This question seems to confound teams considering deep energy retrofits of existing buildings. Since we exist to see buildings perform in operations as they were designed to perform, we have a strong opinion.
Renovating existing buildings has challenges and each building's challenges are unique. Fortunately for our community of practitioners, building owners are beginning to understand why the renovation of existing buildings must include a conversation about the building envelope. There was a time when the immediate response by owners and practitioners was, "the envelope never pays". Well, today the world is beginning to understand that we will not reach our short, medium or long term goals for the built environment unless we attack the building envelope.
Assuming the above, then should we renovate the envelopes of existing buildings from the inside or from the outside? We argue that in virtually all cases, renovation should take place from the outside. Only in cases where owners prefer, or are required, for historic or community reasons, to maintain the exterior look of a building, should deep envelope renovations be done from the inside.
The advantages of pursuing Passive House level performance from the outside are numerous. Here's a partial list of benefits:
- Easier to achieve continuous insulation
- Less risk of losing interior floor area
- Easier to maintain continuous air barrier with fewer transitions required
- Easier preservation of interior features like marble clad walls or architectural detailing
- Minimal disruption of day-to-day operations
- Easier to control hygrothermal performance and location of dew point
- Easier alignment of windows with the thermal barrier without having to make windows smaller
In the end, what we like most about the outside/in approach to envelope retrofits is that it allows for more control. We know from experience that control and simplification of execution translates into lower costs and fewer risks. So, we end where we started. Renovating existing buildings from the outside increases the likelihood of achieving desired energy and indoor air quality performance post-construction.