Building a new home in Climate Zone 5 — Need insulation help
We are building a new home in zone 5 (Michigan) and I would like your recommendations on how you would insulate the walls and roof/attic space.
The home is 3500 sqft with 2750 on the main floor and 750 upstairs. The main living space (kitchen, dinning, living room) is open concept with a cathedral ceiling. Other areas have a more traditional attic space.
The basement will be unfinished and we will use 2″ rigid foam under the slab and on the walls. (unless you tell me differently)
Walls are 2×6 construction.
How would you prefer to insulate this home? We don’t want to do the cheapest form of insulation just to meet code but we also don’t want to spend excessive money without getting much benefit from the cost.
Thanks!
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The best option is to put all the insulation on the exterior. Next best would be a combination of exterior and interior insulation. How do you feel about those two options?
This recent guest blog https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/airport-house-walls-and-insulation describes how I decided to insulate the walls in a house near Flint, MI. Basically, I went with 2 inches of graphite infused EPS over 2x6 walls.
Where in Michigan are you building?
We get a lot of questions here at GBA from homeowners who have problems with their cathedral ceilings (high energy bills, strange snow-melt patterns on the roof, and ice dams). I really think that the best way to insulate a cathedral ceiling is with one or more layers of exterior rigid foam. This article tells you what you need to know: How to Install Rigid Foam On Top of Roof Sheathing.
For advice on walls, you might want to read this article: How to Design a Wall.
-- Martin Holladay
Thank you for your responses!
We are building in St. Johns, MI.
We are using certainteed insulated cedar board siding. R value is 2.2 Is this enough to help with the thermal bridging making the exterior rigid foam less necessary?
If we don't do the exterior rigid foam on the cathedral ceilings what is the best alternative? Pray?
Nate,
If the don't want to install rigid foam above your roof sheathing, you should use one of the methods recommended in this article: How to Build an Insulated Cathedral Ceiling.
-- Martin Holladay
At 2" of foundation foam you won't hit IRC 2015 code-minimum of R15.
Anyone building in zone 5 can get a lot of pointers from this Deep Energy Retrofit guide developed for the state of Massachusetts (which is wholly inside of zone 5.)
https://buildingscience.com/sites/default/files/migrate/pdf/GM_DER_Guide_2013-01-18.pdf
Pay particular attention to the prescribed R ratios on things like unvented roofs, etc.
If you use reclaimed foam from commercial building re-roofing/demolition the costs of doing exterior foam approaches can be dramatically reduced. (I've seen savings in excess of $10K on material costs going that route in deep energy retrofits, and it would be the same for new construction.) Find out who your local reclaimers are, have a place to stash it when it becomes available (even if it's stacked on pallets under tarps on the building site), and you can usually make out just fine if you have a few months of advance planning.
Having crews familiar & comfortable with deeper exterior foam can be important. It's not rocket science, but there's a bit of a learning curve since it's still a bit outside the mainstream for residential construction. A code-min wall in zone 5 is 2x4/R13 + R5 continuous insulation, but most builders opt for 2x6/R20 because it's less labor. But the difference in labor cost between 2x4/R13 + R5 and 2x6/R20 + R10 isn't much, but it's a substantially higher performance. If reclaimed foam is used the material cost can be even less than 2x4/R13 + R5.
Both 2" and 3" are common thicknesses for roofing foam, and an 2x6/R20 + 3" exterior polyiso is sufficient wall performance to hit Net Zero Energy with a solar array that still fits on the roof, if you upgrade the rest of the assemblies appropriately. You could also get that performance point with a 9.5-10" thick double-studwall with dense packed cellulose, which may be easier if you can't find local contractors familiar with installing 3" exterior foam.
A quick guide to the "whole-assembly R" performance needed to hit a Net Zerol benchmark can be found in the Zone-5 row of Table 2 in this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Note, that the R15 basement wall recommendation in that document is now the IRC 2015 code minimum for basements, but most of the other factors are better than current code.
Using these as target values in the planning stages isn't a bad start, and would be both comfortable and financially rational when achieved using lower cost methods.
Are you committed to a full cathedral ceiling that has the same slope as the roof? We used scissor trusses to give us a raised ceiling in our living room. That creates a vented attic that can be insulated with the same materials as a flat ceiling. Our ceiling slope is about half what the roof slope is.
So I have a few answers for you and a few new questions.
Reid-We are doing scissor trusses so we have solved that problem. Thanks for the great thought.
We are building in Zone 5.
We are going to use the Zipp insulated sheathing 1-1/2" R6.6. We also are using Certainteed Cedarboard insulated siding with polystyrene that has an R2.2 value. So this seems to give us R8.8 of exterior foam insulation for the walls.
I need a minimum of R5 exterior with R13 interior. So with the R9 on the exterior how would you insulated the 2x4 interior walls?
Download Beopt, you can run simulation for different wall/insulation and get energy usage/ROI results.
Nate,
Q. "I need a minimum of R-5 exterior with R-13 interior. So with the R-9 on the exterior, how would you insulate the 2x4 interior walls?"
A. Usual choices include fiberglass batts, mineral wool batts, blown-in fiberglass, or dense-packed cellulose.
-- Martin Holladay
Anon3, I have been wondering whether folks trust BEOpt output. As I refine my design, I'm hoping to input more granular data like appliance types, etc, but am not sure whether BEOpt modelling is accurate enough to go through the trouble of doing so...