Envelope
Although the term envelope is not correct (an envelope is where the letters goes in) and the term "thermal, moisture- and air control layer" is more appropriate, we will use the word envelope for the sake of being consistent with what most people call it.
HVAC systems and the envelope both enable us to be comfortable regardless of the environment. both need to function properly. Even the most perfect HVAC system would not be able to provide comfort if the envelope is faulty. for example, at ideal air temperature, humidity, and air velocity we still feel cold standing next to a cold wall radiating heat to the cold surface.
"Build tight, ventilate right" could summarize this page. Any uncontrolled infiltration or exfiltration of air will cause problems ranging from energy losses, condensation, and mold to discomfort of the occupants. The building should be tight be default and any air coming in will be controlled and energy from the exhaust stream recovered by means of energy recovery ventilators (ERV). There is much talk about better insulation and higher R-values. this is good, but in reality most energy gets lost due to movement of air. One can imagine heat traveling slowly through insulation, or jut with wind speed through the wall and one sees how a tight envelope could save energy.
Besides building tight, R-values need to be high. For normally conditioned spaces (meaning air conditioned and heated to 70°F in winter), the R-values should be:
- walls: R30
- roofs: R60, plus reflective roofing material. It is a myth that black roofs gain heat in winter. First, any insulation would prevent from that heat being used, second, the sun is at a lower angle and shines for some hours only in winter, third, the roof likely is snow covered and reflecting sunlight.
- foundations / basement perimeter: R20
- Slab on ground (including under basements): R15 It is a myth that the ground will insulate and "give the heat back". In reality the ground is a huge heat sink.
For less conditioned spaces (i.e. heated vehicle storage at 45°F in winter), R-values can be lower. The R-value should be the true value including any thermal bridges (i.e. studwalls with insulation in between have lower R-value than the insulation alone). At least some insulation layers should be covering thermal bridges completely. Insulation material should be filling any cavities. outlet boxes in perimeter walls, window and door frames, and where walls and roof meet are typical weak points in an envelope. any weak point not only causes energy losses, but also can cause damage to the building causing condensation, cracking etc.
Good building practices employ ICF (insulated concrete forms) for walls and SIPs (structural insulated panels) for roofs. Stick frame is less ideal by default and foamed insulation and blown-in insulation in addition to some foam board on the outside of the studs can be used to reach acceptable values. ICF still would be a better choice due to its inherit tighter construction and fewer thermal bridging, higher mass and structural stability.
Windows provide light for humans and bring in solar energy that can be useful in winter. Unfortunately many of the designs don't employ much engineering and add windows according to "the more the better". Even newer triple-pane windows have much less R-value (R4 is very good) than even bad walls. An energy simulation will provide more insight and these are just some rules of thumb:
- avoid windows on west side to prevent overheating in summer, avoid too many east-facing windows for the same reason
- avoid too many north facing windows. No matter what architects say - the sun does not shine from north.
- locate more windows on the south and provide shading devices blocking sun in summer.
- Provide individual shading devices to enable occupants to block glaring sun
- strategically locate windows high enough to let light penetrate the space as far as possible while avoiding glare
- where day light is present, use daylight harvesting lighting control (dimming photosensor etc.)
Due to usage of the building compromises need to be made, but in general these rules should be considered. Not doing so will dramatically increase the HVAC system size and energy consumption.
All materials should be long-lasting. Unfortunately rubber and asphalt roofs dominate the industry. Those products have to be replaced frequently, are made of harmful material and are made of oil. Replacement in the future will be expensive as oil becomes much more expensive. Metal roofs last many many decades. If applied correctly for over 100 years. Now that is sustainable!