Mark Siddall is a Passive House pioneer in the UK and his latest (collaborative) paper is of course worth reading. It reviews the indoor air quality (IAQ) in residential Passive House dwellings and it contains lots of good news. It confirms much of what drives us to design MVHR (ventilation) systems well in high-performance buildings.
Much of the immediate discussion online relates to just a few comments in the paper about recirculating kitchen hoods, rather than direct extract models, in buildings with MVHRs. When I dug into the details in the paper, I find very few studies that examined this. What we do have is data from Colorado from one (just one) Passive House and nine other homes with MVHR systems that had either no range hood or a recirculating range hood. The study found higher PM2.5 particle counts compared to a conventional home with a direct extract range hood. That’s a mash-up of variables and not enough data from which to draw firm conclusions like ‘we shouldn’t use recirculating extraction’.
I agree with the authors’ comments that it is possible that a recirculating range hood and MVHR extract may not ensure acceptable indoor air quality (IAQ) and further research is needed. This paper is not evidence that one is better than the other.
One more caution: one of the studies referenced did find high nitrogen oxides pollution likely from gas-fired cooktops even with use of an MVHR. They recommended caution regarding combining gas-fired cooktops with recirculating range hoods. In the high-performance projects Sustainable Engineering Ltd designs, we always recommend electric cooktops. If a client cannot be dissuaded from a gas-fired cooktop, we push for direct extraction in the kitchen.
When comparing the measured air flow rates to the UK Building Regulations Approved Document Part F Ventilation Rates, all the certified Passive House dwellings met the required rates but only 47% of other homes with MVHR met the requirements. Another study reviewed 80 homes with trickle vent and either continuous or intermittent extract fan strategies and these registered only 4% compliance with this ventilation requirement.
With regard to relative humidity, Passive House homes rarely exceeded 55% while “in contrast, a number of naturally ventilated homes had peak RH values greater than 60% for a substantial amount of time.” Extended periods of RH above 60% RH often creates conditions favourable to mould growth on surfaces: no good for humans or buildings.
CO2 levels were similarly better. In dwellings with MVHR (again, a mixture of both Passive House and other homes with MVHR systems) having consistently lower levels of CO2.
A review of the indoor air quality in residential Passive House dwellings
Gabriel Rojas, Martin Fletcher, David Johnston, Mark Siddall,
Energy and Buildings, Volume 306, 2024, 113883,ISSN 0378-7788
https://doi.org/10.1016/j.enbuild.2023.113883
Abstract: The Passive House (PH) Standard is a voluntary building energy performance standard focused upon reducing space heating demand to a very low level and therefore considered a viable climate change mitigation technology. Besides comfort and energy requirements, the PH standard also defines criteria with respect to ventilation. However, the question remains, how well do PH dwellings perform when they are occupied? Does the PH approach provide good indoor air quality (IAQ) for its occupants and how does IAQ compare to non-PH homes, in particular, naturally ventilated homes? Additionally, can PH certification improve the quality of installed ventilation systems? This paper summarizes indoor air quality relevant aspects of the PH standard and presents results from measurements examining in-use IAQ in more than 600 PH or PH-like, newly built or retrofitted dwellings. The results reveal that pollutant and carbon dioxide concentration are generally lower compared to naturally ventilated homes, presumably due to the requirement to install a balanced Mechanical Ventilation with Heat Recovery (MVHR) system. Results also suggest that the quality assurance measures of PH certification are capable of improving ventilation and IAQ performance. However, the lack of cooking fume capture requirements in the PH standard, in combination with efforts to avoid energy losses associated with a possible extraction kitchen hood, may lead to elevated particulate matter concentration in PHs. Future research on cooking induced IAQ impairment is encouraged to assess the effectiveness of recently published PH-specific recommendations. Future efforts in empirical IAQ research should also address the lack of high quality IAQ measurement data and the standardisation of IAQ assessment methods and protocols.
Keywords: Passive House; Passivhaus; Indoor air quality; Measurement; Ventilation; Airtightness; Quality assurance
Comments 1
I have in the past critiqued the Colorado study i.a. with a letter to the authors, and they partially agreed with my analysis of the shortcomings. In addition to the sample size of one where an extracting range hood showed good results, relevant context is completely missing. It is impossible to draw a conclusion from this research, in my view. As for NOx: I did not measure concerning levels even in houses with no mechanical ventilation. I do agree that induction cooktops are preferable for a number of reasons, but I cannot base this on IAQ considerations. There is quite a variety of forms, speeds and configurations of rangehoods on the market. Research on rangehood configuration and capture effectiveness for gaseous pollutants and particles is needed, for recirculating as well as extracting options to be able to make a recommendation.