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Four Superinsulated Walls (PTC's Continuing Education Tour, Part 3)

Fat walls mean skinny energy bills
January 20, 2022

We talked about airtight walls in the last hour, this hour we’re going to talk about superinsulated walls.

There are a lot of ways to build superinsulated walls and I’m going to look at four of them.

Continuously wrapping the outside of the house with some sort of insulation, like foam boards or rock wool boards.

Adding a second wall on the inside so that you can blow TONS of insulation into the cavity.

And thickening the wall on the outside so that you can blow TONS of insulation into the cavity

Hyperbolizing a pretty standard system to make people pay attention to it.

They are four of the many ways to go about super insulation and they illustrate strengths and weaknesses.

First, we’ll look at continuous exterior insulation

People used to call this outsulation, which I resisted with all of my might.

Lately, they’ve been calling in continuous insulation, which I like better because it says what it is without inventing dumb new words.

I still usually call it exterior insulation because I am stubborn.

As noted in the CI moniker, continuity is key. We remember from the first segment about building science that heat is a shape-shifting intruder who will find the easiest path to screw you that it can.

If everything else is buttoned up, the easiest path is through the studs. And remember that when thermal bridges are accounted for, R-19 insulation is effectively only about R- 15, or about 25% less than you think it is.

25% loss is a pretty big number.

If you don’t think 25% is a lot to throw away, then please withdraw 25% of your retirement account and send it to me.

Anyway, now we’re going to go into detail about continuous exterior insulation

You may have detected a theme here, where I show an animation of the perfect-world concept, and then show a video of a real-world jobsite.

That is not going to change in this presentation.

Here’s another early animation that I made right after we launched protradecraft. It is based on jobsite guidance from a remodeler whose done this over and over, and who actually has done it on multiple houses owned by Joe Lstiburek and Building Science Corporation.

So not only does David know what to do, he knows why and where the beer is at Joe’s house.

Copyright 2015.

We’ve been talking about this for six years at Protradecraft, but we were doing it for another six at least at Fine Homebuilding and green building advisor.

Let’s look at how guys with tattoos do that in real life.

So this is the builder/remodeler who works for Dr Joe. This is not one of Dr. Joe’s projects, but David uses the d=same details he learned from building for BSC.

It’s a typical split-entry ranch that was gutted and retrofitted into a weird modern house.

Basically, the same thing we saw in the animation:

  • Flash the window opening
  • Install the window
  • Integrate the window with the WRB, making sure everything seals back to framing and leaving no layers open at the edges.
  • Install the first layer of foam, not taping the seams, and using offcuts to fill the pieces when you can…

… because in this video, they’ll install bigger pieces of foam and tape the seams and corners.

They’ll also install furring strips for siding attachment and drainage behind the siding.

Now let's head to the second method, double-stud walls

You can do this in many ways.

I’ve seen photos of people lifting double walls a foot apart, and I always wondered how that frame them like that on the ground.

Its probably not very difficult, and was expecting to see it happen when I visited a jobsite in Maine where they were building a double wall house,

But what I saw was that they just framed the outer walls with 2x4s, stood them, set trusses, got the house dried in, and then went back and framed the inner walls.

Which is a lot faster to dry in than framing twice as many walls at the get-go.

I actually built a double-wall structure one time back in the nineties. The prevailing advice in the hippy homebuilding journals was to use a 2x8 bottom plate and stagger the studs.

The insulation was then woven between them.

This is NOT the best way to do it at all, but I was building a practice space for a rock band, so it was a sound-proofing thing, not an energy efficiency thing. The building was pretty quiet.

For thick-wall buildings where people live, the double stud wall is usually thicker than 7 inches and staggering studs is not necessary.

In fact, it’s easier to not stagger the studs.

(Video of Ben Bogie framing double-stud walls)

So, frame the outer walls with 2x4s, which makes them easy to lift, and then transfer the layout inside to the wall plates and frame the inner walls, also with 2x4s making the window and door openings slightly larger, to allow for humpy lumber.

