Mechanical Ventilation in a Tight, Permeable House
Ok, I am going to thoroughly expose my lack of knowledge with this question (no claims to any expertise on my part), but I’m hoping that some of the experts here can comment on something I am thinking about.
I’m contemplating theoretical house designs, and am currently thinking about a home that is fairly air-tight – say 1.5 or 2 ACH50, with thick (14″) in which all materials have a perm rating of 8 or higher (though airtight and weather sealed), the only non direct-vented appliance would be a gas range, which is vented by a range hood when in use, and all materials used inside are low or zero-VOC, interior walls are covered in lime or earth plaster, and the floor plan is quite open. In case it’s relevant, let’s say the wall is approximately R-30.
My question is whether ventilation beyond the code minimum would be necessary in such a configuration? The thing that gets me thinking about this is the purpose behind ventilation:
1) Removal of Indoor Pollutants: Ventilation is advocated for the removal of VOCs, Carbon Dioxide, and other harmful gasses. With relatively permeable walls, would some of these vapors be able to work their way out of the house through the walls? I realize that perms are a rating specific to water vapor, but is it not likely that limited amounts of other gasses, such as carbon dioxide and VOCs would follow the same principles? This would relieve some of the pressure on the ventilation system to remove those elements.
2) Control of humidity: Between the ability of the walls to allow water vapor to escape, and the natural ability of the plaster on the interior to absorb and release moisture to maintain a consistent level, would humidity still require additional ventilation to keep under control?
3) Control and filtration of incoming air: This is an important feature of a whole house ventilation system, HRV, or ERV. Being able to pre-condition the air coming into the house to avoid heat loss and filter out any contaminants from outside is a big reason to mechanically ventilate. I wonder, though, even in a house that has a full ventilation system, if some amount of untreated air still makes it in through cracks, etc, and how that volume compares to a house that is ventilated only to meet the code minimums?
4) “Fresh Air” supply: A key point of ventilation systems is to bring in “Fresh Air”, though I’m unclear what makes air fresh – is there some additional characteristic of fresh air other than having had the above points applied to it? Are we talking about bringing in additional oxygen molecules? If so, I’m not certain whether the setup I am describing would provide adequate fresh air – though I’m not adverse to opening a window occasionally.
Anyway – I would appreciate some thoughts or considerations that I might not have brought up. Thanks!
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