Moisture is a hot topic in winter: it affects new and old housing stock when not managed correctly, resulting in higher energy bills, mould and bacteria growth.
Mould can degrade a structure rapidly and can have severe health effects once it establishes itself and contaminates the air you breathe. It needs a water source to grow and that generally comes through external leaks or internal humidity levels becoming saturated and condensing as the air becomes cooler. With indoor temperatures generally sitting around 20 degrees it’s a perfect environment for Mould and bacteria to establish.
External leaks in the cladding system and moisture barrier can create the water source needed for mould/fungi growth as the water makes its way into a building structure and cannot dry out.
Internal moisture generally comes from vapour build up as it is constantly being added to the internal environment through living conditions.
Human bodies release vapour through breathing and sweating, add that with boiling the jug, cooking, showering, washing and drying clothes etc and we are talking litres, it will at some point reach a saturation point if not managed. Cool air cannot hold the same amount of vapour (measured in Relative Humidity RH) as warm air so as the temperature cools it will condensate on cold surfaces like glazing or in a wall assembly on products with high thermal conductivity.
With the upgrade to the H1 (NZ insulation compliance requirements) in 2023 new homes subsequently became more airtight and the more airtight a home is the better, but the physics of the internal vapour change again and require additional management. This is where the H1 regulations have fallen short, like they did in the 1978 H1 upgrade, which altered how moisture was transported and could dry out with the methodology at the time, and in some cases contributed to leaky homes. Increasing insulation values and using better wrap systems are a great step forward in building better and more efficient homes that will last longer, but have created the need for mechanical ventilation to manage the internal vapour. Building and energy costs will continue to rise and a home that performs better is healthier, more comfortable, more efficient and lasts longer. Looking at the 20+ year cost of a home its a much more sustainable option to invest in building better.
You may have heard from people in the 50+ age group or be in that age group that grew up in an old cold home – that was fine. In older homes up to 1978, insulation wasn’t required and that old housing stock had building envelopes with materials like timber weatherboards that breathed quite freely and any leaks or internal vapour had enough airflow to be managed. Although very drafty, cold and inefficient with energy costs they were not creating perfect conditions for fungal/ mould and bacteria growth. As insulation was introduced with upgraded building code requirements the physics of the vapour transport changed. Accompanied with building materials like fibre cement and plaster (with high thermal conductivity values that bought the cold/ dew points inside the building envelope) became popular. Substandard detailing and poor installation added to the fire and problems appeared. Both leaks in external envelopes and dew points in wall assemblies or ceiling spaces that couldn’t dry out became common and then mould growth set in.
But, there are lots of easy and helpful things you can do to improve the vapour/moisture management in your existing home.
- Have extraction fans in all wet areas and keep the doors closed when showering or doing the laundry
- Make sure the kitchen extraction fan is on even when boiling the jug if possible
- Don’t use portable un-flued gas heaters as they constantly release vapour
- Wipe down cold surfaces frequently like windows or any surface that is prone to moisture build up.
- Cover up fish tanks, toilet lids or any water source that can evaporate.
- Dry clothes outdoors
- Clean surface mould with a clean cloth and detergent then vacuum the surface and dispose of the cloth after wiping.
- Choose building products with low thermal conductivity values
- Open windows when practical.
In summer with a breeze opening windows is a great way to ventilate and cool a home naturally...
but it’s not always the ideal thing to do. The ability to control temperature and air quality is reduced. Depending on location external air might carry pollutants, allergens, or insects. And if there’s no wind you don’t always get the air exchanges you need depending on the occupants and size of the home to keep the relative humidity at healthy levels. In winter all the energy used in heating can be lost and it becomes an expensive exercise. Wet area extraction fans also help but as they extract air out of the building, air is drawn in through walls, windows, ceilings etc to take its place. Especially in older homes that air can be contaminating as it is drawn through over dirty insulation, vermin, potential mould/rot etc.
The best way to consistently control internal vapour is with a good ventilation system
and Mechanical Heat Recovery Ventilation systems (MHRV) make the most (financial and practical) sense. There’s a lot of different options but a balanced system will give the best results and is a fundamental principle of the passive house standard for a reason. They extract the same amount of air over a heat recovery system that heats the filtered fresh incoming supply air to within 70-98% of the temperature of the extracted air. A well designed system will deal with all that internal vapour, supply fresh filtered air and keep the energy required for heating and cooling to a minimum as it’s not being completely lost. A high performance home will take care of the wet area extraction as well. With an older home if you have moisture issues it won’t fix what is already happening in a structure if it has started but you won’t be feeding the water source needed for mould to grow rampant internally and the air quality will improve as it is constantly being exchanged. In summer if there are a lot of bugs around or no breeze there are summer bypass modes which turn the heat recovery part off and bring the night air in to help with cooling.
Not all MHRVs are created equal though and there is a lot to consider to get the best results. A professional design will calculate supply and extract air flow rates and determine the size of a system. Good systems are calculated on 1ACH (Air Change an hour) and others work on .5ACH or less. Ducting should be in the building envelope and/or insulated so the energy isn’t lost in a roof space and exposed to outdoor temperatures causing condensation. There are also different levels of filters. With older homes Decentralised MHRV systems can be a cost effective option to consider – they are generally through wall units that work in pairs (as one unit extracts the other is supplied for a period of time and then they switch over). They will not give the same results as a centralised system but can still improve the indoor air quality significantly for a lower cost.
External moisture from wind-driven rain/leaks can be addressed with a high-performance vapour permeable weather & airtight building wrap as a secondary line of defence. Not a system with substandard tapes that peel off with temperature change and staple penetrations all through them that water can enter through. Added with a good cross-ventilated cavity system, and smart envelope detailing. It’s a small upfront cost to upgrade to a premium weather tight barrier system that is fully taped and sealed but the long-term payoff is huge. As cladding naturally degrades over time, water will eventually find its way in – so having that reliable secondary line of defence is important.
All homes have a lifespan and the best thing to do will depend on many specific individual factors like what value the building materials still have, how the home was built, the current health of the home and your budget. Building new you get the chance to get it right from the start but renovating the right way can also make huge improvements and it might make more sense economically as the most sustainable house is the house that’s already built. Picking the right team from the architect to the builder and subcontractors that understand the science behind how things work and implementing the correct installation is crucial. By energy modeling a house in the design phase through PHPP or WUFI software, it is possible to calculate the moisture transport and energy consumption so there’s no guesswork. Product selection payback periods can be calculated and cold spots that create moisture issues/ dew points can be eliminated creating healthy homes that will last.
Whether you’re looking to renovate your existing home or build new, one of the most important things to think about is building a team that will work collaboratively on a science-based approach to get you, the client, the best outcome possible.