There’s a variation on this method that I also saw in Maine, at a Passive house construction site. The outer walls were made with SIPs and buttoned uptight. The roof sat on those, and then inside of that, the crew built 2x4 walls and blew in about 9 inches of insulation. So the exterior shell was tight and complete before the rest of the walls ever needed to be framed.

The third method is a lot less common but totally valid

It is a modification of what people call a Larsen Truss, which is basically 2x2s and plywood webbing nailed to the outside of a wall to make room for more insulation.

Ben Bogie is also the carpenter on this project, a passive house retrofit on a 1930’s cabin basically.

So this is a method which you will NOT find in the building code, but you wouldn’t have found exterior insulation in the building code a while ago either when we started covering it in green building advisor and Fine Homebuilding.

But exterior foam is in the code now, so the builders who paid attention six years ago already have their systems in place for meeting higher R-values and things like ‘Effective’ R-value.

The point of the exterior I-joists is not to waste time, but to show a valid solution to the control layers that Dr. Joe defines.

As crazy as this seems, when you watch it done, it goes quickly and delivers high performance with off-the-shelf materials.

One good thing about this system is that the i-joists disrupt the thermal bridge across the floor and ceiling assemblies too—because you can go from mudsill to top plate, whereas double-stud walls can only fir between the subfloor and ceiling joists.

It is really just a different approach to exterior continuous insulation. In the first approach, the rigid foam provided the structure that the insulation needed to go from top to bottom.

In this approach, the insulation is cellulose, which really sucks at being structural. So they use engineered framing lumber to make a place for the insulation.

There are a lot of reasons why you might want to do this, I suspect the main one here was that the architect and client wanted to reduce the greenhouse gas production that would be introduced by adding 10 inches of foam to the walls and roof.

Not to mention where they’d find long-enough screws.

Awesome Wall by OA Design+Build+Architecture

So this method is fairly simple, even if a little different.

Here is a wall design that was invented by a local guy, Michael Anschel, OA Design+Build+Architecture, in Minneapolis,

Michael wanted a wall that would satisfy Dr. Joe’s control layers and be easy enough to install that people in the field could make mistakes, as they sometimes do, and the wall design could overcome those mistakes.

I’ve worked with Michael for a long time and can tell you he’s a great builder. We just finished a really cool show called Building Resilience, which you should look for.

This so-called Awesome wall uses continuous exterior insulation outside the framing and a closed-cell spray foam flash coat between the studs. The spray foam does a great job of stopping airflow and adds 13 Rs to the wall.

Outside the continuous insulation is a drainage mesh rain screen product that is available from at least a few manufacturers. One advantage that entangled mesh has over furring strips is that air can move up, down, and sideways without being restricted by solid furring strips.

With furring strips, you have to leave space at the top, for air to move sideways or else, it’s trapped.

We saw that in the earlier video.

And you have to do this at windows, too. Not that it is difficult, just that it is more opportunities for people to space out and make a mistake.

Another thing that this builder really likes about this system is how it flushes out with the 3-inches of foundation insulation you all need to install.

With this 2.5-inch exterior insulation, if he bumps his sill plate out a half-inch, the face of the sheathing lines up with the face of the foam, so it just eliminates a lot of little math headaches.

I wanted to add this video because it puts a lot of this stuff in the context of a whole house that is complete.

It also includes some talk about rain screen systems and pulls siding and windows into the picture.

Anyway, he puts the whole package in context, the exterior insulation, the rain screen, spray foam between the studs for air sealing. He calls it the awesome wall, because he tends toward hyperbole, but he is also a great builder, who is not out to lose money on callbacks, rotten houses, and bad press.

This house was basically a flip for Michael. They bought a hundred-year-old house, added some square footage, fixed the layout, added a couple of bathrooms, tightened it up, and sold it.

So, four approaches to wall systems that stop water, air, and heat. They all do it differently, and they are all effective.

They all have some challenges, and as always, it depends on the house, the crew, and the materials at hand.

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