Two good Exterior Insulation Systems: 7 Minutes of BS (Building Science)

In this episode of 7 Minutes of BS, we continue the discussion with engineer Jonathan Smegal of RDH Building Science Laboratories to break down the best exterior insulation retrofit systems based on real-world testing on five identical student housing units.

Jonathan walks through what separates a good system from a great one — fewer layers, less thermal bridging, lightweight claddings, and a dedicated air and water barrier at the sheathing layer. His preferred approach: remove the cladding, add sheathing if needed, apply an air- and water-barrier, then install exterior insulation with strapping on top.

Dan also makes the case for EIFS (Exterior Insulation Finishing Systems), which can be applied directly over existing brick — keeping thermal mass within the envelope, reducing landfill waste, and nearly eliminating thermal bridging when the foam is applied adhesively.

Key takeaways:

• All five tested systems performed well for energy efficiency and durability
• Fewer installation passes = lower cost and less complexity
• Air and water control belong at the sheathing layer, not split across multiple materials
• EIFS offers exceptional design flexibility and near-zero thermal bridging

TRANSCRIPT:

There are multiple ways to do this, and they all will work. 

Today, we're continuing the conversation from last time, when Jonathan described a test in which they were able to monitor.

Five exterior insulation retrofit strategies were applied to five identical student housing units. 

1. BASF used a combination of spray foam on the exterior and neopore, which is EPS insulation, as the exterior continuous insulation.
2. Rockwool used a semi-rigid stone wool with a vapor-permeable membrane on the OSB.
3. DriveIt did an EIFS system.
4. Owens Corning had an XPS solution with taped XPS as the air-tightness layer
5. PlastiFab Insulspan had more of a nail-base product. 

Last time, Jonathan told us about each system and reported on their performance.

All of the five systems worked well when it comes to efficiency and durability...

This time, we're gonna talk about our favorite systems,

...but some of them were a little more. Onerous in their installation. They required more effort, more passes. Like every time you have the contractors go around the building again to do another layer of material, it gets more expensive. 

Nobody likes to see workers walking around the house. But seeing them do it over and over is downright painful,

...and so the idea is to make it as simple as possible while still maintaining the energy efficiency and durability.

We tend to like the least number of layers possible. We like adding, strapping with long screws so that we don't have thermal bridging through the exterior insulation.

If I think about what I see in general in, in deep energy retrofits, oftentimes people will attach the exterior insulation in a way that they end up reducing the effectiveness significantly with. Zed girds or other materials? Some clip systems are pretty energy intensive, so we like to use long screws.

We like to use lightweight claddings and we like to use as few layers as possible, but whether or not that is Stonewall insulation or XPS or polyiso, I like to see board foam insulation put up with strapping, put over top of it, and a bare barrier put behind it. 

That sounds a lot like the wall system. We usually talk about when we're talking about residential exterior insulation: house wrap, exterior foam, furring strips, and cladding.

There are multiple ways to do this and they all will work, but what we generally like to see is you remove the cladding, you get back to the sheathing if there is sheathing. If there isn't sheathing you, we like to add sheathing. We apply an air and water barrier on top of that, and then we apply the exterior insulation over that with strapping.

So the exterior insulation doesn't need to be a water barrier. It could be vapor permeable. It could be vapor impermeable, and then you apply the cladding to the strapping. And so that is. Generally speaking, a very good strategy. It addresses air tightness because you get to put that membrane on first, whether it's mechanically adhered or self adhered, and you get to detail all the corners.

You get to tie that into the windows.

Air and water control at the house wrap layer

Air and water tightness is completed at the house wrap layer, so everything outside of it can focus on doing its own little job instead of having a smorgasbord of responsibility. While technically Jonathan just picked a winner, he's quick to point out that really they're all winners.

I wanna be insulation agnostic. We work with all the ins, installation manufacturers, we like them all. Insulation is good. Some have more limitations than others. And depending on if you're looking for vapor permeable or vapor impermeable, we know that they all can work, especially if you keep the water outta your wall.

Which I mean, that's the goal. 

And I gotta say, while I like the system Jonathan picked. I also think there's one worth talking about: the EIFS retrofit, exterior insulation finishing systems. These student houses were brick clad and while many of the systems removed the brick to get back to the wall sheathing.

The EIFS approach added insulation to the exterior of the brick. It brought all that thermal mass into the thermal envelope, and it keeps a lot of bricks outta the landfill. I just kind of feel like if you're gonna use forever materials to make buildings, you should use 'em forever.

And that's a good point. I temporarily forgotten about the Eves because Eves is really flexible in terms of the finishes.

You can do almost anything you want with it, depending on. You can add features because they just put on more EPS foam to make it look like, give it more texture. You can have different coatings, different colors, like you can, the EAs is so flexible in its finishes, and oftentimes, I'm trying to remember if the eaves in this case, or if the foam was mechanically adhered or, or glued.

If you glue it up, then you have almost zero thermal bridges through your insulation layer. If that's an appropriate design for that building, then you've reduced thermal bridging even further by adhering it to that. Now, each always consists of a fluid applied air and water barrier behind it. 

A fluid-applied air and water barrier is better performing than a peel-and-stick weather-resistant barrier. And that's better performing than a mechanically fastened WRB.

That's a continuous adhered air barrier. It's gonna work very well. We like to see adhered air barriers 'cause they're stronger and generally work a little bit better. Eves is a great option. The only challenge with the eve, frankly. For my neighbor's houses, they have these woodpeckers or birds that like to live in it.

So you gotta get an appropriate coating for that. The companies have them and deal with that. But other than that, each is a great solution. Like you said, you don't have to remove the brick and you can have high flexibility. It's still a drain system and there's less thermal bridging, so it's great. 

And there you have it: two exterior insulation retrofit systems worth talking about. 

But again, there are a lot of ways to do it, and many of 'em can work quite well if you have skilled craftspeople doing the work. Today's craftspeople not only know how to build cabinets but also understand air, water, and thermal control layers. Remember, you get paid for what you do and what you know. And now you know more. Seven minutes of BS is a production of Endeavor Business Media, a division of Endeavor B2B.