Tuesday, January 15, 2013

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


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  2. No problem Negi - happy to help in spreading the word on the quality of passive house construction

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