How to insulate house walls from the outside

Adding extra insulation to the exterior walls of an older home when renovating or remodeling is a great way of improving a home's walls thermal performance and reducing energy use while reducing heating bills, carbon footprint and improving a building's comfort level. It may look easy to go buy some packs of rigid insulation panels down at the local hardware store and screw them to your walls, but unless you understand how your existing walls are built and how they "work" in terms of keeping moisture, heat and cold where they should be for durability, the insulation panels might not be the only things you're screwing up!

Adding insulation to the exterior of a home correctly starts with establishing what climate zone you are building in and how the existing wall is built to make sure you are not creating a condensation point at a dangerous place within the wall assembly, or preventing internal moisture from the occasional leak in the exterior skin from running away, so the wall structure can dry out. 

Walls need to be built to meet their specific climate needs. All too often ‘the best wall assemblies’ migrate from their own climate to one where they do not belong, like putting interior vapor barriers on homes with aircon in hot humid regions, which simply should not be done if you want your house to last. Durable, healthy and sustainable homes need careful research and planning - as changing one element without considering how this changes the dynamics of the building envelope is a potential recipe for disaster.

The best pre-emptive strike against the possibility of a nightmare exterior wall renovation and insulation project is educating yourself - so you don’t get talked into the wrong wall system by a poorly-informed general contractor. Start by watching our ‘building science made easy’ video below. 

What is the right amount of insulation for walls? 

Determining your climate zone will help decide on an appropriate amount of wall insulation for your home, whether you are in a heating or a cooling climate. Next, compile a short list of the types of insulation that best suit you by determining what your biggest concerns are. Consider some of the following: Cost, durability, fire-resistance, insect-resistance, low GWP (global warming potential), made with recycled materials, made with natural materials, which ones are best for sound proofing, or for reducing off-gassing to protect indoor air quality. 

See our page on choosing the right building insulation for the right application

Once you determine the approximate amount of insulation you want to install on exterior walls and the type(s) of insulation you are open to using, then you need to find a way to attach /apply it securely to the outside of the walls in a manner that preserves it's R-value or thermal insulating qualities. 

How to avoid thermal bridges when fixing exterior wall insulation in place

For insulation to work properly on exterior walls and have a long functional lifespan, the wall system needs to have all the appropriate components, including an air barrier, a vapor barrier /vapor control layer, well balanced insulation, and be attached in a way that doesn’t compromise the R value of the insulation. But you already know that from the building science video right? Didn’t think there would be a test so soon did you ?? heh heh

Here is a page on thermal bridges: what they are and how to break them in buildings

But for those not clicking their homework assignments, here’s a quick summary – metal is a conductor, and when you have metal that passes from one side of a wall to the other, you have a thermal bridge, which is a path for heat to leave your home, and money to leave your wallet. To those of you living in Toronto or New York in older homes, this also applies to the elegant brownstone rowhouses, townhouses, and tenements that were built over a hundred years ago from brick and stone - neither of which are known for their outstanding insulating properties either. 

Of course we all want to limit using energy (and losing money) while decreasing our carbon footprint as much as possible, so be mindful of the way you fasten any insulation when refinishing and upgrading outside walls - as in a perfect exterior wall assembly the insulation layer should be continuous, unbroken by thermal bridges. Below are a few methods for improving an older home's insulation levels and attaching exterior wall insulation successfully.

Larsen trusses with cellulose or batt insulation

A Larsen truss wall is one with engineered I-joists attached to the outside of a stud or brick wall, which then creates a cavity that can be filled with additional insulation. You can add batt insulation, dense pack cellulose, rigid insulation panels or spray foam if you choose (spray foam is our least favorite but it can be done so we mention it, though we always encourage readers to choose spray foam with the safest blowing agents

Larsen truss walls can be made in a variety of thicknesses, which is a really nice advantage to the design as it can be done in a way that has the best return on investment for any climate. Below is a photo of a Passive House using Larsen trusses on the exterior of the wall (see video), that will then be filled with dense-packed cellulose insulation

Larsen trusses with dense pack cellulose insulation for Passive House
Using Larsen trusses with dense pack cellulose insulation for a Passive House

To go back to thermal bridges – wood does act as a thermal bridge, but not nearly as much as metal, and with engineered I-joists, at only 5/8ths of an inch, the conductivity of the wood isn’t a great concern and becomes even less so the deeper it is. 

Fiberglass thermal spacers or "Cascadia Clips"

The purpose of fiberglass insulation fasteners is to attach the clip itself to the frame, which then supports and secures the insulation with a material that has a low conductivity to avoid heat loss, fiberglass clips also provide a solid surface for fastening strapping (furring strips) and exterior cladding.

Cascadia Clips from Cascadia Windows in Vancouver are the biggest name in fiberglass thermal spacers you will find across Canada and the U.S. so the concept has in a way adopted the brand name, but there are others brands of wall insulation thermal spacers such as Armatherm Z Girt available, pictured below.  

use fiberglass spacers to avoid thermal bridging when attaching exterior insulation
Fiberglass spacers break thermal bridges when attaching exterior insulation 

 

Also, if installing a final finish to exterior walls over additional wall insulation, see our page on how to install exterior siding so walls can dry.

