What is the Best Insulation for External Walls?

Proper insulation of external walls is crucial for maintaining a comfortable and energy-efficient home.  However, selecting the right type can feel overwhelming.

In this article, we’ll explore various types of insulation and their benefits. Read on to find out what fits your home.

Types of Insulation Materials for Exterior Walls

Fibreglass Insulation

Fibreglass insulation is made by spinning or blowing molten glass into fibres, with most manufacturers incorporating up to 40% to 60% recycled glass content. 

Advantages of fibreglass insulation include:

• Wide availability
• Ease of installation
• Lower cost compared to other insulation types

However, it does have some drawbacks:

• Allows more air infiltration than rigid foam boards unless installed meticulously
• Fibres can settle over time, creating air pockets that reduce effective R-value
• Minimal resistance to moisture

Fibreglass works best for wood or steel framed walls in new construction when wall cavities are accessible before drywall installation. It’s also viable for insulating older homes during renovations that involve opening up existing walls.

Fibreglass batts are the most popular residential exterior wall insulation. This popularity stems from the balance of good insulating performance and affordable pricing.

Mineral Wool

Mineral wool insulation is formed through spinning rock fibres into batts or rolls. It usually has a higher density than fibreglass.

Advantages of mineral wool include:

• Better fire resistance than fibreglass
• Greater resistance to moisture
• Does not settle as easily over time

Disadvantages include:

• More expensive than fibreglass
• Requires precise installation to prevent air infiltration

Mineral wool batts provide good thermal and sound insulation performance for exterior walls. Their density helps reduce air movement through the insulation.

Rigid Foam Boards

This category includes insulation boards made from materials like extruded polystyrene foam (XPS) or polyisocyanurate. Rigid foam board is the most efficient as continuous insulation on the exterior side of walls. They minimise thermal bridging through framing.

Key advantages are:

• Does not allow air leakage
• Can be used continuously on exterior walls without thermal bridging
• Higher R-values per inch than other options

However, drawbacks include:

• More costly than fibreglass or mineral wool
• Requires precise installation without misaligned joints or compression

Rigid foam boards create an extremely effective insulating and air barrier for exterior walls when installed properly with tight seams. The lack of air infiltration maximises the insulation performance.

Spray Foam Insulation

Spray foam insulation is a unique product that is applied on-site, conforming to the surface where it is sprayed. It creates a tight building envelope which enhances energy efficiency by insulating, sealing gaps, and forming air, moisture, and vapour barriers. 

Advantages of spray foam include:

• Excellent insulation and air sealing abilities
• Prevents any air infiltration or moisture movement
• Structural enhancement by strengthening the building envelope
• Effective in irregular shapes and hard-to-reach cavities

The downsides are:

• It is typically the most expensive insulation option
• Professional installation is required
• Expanding foam can overfill and distort wall cavities if not applied properly

Spray foam provides superior air-sealing and insulation performance for exterior walls when installed correctly by trained professionals. 

It maximises energy efficiency and comfort by eliminating any air leakage through the insulation. Spray foam essentially transforms exterior walls into an uninterrupted insulation and air barrier system.

Blown-In/Loose-Fill Insulation

This category is a versatile type of insulation suitable for various applications like attic, basement, and ceiling insulation.

Blown-in insulation works best for retrofitting insulation in existing exterior walls by drilling access holes. It can also effectively insulate irregularly-shaped cavities and hard-to-reach areas. However, the insulation must be dense-packed to prevent settling and maintain effective air sealing over time.

Understanding Insulation Performance

The effectiveness of insulation materials is primarily measured by their R-value which represents the resistance to heat flow. The higher its R-value, the greater the insulating power. 

However, other factors like air sealing ability, moisture control, and proper installation impact overall insulation performance.

R-Value Comparison

The table below shows the typical R-values per inch of thickness for common insulation materials:

Air Sealing and Moisture Control

In addition to R-value, preventing air leakage through the insulation is crucial for energy efficiency. Air barrier materials like rigid foam boards and spray foams provide superior air sealing compared to batts or loose-fill insulation. Proper installation techniques are also important for all insulation types.

Moisture accumulation within insulation can degrade its performance. Vapour retarders, like faced batts or spray foam, help control moisture movement. In humid climates, vapour permeable insulation options allow drying to the exterior.

Other Performance Factors

Other considerations include fire resistance, environmental impact, and ease of installation when choosing insulation. For example, mineral wool, cellulose, and foam insulations offer increased fire protection compared to standard fibreglass batts.

While R-value is the main insulation metric, these additional factors related to air sealing, moisture management, and product characteristics impact the overall real-world performance of different insulating materials.

Choosing the Best Insulation for Your Needs

Deciding on the best exterior wall insulation depends on your specific project requirements and goals with several insulation material options available.

New Construction vs Retrofit

For new home construction, blown-in insulation like fibreglass or cellulose can easily insulate open wall cavities before drywall installation. Batts like fibreglass or mineral wool are also common choices in new builds.

Blown-in loose-fill or spray foam are ideal options for filling enclosed wall cavities through drilled holes or by removing exterior siding/interior drywall when insulating existing homes during renovations or retrofits. Rigid foam boards can also be added externally during re-siding projects on existing homes.

Climate and Regional Factors

The local climate is an important factor when selecting insulation R-value. Higher insulating values from spray foams or rigid boards can maximise energy savings for colder climates. In humid environments, vapour permeable insulation that allows drying to the exterior may be beneficial.

Cost-Effectiveness and Installation

Upfront insulation costs vary considerably – fibreglass and cellulose are typically the most affordable materials, while spray foams are premium choices. It’s wise to calculate potential long-term energy savings against the installation costs for each option.

The installation method itself also impacts the insulation decision. Fibreglass batts are relatively DIY-friendly, while spray foams require professional contractors. Blown-in installation often necessitates hiring insulation experts as well in existing construction.

Environmental Impact Considerations

Insulation with recycled content like cellulose or mineral wool may be preferable for homeowners prioritising environmental factors. Products with low embodied carbon like fibreglass could be considered over petroleum-based rigid foam boards and spray foams.

Conclusion

There is no one-size-fits-all solution when it comes to the best insulation for external walls. The optimal choice depends on carefully evaluating the specific requirements of your project and climate.

Proper installation following manufacturers’ guidelines is vital for realising the full insulating potential regardless of insulation type. Even the highest R-value insulation underperforms with poor installation allowing air leakage and moisture issues.

The best approach is consulting insulation professionals to assess your project needs, regional climate data, and cost-benefit analysis for the various insulation options. You can select an insulation system that delivers superior energy efficiency, comfort, and long-term value for your home’s exterior walls with their expertise guiding your decision.