Reply To: Internal moisture and ventilation systems

[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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