Attaching insulation with metal screws

Above are a couple of ways to attach exterior insulation and maintain the highest possible R value by breaking thermal bridges, but the reality is that most exterior insulation is going to be a few inches of rigid insulation boards screwed or nailed to a wall with strapping. It’s not perfect and it does reduce the overall performance of the wall, but it’s not the end of the world, so don’t panic if you’ve already done it or are planning on doing it, we’re just trying to outline best practices. 

We built our LEED V4 Platinum Edelweiss House using rigid Rockwool or Roxul insulation panels attached with strapping and screws (which is in the building science video above), and what we did to break the thermal bridge was to do it in 2 layers of 4 inches of rigid Rockwool so there would not be a single screw passing through the wall assembly.

It worked great in terms of energy efficiency and durability, though it’s not what we would call the most efficient use of time as doing two layers of insulation instead of just one meant double the work. In hindsight it is unlikely we would ever repeat that process, but that’s how we learn! You can see the whole Edelweiss LEED Platinum v4 certified Demo House being built in our YouTube green building video series.

One alternative "mainstream" system that we like for attaching rigid exterior insulation on the outside of walls and avoiding thermal bridges is the ThermalWall PH panel. It is a rigid EPS foam insulation panel that comes in various thicknesses, which is in itself not a new concept in wall insulation, but the interesting part is how simply and quickly it is fitted. It was originally invented by Legalett as an effective way to increase total exterior wall R-values to Passive House standard easily taking a standard code compliant R-24 wall up to a R-52 high performance envelope. 

Even Lloyd Alter of Treehugger magazine wrote when reviewing the system; "there are some serious advantages to foam that can make a TreeHugger think twice, especially when one is talking Passive House, where one needs a lot of insulation and avoiding thermal bridges is a very big deal.... I do often foam at the mouth about foam insulation and have always promoted alternatives. But this system really does provide a continuous wrap of effective insulation ... to the roof. It is going to be pretty airtight too. It is such a simple system that makes a very good case for the stuff" which we agree with.

There is a galvanized metal rail embedded inside the panel, and when the ‘snap track’ is removed the metal rail is exposed. From there you can screw it to a stud wall or into masonry, then you replace the snap track and fasten your exterior strapping to the same metal rail. 

ThermalWall rigid EPS insulation board for insulating the exterior walls of homes
ThermalWall PH rigid EPS insulation board for insulating the exterior walls of homes

 

The advantages are plenty here – panels are between 4.5 inches and 8 inches thick, the exterior insulation is continuous, and they can be attached with 6-inch #10 screws from 4 to 6 inches long. Having a central screw that attaches the panel to the wall and a separate screw attaching exterior furring strips to the rail breaks the thermal bridge. An additional benefit is that screws of that length are still quite affordable. From experience, over 6 inches long you start getting into ¼ inch diameter truss head screws and they start to get really expensive - which when insulating large exterior wall areas starts to add up in cost.

We realize that the mere mention of any kind of "Styrofoam" or foam insulation (ThermalWall PH is made with EPS / Expanded polystyrene) can upset the sensibilities of hard-core environmentalists, but it is worth keeping in mind that EPS insulation panels are mostly made of air, it is recyclable, the plastic content is a petroleum byproduct rather than primary material and it could be plant-based if it weren't for cost and the carbon footprint of doing so, and it works well when you use it for the right applications. It’s not "perfect", but then what in the building industry is? We've found that EPS's reputation as somewhat of an environmental "no-go area" is often based on a misunderstanding of what it is, and isn't, so we'd recommend remaining open-minded until you've got all the facts. 

Even the greenest natural home insulation products have a carbon footprint of some kind, for example a strawbale house or straw insulated SIPS panels has diesel fuel in its history from fertilizing, irrigation, harvesting and transportation. And a lifecycle analysis of EPS (expanded polystyrene) insulation is not nearly as a bad as its reputation in some quarters of the green building industry would have us believe - but then a lot of the problem is it's usually thought of as Styrofoam in North America - which it may surprise you to learn it most definitely isn't.  Just putting that out there, but we always welcome (friendly) dissenting voices and discussions in the comments section below! 

The risks of condensation when insulating walls

The most important thing to remember when insulating exterior walls during renovations, is to be sure not to trap moisture between two vapor barriers. If a home already has a vapor retarder or vapor barrier, it’s important not to add another vapor-impermeable layer on the opposite side of the wall that could trap moisture in the middle along with organic materials such as insulation or wood framing members. 

Some rigid insulation boards have integrated air barriers, some have vapor barriers, some have both and some have none. Some insulation panels  (like EPS, XPS and polyiso) can act as a vapor barrier, which can be a good thing or sometimes a bad thing, so be careful!

This is why insulating exterior walls can sometimes go terribly wrong when "poor" insulation products are chosen for any given application. Don't just walk into a building supply store and pick the cheapest rigid insulation panel, that could be a disasterously expensive decision. This page will help explain: 

The differences between Polyiso, EPS & XPS & Styrofoam

This is why deciding on a wall assembly for renovations has to be considered carefully to ensure that walls are able to dry out. Be sure to either have your wall assembly designed by a professional, but better still, get an understanding of the science behind it yourself, that way you will be able to spot anything that seems fishy with enough time to change it. Check out some of the links below, and if you have any questions that aren’t answered they look in our discussion forum and maybe ask a question of your own here

Now you know more about how to insulate house walls from the outside and the pitfalls to avoid... 

Find more pages about how to build energy efficient and durable walls here : 

Find more about green home construction in the EcoHome Green Building Guide pages