I've always been really curious to know how ventilation is supposed to work.
In an RV if you're cooking or venting in the shower the directions pretty clearly say something to the effect of "open a window"—those things are sealed pretty tight and the air you're pushing out needs to be replaced with something...
So when you are taking a shower and use a ceiling vent (louvered, I assume) where does the replacement air come from if you're sealing up the house? Otherwise you'd just be creating a pressure difference and the fan wouldn't be moving a lot of air. What am I missing?
Air-tightness is a spectrum - a reasonably tight house still leaks enough to make up most ventilation needs introduced by an exhaust only ventilation system.
Very tight homes may need air inlets (literally fancy holes in the wall somewhere), or a balanced ventilator system. Appliances that can draft are also an issue. Some are generally discouraged, like drafting water heaters, and some will require their own air inlet, like wood burning fireplaces.
The issue with a whole lot of "new" and "energy saving" houses is often connected to poor ventilation.
Set aside for one moment the vapour from the shower, and from either the dryer or coming from the natular drying of washed clothes, there is the human generated vapour and that created by cooking.
In modern houses (without an efficient ventilation system) the airtightness causes condensation/moisture (usually in corners of the rooms or wherever there is a thermal bridge) which usually evolves in this (or that) form of mould or mildew.
Some (not all) heat insulation materials make the walls (besides airtight) also watertight, which makes the issue worse.
The idea of creating openings to allow the air in, which may be good in - say - a kitchen where the air also exits normally via the extractor hood or chimney (at the cost of some energy/heating/cooling dispersion) does not work in other rooms, where the openings most of the time either do not guarantee the circulation of air or behave as "heat dispersion" holes, negating all the savings that the airtightedness gave you.
Not only, people breath and emits, beside vapour, C02.
I know that it sounds obvious, but if you don't let (fresh) air in, before or later you will find yourself in a low-oxygen environment.
Having "normal", not airtight, windows and doors makes continuous ventilation of the interior of the house a "natural" phenomenon (with a cost in energy).
If you make an "airtight" house, additionally "watertight" (as an example using polystirene or similar for heat insulation) you are essentially living in a unhealthy environment, unless you provide a suitable ventilation system, and a ventilation system without an air-to-air exchange (to re-use part of the heath or cold) will negate most of the saving effects of the added insulation/airtightedness.
So, as pointed out by PoachedSausage, heat recovery ventilation is an actual need in energy saving houses, but it is still rather uncommon.
It should be noted that pretty much any mold related repair is going to potentially blow away years of energy savings, so you really need to make sure your moisture control situation is in order.
My rented property has a master bedroom that is west facing & has a massive tree in in front, that wall never gets the sun so consequently it gets surface moisture, it's a pita every winter, this year I redecorated and am getting a heavy grade dehumidifier to see if that helps, also shifted the heater during decoration so it sits at the bottom of that wall.
In an RV if you're cooking or venting in the shower the directions pretty clearly say something to the effect of "open a window"—those things are sealed pretty tight and the air you're pushing out needs to be replaced with something...
So when you are taking a shower and use a ceiling vent (louvered, I assume) where does the replacement air come from if you're sealing up the house? Otherwise you'd just be creating a pressure difference and the fan wouldn't be moving a lot of air. What am I missing?