"And yes, houses have "breathed" naturally for millennia.
Now, how many millennia-old homes do you have examples of?

Are you going to be building in concrete and stone? Or using something with a wood component?"


There are 1000 year old wooden stave church buildings in Norway that "breathe" just fine, as well as wooden buildings in China of similar vintage.

https://www.asiaculturaltravel.co.uk/the-yingxian-wooden-tower/

https://www.fjordnorway.com/things-to-do/culture-and-heritage/stave-churches

But I'm not sure what the point is.

Buildings don't need to "breathe" air, but people do.

Air leaks in building assemblies are a primary cause of moisture problems in colder climates, or in air conditioned buildings in warmer climates. The higher the insulation values, the greater the risk, since there is less parasitic heat available to help dry things out. Making the house reasonably air tight mitigates against these risks even if it's not changing the energy use picture very much, and may require active ventilation to optimize the health and well being of occupants who DO need to breathe.

"There are 1000 year old wooden stave church buildings in Norway that "breathe" just fine, as well as wooden buildings in China of similar vintage. "

Yep. And we don't actually build or live in buildings like this anymore. Lots of new-growth wood, sheetrock, etc. Stuff that's notoriously sensitive to moisture issues.
I mean, we ARE talking about a home here. Right? Not a church. Not a shack, not a barn. Right?

So you throw in a modern HVAC system and modern building materials and you start developing problems with buildings that open.

And YES, you can do the Rube Goldberg whack-a-mole and try to MITIGATE each of the problems brought about by the combination of obsolete building methods and modern building materials.
And if you get it wrong, you wind up with a building that either rots out from under you, makes you sick, or both.

I'm not saying you have to be down at 0.5 ACH


"Not sure what the point is."

Then why did you go on to make my point for me?

"Air leaks in building assemblies are a primary cause of moisture problems in colder problems or in air conditioned buildings in warmer climates. The higher the insulation values, the greater the risk."

You then talk about reasonably airtight.

7ACH isn't "reasonably airtight". And in the end, you'll wind up spending far more in terms of simple climate control than you ever would with a tighter home and mechanical ventilation. While at the same time, opening yourself up to moisture and air quality issues.

is there anything out there showing the accuracy of beopt compared to IRL? Im using it wrong or something but the results seem suspect.

examples. take a south facing house where all the windows face south and are shaded and turn it west, and annual use increases only 100kwh???????? this is with no window coverings either

change roof or walls from light colored with low absorptivity to a dark color with high absorptivity and the annual consumption barely changes.

I highly recommend you hire an experienced energy modeler/HVAC designer to run this for you. You may well be doing something wrong.

But even if your numbers are correct:
If you like a home that breathes like they have for millenia, buy an existing home.
Remember the mold problems that started in the 80's and continued into the 90s? That is what happens when you do it wrong. Those homes still "breathe" by today's standards. Want to live in one of those?
If you want your home to breathe, live in a cotton tent. If you want your home to be comfortable and healthy, build tight and ventilate right. The latter approach will naturally maintain humidity levels that promote comfort and health. The home will far outlast a leaky home, and have fewer maintenance requirements. But maybe the only thing that matters is your electric bill. If that truly is the most important thing, you are on the wrong forum.

BEopt is really more for just getting a feel for what affects energy efficiency. BEopt is an overly simplified energy efficiency modeling tool and I can’t imagine that anyone would actually use it design a building...and certainly not a passive solar cooled/heated building. If that is your objective, perhaps give our suite of passive solar design calculators a try and please be sure to carefully read the associated instructions before using them:

Borst Passive Solar Altitude Angle Calculator

Borst Passive Solar Roof Overhang Design Calculator

Borst Passive Solar Fenestration Exposure Calculator

Borst Passive Solar Heat Gain Calculator

Borst Passive Solar Thermal Mass Performance Calculator

These calculators are really engineering software tools and they do require some thought before using them. However, the associated calculator instructions are very detailed and many folks have successfully used them. And I would say that if you hire someone to do this for you, they should be an engineer or at least have that aptitude.