Insulation: comparing prescriptive guidelines beyond efficiency
Since I don’t know a lot of the guidelines for PGH, this will mostly talk about walls, but I’m not intentionally excluding roofs or slabs, I just haven’t read the PGH book yet.
I understand that in Climate Zone 5, going past R-22 to R-25 there are significantly diminishing returns for wall insulation, especially for ROI at current insulation and energy prices.
Using double versus triple glazing as an analogy (which may or may not be appropriate since that’s an R-value improvement at the low end), I’m curious to know if there are other benefits to going past this level of wall insulation that aren’t easily captured by ROI.
For example, building envelope forgiveness/durability (although this obviously depends heavily on the assembly order), hedging against extreme energy prices in the future (looking at the UK right now where some people are paying 80% more than they did last year), acoustic comfort, thermal stability due to increased mass, etc.
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For the various Climate Zone 5 recommendations I’ve heard of:
Here’s what I heard of before for climate zone 5, mostly from Straube and Lstiburek (and people quoting them):
R-10 under foundation floor slabs; R-20 foundation walls; R-40 house walls, and R-60 ceilings or roofs.
I haven’t read the PGH book yet but I’ve heard it’s more like R-25 for the walls in CZ 5. Not sure about the other figures.
This lines up closer to BC’s step code which is looking at R-22 to R-35 walls.
https://www.bchousing.org/publications/IG-R22-Effective-Walls-Residential-Construction.pdf and they have much wider R-value examples for roofs: https://www.bchousing.org/publications/Illustrated-Guide-R30-Effective-Vaulted-and-Flat-Roofs.pdf
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Andy, a lot of people reduce the holistic nature of PGH down to its prescriptive thermal recommendations for climate zone 6, which were simply repeated from this at-the-time freshly released research paper by BSC: file:///C:/Users/Admin/Downloads/BA-1005_High%20R-Value_Walls_Case_Study%20(8).pdf.
As seems to be the answer for most building science questions, the real answer is, "it depends." In the book we cite the report above, which for climate zone 5 lists R-8 under the slab, R-15 basement walls, R-30 walls, R-50 compact (vaulted) roof and R-65 vented attic. Actually in the book we adjusted those last two values for consistency but it's really not all that important once you're into that range of R-values.
Those values are selected in part for thermal performance and in part for cost/benefit, but of course the construction system makes a difference. We now know that the embodied (up-front) carbon emissions also matter, greatly. My approach is to use energy modeling to fine-tune the numbers above, and in most cases I find that I can justify somewhat lower values for ROI purposes, and when forced to use environmentally damaging materials such as foam I keep the insulation levels as low as I can justify. There is really no single answer that works for every situation.
The more insulation you have, the less thermal energy you have to dry the assemblies, which isn't an excuse to skimp on insulation but it means that getting the details right are more important than on poorly constructed homes. Once you have put in that extra effort, you will have a more durable and resilient home than most code-minimum homes.