Super Insulated Walls & Roof – The Postgreen Homes Difference
We recently started a new series of blog posts highlighting “the Postgreen Homes Difference.” Getting back to our 100K geek blogging roots. Getting into the nitty gritty. See our ABOUT page to get the full list that will eventually have links back to a bunch of these detailed posts.
What to post on the first one? Why not start with our super insulated walls and roofs. Yes, they have much insulation. Twice a normal house. Maybe more. We’re going to keep it simple. We’ll get into thermal bridging and air sealing and glazing on later posts. Hold your building science geek pants on. You know who you are.
High R-Values Doubling Code Requirements
The R-value code requirements in Philadelphia (Climate Zone 4) are R-13 for walls and R-38 for ceilings or roofs. Newer code requirements would increase the wall values from R-13 to R-20. In general we are aiming to double code requirements for wall and ceiling/roof insulation in our homes. This means we are aiming for around R-40 in our walls and R-70 in our roofs.
We did not come up with these figures arbitrarily. There are a couple of key reasons that we pack this much fluffy stuff into our walls and roofs. Let’s make a bulleted list of them:
- This amount of insulation is the backbone of our strategy to cut energy usage by a minimum of 50% over a code house. That means a HERs rating of 50 or better.
- Passive House Modeling – We’ve modeled multiple projects using the Passive House Performance Spreadsheet with a number of certified consultants. They all lead us to this level of insulation (among many other details) in order to achieve the stringent Passive House standard for extreme energy efficiency.
- The folks at Building Science Corp (who we greatly respect and are responsible for many lost sleeping hours researching their writings) recommend R-40 walls and R-60 roofs as the best bang for the buck in a low-energy house. They are smarter than us and we listen to people like that. There are a lot of them out there.
- It makes for a substantial looking, thick facade wall that no one else is crazy enough to build and makes a nice shelf for plants and cats.
Better True Wall R-Values Boosts Super Insulated Performance
A well insulated wall is not just about a single R value. A lot more has to do with the framing of the wall and the install of the insulation product than most realize. We take these details seriously. Here’s some more knowledge nuggets on the subject.
A typical wall in new construction homes is made up of a 2×6 wall framed on 16″ centers with R19 fiberglass batts. Now this will be advertised as an R19 wall, but in reality about 25% of the wall is made up of solid wood studs from drywall to exterior sheathing (plywood or OSB). That means the true R value of the entire wall is about R-13.7 or a reduction in 28% from what you think you’re getting in that wall. Wood studs are only R-1 per inch where the fiberglass batts are typically R3.4-3.8 per inch.
We want to improve upon this, so we first start with advanced framing by building our walls with studs every 24″ instead of 16″. We then incorporate a lot of “Advanced Framing Techniques” like two-stud corners, right-sized headers and fewer studs on the sides of window and door openings. This gets our framing factor down to about 16% rather than 25%. We then put in a more efficient insulation product at R-3.8 per inch to obtain a true wall value of close to R-16.8. That’s a 23% increase in your whole wall R value in the same thickness right off the batt! Sorry, I couldn’t resist the pun.
The next step is to add a layer of continuous insulation that is not interrupted by any framing. We do this either by adding continuous rigid insulation to the outside of the wall or by constructing a double stud wall that has a minimum of 2″ of nothing but insulation in between an outer wall and an inner wall. The lowest this continuous layer ever is, is 2″ of low density insulation for an added R-value of 7.6. Add that to our R-16.8 advanced frame wall and we’ve boosted our true wall R value by 45% to 24.4. That’s almost double a typical wall and we’re not done yet.
Most of our walls contain double studs with an added layer of insulation. This extra layer adds R-11.1 to the wall assuming it’s a 2×4 wall on 24″ centers. That brings us to a minimum of R-35.5 for one of our most typical walls in terms of the true whole wall R value. At most parts of the wall, it will be R-41.8 where there are no studs. This is the figure we are using in marketing materials where others are using a R-19 or R-21 fiberglass batt.
To recap, our R-41.8 wall has a true wall value of R35.5 compared to the standard R-19 wall that actually only has a true wall value of 13.7. That’s well over twice the insulation value at 2.6 times the R value in our walls than a standard new construction home. This is a lot of geeky wall stuff, so thank you if you are still reading this. We got all of this data from Building Science Corps case study on high R walls.
Types of Wall and Roof Insulation Used
Each of our projects are slightly different from each other including their wall construction and insulation type. We’ve used a variety of insulations and often combinations. Here is a brief list below.
- Cellulose – Basically recycled newspaper. Our favorite insulation with high thermal mass, sound and air blocking properties and fills all of the cavities in our cavernous walls.
- Rock wool – More popular in Europe or in commercial fire stopper applications. Far superior to fiberglass batts for many reasons.
- Recycled Fiberglass Batts – Sometimes we use fiberglass batts, but they are the recycled type with no paper facing that pressure fit nicely to fill the walls like the pic to the right. We use these a lot inside and on the party walls for sound proofing as well as on the interior layer of an externally insulated wall.
- Open Cell Soy Spray Foam – No oil, but still breathes and fills the cavity at a fraction of the cost and worry with closed cell spray foam.
- Exterior insulated sheathing – This can take many shapes, but it’s typically poly-iso rigid insulation with R values approaching R-7 per inch and we like to get R-15 on the outside if possible.
Thanks for reading. Now imagine it’s the 1800’s and thick is in. It shows your affluence. Buy a Postgreen Home and be proud of your new thick, super insulated walls and flaunt them at your next dinner soiree.