Internal moisture and ventilation systems

[Sally Blackwell]

As the weather gets (marginally) colder are you fielding more enquirers about dampness and ventilation systems. Lois Easton has just posted a fantastic blog on the topic. Take a look and add to the conversation.

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[Phil Squire]

Hmmm, I think this issue identifies some of the disconnect between the science and homeowners perception of the benefits. Most people I talk to who’ve had a positive pressure system rave about it. And some claim that it does indeed warm their home up on clear days during winter, spring and autumn.

I know that my roof space is very warm in winter as is my front north facing room. And if I transfer the heat from these spaces to the colder south rooms then they warm up. As a homeowner that makes me happy and seems to make sense.

Also external air is always (mostly always) at a lower absolute humidity than internal air so introducing it through positive pressure will reduce the relative humidity of the internal air usually to a point where the surface temp of windows is now above the dew point of the “drier” air. How many times have you seen dew on the outside of windows?

I guess I’m seeing that the issue is complex and homeowners perceptions are based on what they see and what they are told by their friends and salespeople.

If anyone can do the calculations to work out the cost-benefits for a positive pressure system I’d be much obliged. I imagine a programme like Accurate that also modeled humidity would be the best bet. If you set the parameters to maintain a certain humidity as well as internal temperature and were able to adjust for various glazings, insulations levels, etc, we might be able to determine the costs to achieve the perfect indoor environment.

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[Vicki Cowan]

Hi Phil

I really recommend you read the research that’s been done by Otago University and Beacon on these systems.

External air doesn’t almost always have a low absolute humidity than internal air .  You’ll find Lisa French’s (now Burroughs) research on this here:  http://beaconpathway.co.nz/further-research/article/testing_ventilation_systems

Essentially because dew forms on the ground overnight – and then evaporates during the day, there’s a massive increase in moisture in external air in the morning – and this air then moves into the roof space.  As a result air the in the roof space during the day is often more humid than that in the house.  And of course it’s much colder in the roof space at night.

What Lisa’s research found was that in many cases positive pressure systems were pushing damp air (sometimes warmer) into the house during the day, and cold air during the evening/night time.  So the systems aren’t drying out the house.  It’s just the speed of the air movement that stops the condensation settling on the windows.  And I’ve been to houses where even this hasn’t been enough.

So there aren’t humidity benefits from positive pressure systems.  There might be condensation reductions because of airflow, and even then not in all cases.

I know Massey University did some research on the systems as well, and did find some benefits in relation to contaminants in air quality.  But they were looking at houses where, for example, the households heated with unflued gas heaters.  Surely I say, just removing the unflued gas heater and installing a decent heater would deliver less air quality contaminants (nitrogen oxides in particular), less moisture (unflued gas heaters deliver about a litre of water per hour into a house) AND a warmer house – and you could put in a reasonable heating system for the price of a positive pressure ventilation system.

 

 

 

 

 

 

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