Allison Bailes, a building scientist, writes about home energy efficiency, HVAC, insulation, and building science.
Commenters recommend it as a source grounded in real science on HVAC and insulation, including the diminishing returns of added R-value.
> Also just a guess: is the story with heating that your heat pump is sized based on your max heating load, and so losing up to 25% of its capacity on the coldest days means it would never be able to catch up?
The unit I have has a HSPF2 rating of 10.5. It's very efficient at the transfer of the heat available. Heat pumps can extract more energy than the consume. New heat pumps can operate over 300% efficiency. The best furnaces are generally in the mid-90 percentages.
I have a dual-fuel system and can set where the heat pump locks out. The system is rated to a COP of 1.75 at 5F. Meaning it's still over 100% efficient at that temperature. The unit will operate down to -22F but would be at, likely, less than 50% capacity of output. To save on wear and tear of the unit, however I have my setpoint lockout at 15.
It would have to be significantly below 0 for days on end to have a problem. But even then the furnace would be providing primary heat. There are people in my area that heat their entire home with ductless heat pumps [0]. My unit is rated at 48k BTU. The furnace at 100k BTU. I only really "need" the furnace for emergency. But we do have sub-zero highs rather often and during those days the heat pump would be running continuously.
I also have an 80 gallon hybrid heat pump water heater which has saved us thousands over the last few years. 98% of the year it only uses the heat pump.
[0] https://www.energyvanguard.com/blog/can-a-heat-pump-work-in-minnesota/
Not in a way meaningful to assessing infectious risk, no.
I consider outdoor air to be unshared, except in cases of large dense crowds (such as say outdoor festivals or sporting events).
I consider risky shared air to be indoor air with one or more other individuals that are not known to be taking infection-prevention precautions.
One can measure CO₂ as a proxy to rebreathed air fraction.
For example, a CO₂ reading of 2300ppm (common in a small or medium room with a few others, or larger rooms with a crowd or conference room, or in a car) means 5% of your air is rebreathed (5% of your intake is output from another person's lungs).
A way to think about this is we take ~20 breaths a minute on average. So in that scenario, it would be equivalent to one breath every minute coming directly from someone else's lungs. If they happen to be contagious with an airborne contagion (such as Covid, or influenza, or RSV), there's a high likelihood that you will catch it if you're spending more than a short time in that environment.
There are nuances, such as maybe the air is being scrubbed (eg by a HEPA filter) which won't affect the CO₂ levels but will drastically lower the infectious risk of that environment.
More reading: https://www.energyvanguard.com/blog/what-a-carbon-dioxide-monitor-tells-you/
> No, this is Australia. Houses are more like tents than actual buildings.
Pre-AC being ubiquitous (or being invented), the way for people to cool off in summer was to have airflow. Insulation was a more of a cold climate thing, as keeping the heat in / cold out was more intuitive when burning wood or coal: thermal boundaries were more 'needed' when there was snow outside.
Having thermal boundaries in your structure didn't make sense when it was as hot inside (due to lack of AC) as it was outside, and it has only been in recent years that people clued into that fact. But unfortunately there's a large stock of housing around that was built pre-clue in.
Even now there are people who insist that houses need to "breathe", i.e., have holes / be drafty. They do not:
* https://www.energyvanguard.com/blog/Myth-A-House-Needs-to-Breathe
* https://www.greenbuildingadvisor.com/article/most-houses-that-breathe-arent-very-comfortable
* https://www.greenbuildingadvisor.com/article/buildings-dont-need-to-breathe
Buildings need to be as air tight as possible, and be ventilated through mechanical means so that stale air is vented out and fresh air is brought in after being filtered and tempered (via an ERV).
It depends on your goals. One saying is that "a good window is still a poor wall". A quality double glazed window still has about 10x the heatloss of a well built wall. Here's a summary with a link to more: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/55519/A-Good-Window-Is-Still-a-Poor-Wall
Shades can help with privacy, but what if you want the privacy at the same time you want to be capturing heat? And if requires active control --- well, that's by definition not passive. Interestingly, the new movement in high efficiency houses is frequently for low solar gain windows: well insulated windows with special coatings to admit less heat while still admitting light.
From another standpoint, gigantic expanses of windows are rarely the best way to build a pleasant room. As well as being a rare crossover between Computer Science and Building Architecture, Christopher Alexander's "A Pattern Language" has lots to say on this. Some excerpts here: http://kk.org/cooltools/a-pattern-language/
Which is to say that opaque heat-gathering walls serve a purpose that is separate than that served by windows. Both have their place, and they don't tend to replace each other directly.
> It was set to 69 and held steady at 67 running for 8.5 hours. It wasn't until the temperature outside broke 36 that the inside temp slowly started to climb.
This may be normal. Standard practice, per ASHRAE, is to design for 99% of days, and for the remaining 1% (~4 days per year) the system may have to run 24/7 (either for heating or cooling):
* https://www.energyvanguard.com/blog/we-are-the-99-design-temperatures-oversized-hvac-systems/
* https://www.ba-inc.com/hvac-systems-what-is-a-design-day/
This allows for equipment that is "right-sized", as issues can arise (including premature death of equipment from short-cycling) if they are oversized:
* https://www.energyvanguard.com/blog/why-an-oversized-air-conditioner-is-a-bad-idea/
* https://www.cooltoday.com/blog/3-problems-caused-by-an-oversized-air-conditioner
* https://bryantlincoln.com/understanding-the-risks-of-oversizing-your-hvac-system/
By designing for 99% of the time, your gear should be just enough to handle things.
There are multiple articles readily found about complications of using positive pressure:
https://www.medscape.com/viewarticle/581344_7
http://www.mdedge.com/ecardiologynews/dsm/5224/hospital-medicine/complications-mechanical-ventilation
https://www.ncbi.nlm.nih.gov/pubmed/2199002
To my mind, this is actually related:
https://www.energyvanguard.com/blog/54084/Why-I-m-Not-a-Fan-of-Positive-Pressure-Mechanical-Ventilation
One of the problems you run into with any lung related mechanical devices is the need for proper sterilization. From what I gather, this is much less of an issue with negative pressure ventilation.
People in iron lungs are people with fundamentally compromised immune systems. Exposing them to germs forced into their lungs by positive pressure while potentially damaging their lung tissues in the process strikes me as a great way to give them a deadly infection, sooner or later.