Air Quality & Humidity levels in a Passive House

Air Quality & Humidity levels in a Passive House

One of our main reasons to build a passive house was to live in a building that is really comfortable & healthy. Based on our experiences of living in 1920s, 1940s and 2000s built homes in Ireland, I can say with certainty that the its not something we've found in many buildings

One of the key measure of indoor air quality is humidity which is a measure of how much water is in the air. If humidity is too high or two low then there are definite health implications:

• Too high & mould growth is likely
• Too low, its starts to feel very dry, skin drys out quicker, musical instruments like pianos are damaged!

The consensus is that the optimal range for humidity is between 35 -55%, the figure above is from the PH training package.

Other pollutants in any building include:

• Bad odours – usually in bathrooms
• Formaldehyde & other volatile organics– often off new furniture, ;paints, flooring, varnishes, treated wood, plastics
• Ozone
• Radon – naturally occuring from the underlying rock / soil

Usually in homes, ventilation is supplied via trickles vents on windows , wall vents, bathroom fans, kitchen extractors...or a combination of the above combined with uncontrolled air leakages from the attic hatches, doors, windows, drains, downlighters. In our previous houses this gave a humidity usually in the range of 60-70%, but often as high as 90-100% depending on conditions outside.

 

Photos of the MHRV (during installation) & the filter cartridges

With an airtight building , the air passing though the building is controlled by a heat recovery unit, also called an MHRV , which strips heat out of the stale building air. There is no air cross over / recycling & the air coming in is filtered. These systems need to be commissioned correctly to deliver the correct amount of air to each specific room type. If the overall system over-ventilates then excess air is being drawn though the building resulting in low humidity and higher heating bills.

So...bottom line is what is our experience with the air quality in the house over the last year?

• Humidity levels are consistently between 47-53%, with very little variability; have verified this with a mobile humidity meter ( about €5 on ebay) & and the humidity sensory on the MHRV system, both of which tally
• The air filters on the MHRV need to vacuumed about once per month
• Reckon that is took about 4 months after moving in for the dust to settle from the build process, this was despite the house being well aired before moving in
• The dust in the house is all from internal sources : clothes, furniture. I've heard it said that there is no dust in a passive house & thats unfortunately not true.
• You can & do open windows in a passive house, during the summer I noticed this meant that the dust was a slightly darker colour than the dust during the winter when all the air was filtered
• The air is very consistent, crisp, there are no drafts. If there is a big group, like during the international passive house open day, then I tend to turn the system up a knotch
• With the constant movement of air through bathroom, utility & kitchen setting up a constant flow of air, there is no need to extract fans – the MHRV does an excellent job
  
  
  All in all, we've found the air quality really high, ventilation levels high enough to shift smells, the MHRV does an excellent job. The air quality in a passive house is really unlike any building I've been in

Feedback from International Passive House Open Day

We had 45 guests for the open day in November ,ranging from architects to home owners looking to retrofit their own houses to tradesmen. The visit consisted of a presentation on the overall PH design principles and photos of the build process for this specific house. There was specific interest in the heat recovery system ( which somebody described as looking like something from Star Trek), the windows which are a triple glazed aluclad & the differences between block versus timbre frame construction. 

We presented a 'hands on' number of items such as a sample of the wall buildup, airtightness tapes & membranes, heat recovery filters & live thermal imaging camera images of the building fabric. The was followed by a detailed tour of each type of room in the house, the plant room & the heat recovery installation.  Observations from the visitors included the quality of the air, good natural lighting levels & a very high degree of comfort

If there was a theme to my presentation / talk through the build process, it was that great products , from whatever source, need to be correctly designed, installed & commissioned in order that they work correctly for the home owner over the lifespan of a building. Furthermore , the Passive house building standard is a challenging design & construction standard for all involved but results in a very special living environment for the homeowner.

Questions raised which I'm in the process of answering:

•  Explain the certification process, what 7 who does it involve
• Climate files for PHPP
• Is heat recovery noisey? Can i save money by turning it off? How much does is cost to run?
• What is this supply air heating
• How much to MHRV filters cost & how often replaced?
• What are typcial construction details & why are they important?
• What is the ideal humidity level in a building?
• When is solar shading needed & not?
• Payback period for passive
•  Does it work in practice -predicted v actual data
• How do you airtight around windows
• Heat recovery as bathroom extract, can it reduce humidity.
• Do sliding doors work with passivhaus
• Opening windows in a passivhaus
• Costs.  How much more is a Passivhaus than conventional.
• What are the costs for the certification process alone
• selecting heating, reneables and ventilation systems and choosing between wall build types is confusing and the salesmen even contradict themselves how did you make decisions.
• How does timberframe breath or diffuse moisture is it to a cavity/rainscreen or inside through an intelligent membrane?
• Can you have an open fire or extractor fans in a passivhaus
• Is a passivhaus and a zero energy house the same.
• When contractors see passivhaus in for pricing do they think Ka-ching,
• Why did you decide to pursue such a low energy building?
• Is there such a thing as near passiv.
• Can you use cavity wall for passivhaus.