Does air tightness trump R value to some degree?

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Got some time to invest and improve your thoughts .

Have at it and enjoy … Roy

A perfectly insulated completely air tight house will not need any heat in any climate, only cooling, as the occupants would be providing more heat than required to maintain the house at 70 F. The occupants would also be dead, because they would have used up all the oxygen inside their air tight house, but they would be comfortably dead, as a theoretically perfectly air tight and insulated house will maintain its temperature forever.

So, in order to keep everybody alive, we need to exchange air with the outside, a minimum 0.3 air changes per hour is required by code. For a 1000 square foot house, no basement and 8 foot ceiling, that would be 8000 cu.F * 0.3 cubic feet per hour, which will require about 43 BTU per degree F. If it was 0 F and you wanted 70 F inside, about 3000 BTU.

Pursuing ‘perfect’ air tightness after the fact is probably in many cases a waste of money, a house needs ventilation, whether it is the result of air leaking in through cracks and gaps or supplied by mechanical means, it will still mean bringing in unheated unconditioned air and expelling conditioned air to the exterior.

Older houses were not designed to be air tight, obsessively upgrading air tightness beyond the original design will quickly butt up against diminishing returns.

Having said that, there is benefit in basic fixes, such as making sure that everything is working as originally designed, window frames are sealed, there are at least two panes of glass between outside and inside, whether double glazed, or double windows, doors are weatherstripped and properly hung, storm doors are installed, etc… Mom and Dad, even Grandma and Grandpa liked to be warm and save money on heating same as us.

Approximately 15 cubic feet per minute for each individual and that is not enough to reach a 3rd of an air change. Each human gives off about 240 Btu’s so the more people in the house the warmer it is.:mrgreen: Add all the other stuff in electronics and the home is nice and toasty. All houses loose heat at a certain rate and must replace that heat with something. The slower this happens the better the design.

I think the OP in this link is off in his initial assumptions. The inefficient windows he is attributing air leakage to may actually be causing discomfort due to heat loss through the pane. Sit below a single pane window which is air tight and you will still feel a draft of cold air dropping on you as the air next to the windowpane has the heat drawn from it and it sinks to the floor in a rush. I don’t think one trumps the other, certainly not in all climates.

I disagree with that statement.

There are many more things to consider other than insulation and air tightness such as the structure’s thermal inertia (the ability of a material/wall to store heat during the day/solar loading and give it back at night).
The house strategic placement of windows/doors/walls to maximise the solar loading during winter and yet not be too hot during the summer.

The “theoretically perfectly air tight and insulated house” cannot maintain its same temperature forever as energy is always moving through material/walls from hot to cold to cold until there is thermal equilibrium.

I’m guessing he is speaking of what does not exist: a home where the insulation prevents all heat loss. That’s the reason he stated it would maintain its temp forever. It’s an interesting theory, but one that will never exist in our universe so it has little benefit.

Yes it does produce some convection, but don’t disregard radiational losses that make you feel cooler just being “in sight” of an unshielded window glazing…

Good point. Heat loss is a multi-pronged attack on your utility bill and comfort. :slight_smile: