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Continuous Insulation – Making the Right Choice

If you’ve made the decision to include continuous insulation on your building, it’s time to pat yourself on the back.

The International Energy Conservation Code (IECC) has required continuous insulation since 2012. When the building envelope is insulated on the outside, it not only improves the energy efficiency, it also helps to reduce the possibility of moisture damage through thermal bridging.

WATCH:  ThermalBuck installation using polyiso CI  

Which type of continuous insulation should you choose? Good question.

Each type of insulation has different thermal properties, costs, features, and installation requirements, so you really need to consider what matters most in your application.

ThermalBuck is compatible with all types of continuous insulation, and will simplify the installation process.  It solves many of the common challenges builders find when installing windows with continuous insulation, and make the installation more energy-efficient by insulating the rough opening – typically a source of energy-loss in the building envelope. 


Types of Continuous Insulation

There are three main types of continuous insulation: rigid foam, mineral wool, and cork. The most widely used is rigid foam, which is split into three main categories: EPS, XPS, and Polyiso.

Expanded Polystyrene (EPS):  R-4 per inch

EPS is the most commonly used rigid foam. While it has the lowest R-value, it’s also the least expensive around .31 cents per sq. ft., which makes it a favorite for code compliance within budget. EPS does absorb water, and has the lowest compressive strength of the rigid foams.

Structured Insulated Panels (SIPS) and Insulated Concrete Forms (ICFS) are comprised of EPS. Insulfoam, a division of Carlisle Construction Materials, is largest manufacturer of block-molded expanded polystyrene (EPS) in North America.

It is important to note that EPS should be used over housewrap, and supported by OSB or plywood when used as sheathing.  

 

Extruded Polystyrene (XPS):  R-5 per inch

Many green builders discount XPS right off the bat, because it is the least environmentally friendly option. It contains the flame retardant HBCD, and its blowing agents have high global-warming potential.

However, with it’s higher R-value and moderate pricing at .47 cents per sq. ft. it is widely used. It’s stronger than EPS, and more water resistant, making it a preferred choice for under-slab and below grade applications.

XPS is available faced or unfaced, which affects the vapor permeability. Owens Corning Foamular “pink board” is some of the most widely recognized XPS on the market.

 

Polyisocyanurate (Polyiso or ISO):  R-6.5 per inch

Polyiso is the most expensive of the three types of rigid foam, as much as .70 cents per sq. ft. for a 1″ thick panel. However, the higher R-values often make the added expense worthwhile in the long run.  All polyiso is faced on both sides, most often with foil.

Polysio is considered the most-environmentally friendly of the three foams. It does absorb water, and generally cannot be used below grade. Johns Manville is a leading manufacturer of polyiso foams.  

One of the primary benefits of polyiso, is that it can often be used as a WRB behind your cladding. While it may cost more upfront than EPS or XPS, not installing a separate WRB component can save a considerable amount. It really depends on your climate, if this feature will be a long-term benefit, or present a building science challenge. 

 

Mineral Wool:  R-4 per inch

When you think mineral wool, you think of Roxul, easily the best known brand on the market. Mineral wool is also known as stone wool because it is made of basalt rock and steel slag, making it a favorite choice of green builders.  Easy to work with, fire-resistant, sound resistant, and water resistant. It’s the only insulation recognized by code as a firestop.

ComfortBoard is Roxul’s exterior insulation.  With an R-value of 4, it is lower than most rigid foams, however, there is no reduction in R-value over time, which will happen with foam insulation materials that rely on lower-conductivity blowing agents that slowly leak out or allow air to leak in.

Mineral wool is highly vapor-permeable and easy to install. Roxul is approximately .64 cents per board foot. 

 

Cork:  R-3.6 per inch

Thermacork, is the most widely recognized an all-natural rigid insulation material made from expanded cork. It offers excellent acoustic control, is highly durable, has high vapor permeability, and meets fire-safety requirements without flame retardants. It is labeled Red List Free for use in Living Building Challenge projects.

Cork is by far the greenest of the green in the insulation category, but it’s significantly higher cost and limited availability make it more of a niche product.


It’s important to take your specific climate into consideration when evaluating building materials.

EPS and XPS increase in R-value as the temperature drops.  A great thing if you’re in Wisconsin. However with Polyiso, the R-value actually decreases when the temperature drops. Not such a great thing when you’re in Wisconsin.

To choose the best continuous insulation for your project, analyze all of the variables: performance needs, climate, building codes, and budget.

No matter which continuous insulation you choose, proper installation is critical to achieving the desired performance.

Our energy-efficient homebuilders in Utah, Thomas & Melissa Griffiths, did extensive research on the features and benefits of the different types of exterior insulation, and decided on Atlas EnergyShield Polyiso for their dream home.

Thomas wanted the highest R-value his budget would permit, and appreciated the fact that he could use the continuous insulation as his WRB. To eliminate the thermal bridge around their Alpen 525 Series windows, they chose ThermalBuck. Take a look at their recent ThermalBuck installation below.

ThermalBuck simplifies the installation of windows with all types of continuous insulation, making a truly high-performance building envelope.  To see additional installation steps, strength and installation challenges, view the ThermalBuck Installation page.


For an in-depth conversation about rigid foam types, we recommend the experts at Green Building Advisor. For additional videos of polyiso installation, we recommend this one by Synergy Construction.  Of course we would recommend the use of ThermalBuck over the plywood bucks, to limit thermal bridging and improve the performance of the window installation.

While written in 2010, this piece from Green Building advisor about Using Rigid Foam as a WRB, still has merit.  Updated in January of 2016, ICC document AC71 Foam Plastic Sheathing Panels Used as Weather-resistive Barriers establishes guidelines for evaluation of foam plastic sheathing panels used as water-resistive barriers in combustible construction. See your rigid foam manufacturer specifications for more information.  

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