and your health) in tip-top condition, it is a phrase worth of repetition.
Most of us live in timber framed homes. This means that the structure of the home is made of wood. Wood is hygroscopic, which means it readily absorbs and releases moisture from and to the atmosphere. This means that the Moisture Content (MC) of wood will constantly fluctuate depending on the Relative Humidity (RH) of the surrounding air.
For example, as the Relative Humidity increases the Moisture Content of the wood will increase. This will cause the wood to expand. Conversely as the Relative Humidity decreases the Moisture Content of the wood will also decrease and the wood will shrink. The relationship between atmospheric relative humidity and wood moisture content is referred to as the Equilibrium Moisture Content (EMC). The Equilibrium Moisture Content can be accurately predicted from measurements taken of either the Wood Moisture content or the atmospheric Relative Humidity.
So what’s this got to do with Meat and Poison?
While there is an optimum level for relative humidity in a home to ensure the lumber that supports it is kept in great condition, there is also an optimum level of relative humidity that prevent mould growth.
Mould thrives when given three ingredients, food, warmth and moisture. The most important is the moisture. Remove all moisture and mould cannot grow. Notice I didn’t say mould couldn’t survive. If there is insufficient moisture mould does not necessarily die, but will probably become merely dormant. Active growth can start again, sometimes years later, when sufficient moisture returns. Mould really loves homes with high humidity. Wood however doesn’t. As the relative humidity increases in a home, so the moisture content of the lumber can increase as well. When wood gets to levels of 35-50% moisture content, then wood rotting fungi can start to grow.
Conversely, as the relative humidity in the home reduces the likelihood for mould growth reduces. If you move the relative humidity in your home to levels lower than 30% mould production will be seriously hampered. At these lower humidity levels, the wood will release it’s moisture to the atmosphere and the likelihood of it rotting will be reduced.
So what’s good for the mould, is not good for the wood, and what’s good for the wood is not good for the mould, hence the “meat and poison” reference.
Cracking the problem of humidity
There is another consideration in the equation of humidity-mould-wood that needs to be addressed. Wood is strongest when it’s moisture content is around 10-15%, either side of that and the strength starts to diminish, but more importantly, as from earlier on, as wood releases it’s moisture, it shrinks and as it absorbs moisture it expands. But the strength of wood is also affected by it’s moisture content. Drywall and plaster do not undergo these shrink-expand characteristics but they suffer from them. Also the way in which wood expands and shrinks is different depending upon the direction and orientation of the grain. (The scientific term for this type of expansion is anisotropic).
Longitudinal shrinkage (that is shrinkage along the grain) is so small (typically around 0.1% to 0.2%) it is normally inconsequential. For example: of an eight foot length of stud-work the amount of expansion and contraction would range from 1.5mm to 3.5mm. Tangential shrinkage (shrinkage across the tangential plane of the lumber) and radial shrinkage (shrinkage across the radial plane of the lumber) can amount to anything between 2% to 15% (from 1mm to 7mm!)
This means that while tiny cracks of less than 1mm may appear at the top and bottom of drywall on an eight foot high drywall assembly, vertical cracks of a larger proportion and higher frequency are more common when levels of humidity in the home are allowed to fluctuate wildly.
Nearly all walls in a timber framed property are made up the same way. That is long lengths of 2″ lumber either 4″ or 6″ wide cut and placed to make a frame. These “bits” of lumber have different names, you might hear the terms studs, noggins, sill plate, top plate, header, sill, trimmer, etc., but essentially the walls are exactly what I said at the beginning of this paragraph.
If you imagine the amount of shrinkage that can go on behind the scenes is a wall frame, it is very quickly apparent why cracks appear in drywall so readily.
So what is best?
Obviously, having seen that there is a strong correlation between moisture and mould growth, one would want to keep relative humidity levels at their lowest. But then recognising that lumber shrinkage may leave you with cracks all over you drywall might not encourage you to reduce the moisture in your home too much. There is a balance you can strive for however.
Most mould remediators will tell you that keeping your humidity levels in the home to below 50% is sensible. This goes for the preservation of wood and the ability to help reduce mould growth. But lowering the humidity to much can have a detrimental effect on your comfort.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), thermal environmental conditions for human occupancy has defined a comfort zone for summer and winter season. The definition of comfort zone is rather complex, but it can be more easily explained by the graph which shows an estimation to a comfort zone which depends only on relative temperature and humidity.
Here we see that depending upon the season, relative humidity can fluctuate between 23% at the lower end and nearly 80% at the upper end. At 23% the lumber in your home starts to rapidly release water, dry out and shrink, and at 80% not only does the reverse happen, but the conditions are ripe for mould growth. A fluctuation of 57% of relative humidity, although being comfortable to humans, would obviously have a huge impact of the home itself, both from a cosmetic and a structural point of view, let alone a health perspective.
Limiting the fluctuation of the relative humidity is obviously the way to go, and picking something close to the centre of the comfort zone would appear to be the best case for limiting mould, reducing the expand-shrink problems of wood and at the same time provide the most comfortable environment to live in. Unfortunately the old methods of humidification in the winter, and humidification in the summer using stand-alone units would not be the best option for whole house conditioning so investment in a whole house humidification system may be the best choice for protecting your home, protecting your health and allowing a comfortable living accommodation all at the same time.
While whole house humidification is a fairly simple and inexpensive install, whole home dehumidification is a bit more complex, and consequently more costly. Canadian seasonal humidity fluctuation can range from 50% to 94%, so installation of both is probably the best option for a combined solution.
Too moist or too dry?
As part of our home inspection services we carry equipment that allow us to measure both moisture content and relative humidity in the home. If you are concerned about the humidity in your home ask our inspector if they can run a quick check for you. It’s outside of the normal standard of practice for home inspections, but if you have a concern, and we can at least help you investigate it, we will be more than happy to do so. If it is done as part of a home inspection there is normally only a small charge (to cover the extra time required). If you want us to come out specifically and solely to give you humidity reading it would require a time and mileage charge.