27th Nov 2023 -

Improving the efficiency of existing buildings through retrofitting is important to reduce energy consumption. One of the most effective ways to upgrade energy performance in older structures is through proper insulation.

Foam insulation products provide an ideal solution for retrofit projects. Unlike fibreglass batts and other insulation types, foam boards and spray foams can insulate and air-seal difficult spaces in finished areas. 

Retrofitting with foam insulation allows you to improve thermal performance without damaging existing walls, ceilings or finishes. In this article, we explore the types of foam insulation best suited for retrofit projects.

We also look at how they can help make existing structures more efficient and comfortable. Whether it is a small project or a whole-building approach, foam insulation is the go-to solution for enhancing energy performance.

What is the purpose of foam insulation in retrofitting?

Adding foam insulation is a common method for retrofitting and weatherproofing older homes and buildings. There are a few main reasons why foam insulation is well-suited for retrofit projects. 

  • Fills gaps and seals cracks: Expanding foam filler adheres to surfaces, allowing it to get into hard-to-reach spaces in walls and lofts. It helps seal air leaks that contribute to energy loss.
  • High insulating value: Foam insulation provides effective insulation with less thickness needed compared to other types like fibreglass batts. This allows adding insulation without losing as much interior space.
  • Acts as an air barrier: Closed-cell spray foam insulation is air-impermeable, preventing drafts and further reducing energy costs.
  • Insulates irregular spaces: Its flexibility works well as foam pipe insulation as well as for other obstructions that would be difficult to insulate neatly with batt insulation.
  • Structural strength: Rigid foam boards add stability and improve structural integrity in retrofits.
  • Resists moisture: In particular, closed-cell spray foam is water-resistant. It does not easily absorb moisture or allow condensation to build. 
  • Long-term durability: Properly installed foam maintains its insulating value for years with minimal decomposition.

Types of foam insulation explained

The best type of foam insulation to use depends on the specific project. Here are the types of foam insulation which are best for retrofitting existing buildings.

1. Spray polyurethane foam

This is a two-part foam made by combining methylene diphenyl diisocyanate (MDI) and polyols. It reacts and works as expanding foam insulation to fill cracks and voids around windows and walls. Also, it comes in open-cell and closed-cell spray foam variations. Open cell is more flexible and breathable while closed cell provides higher R-value per inch.

2. Rigid foam boards

Rigid foam boards like extruded polystyrene (XPS) and expanded polystyrene (EPS) are useful for exterior retrofits. They can be installed on a floor and the exterior of walls to add a continuous layer of insulation. Rigid foams are moisture-resistant and provide good insulation value.

3. Injection foam

Foam like polyurethane or icynene can be injected into wall cavities through small holes to insulate and air seal. This creates retrofit cavity wall insulation without damaging drywall or finishes.

4. Spray polyisocyanurate (PIR) foam

This type of foam insulation spray is applied as a liquid. It expands 110 times its volume in place to form an insulating and air-sealing barrier. PIR has the highest R-value per inch of standard spray foam insulation. It also provides structural enhancement and is fire-resistant.

5. Foil-faced foam boards

Foil-faced polyisocyanurate or polyurethane boards are particularly useful for areas such as basement walls and other places where moisture may be an issue. The foil facing acts as a vapour retarder. This means it reduces the speed of water vapour travelling through the material.

What does foam insulation offer compared to other materials?

When it comes to retrofitting existing buildings for better energy efficiency, foam insulation stands out as a superior choice. Below, we detail how foam insulation outperforms its counterparts.

  • Air sealing: Polyurethane spray foam and injected polyurethane foam expand and cure in place. It seals any cracks or gaps and also prevents heat loss/gain from air leakage. Fibreglass batts and blown cellulose have no inherent air-sealing abilities, allowing drafts in a retrofit.
  • Contact insulation: Rigid foam boards like XPS and EPS form tight, continuous insulation levels with no gaps when installed on exterior walls or roofs. The closed-cell structure of polyurethane spray foams also minimises gaps. Fibreglass batts and blown cellulose can leave empty cavities and compressed areas.
  • Versatility: Spray polyurethane foams and injectable icynene foam can fill the smallest, hard-to-reach spaces in finished walls and lofts in retrofits. Fibreglass and cellulose often require extensive air sealing beforehand.
  • Higher R-value: PIR rigid boards provide up to R-8 per inch. Closed-cell spray foam provides R-7 or more per inch. The same thickness of fibreglass batts or blown cellulose provides less than R-4 per inch.
  • Vapour retarder: The closed-cell structure of polyurethane foams and foil facings on rigid boards serve as vapour barriers. Fibreglass and cellulose allow vapour transmission without an additional barrier layer.
  • No settling: Cured polyurethane foam and rigid boards maintain consistent, long-lasting thermal performance. Blown-in fibreglass and cellulose are prone to compression and settling issues over time.

How much does retrofit foam insulation benefit energy efficiency?

Foam insulation can make a big impact on a home's energy efficiency during a retrofit. Properly insulating with quality foam during a retrofit can pay back through energy savings in just a few years. You can maximise efficiency gains by meeting recommended R-values based on location.


  • Aim for R-13 cavity foam wall insulation in exterior walls.
  • This usually requires 3 to 4 inches of high-density spray foam or rigid foam panels.
  • Foam insulation provides a complete air seal and thermal barrier.
  • Significantly reduces conductive heat transfer through the wall frame assembly.
  • Prevents drafts for increased comfort and lower heating/cooling costs.
  • R-13 is the recommended minimum level for impactful efficiency improvements.


  • Reach for R-30 foam loft insulation or higher.
  • You will typically need 9 to 10 inches of spray foam or rigid insulation panels.
  • Ensures complete coverage and air sealing of loft space.
  • Minimises heat transfer through the roof and prevents ice damming.
  • A high R-value is critical since lofts can otherwise easily lose heat.
  • Properly insulated lofts can reduce heating and cooling costs by up to 30%.

Rim joists

  • Add R-10 to R-15 foam insulation. 
  • Foam insulation board sheathing or spray foam are the best options. 
  • Rim joists commonly allow air leakage and drafts. 
  • Foam air seals and provides an essential thermal break. 
  • It prevents conditioned air loss and uncomfortable drafts. 
  • Insulating rim joists can improve efficiency by 10% or more.


  • Use 2 to 3 inches of foam insulation on foundation walls. 
  • Creates an insulated thermal envelope in conditioned basements. 
  • The foam prevents the "cold basement effect" from concrete walls. 
  • Secures R-10 insulation for noticeable efficiency gains. 
  • Improves temperature control and comfort in basement spaces.


  • Use 1 to 2 inches of foam insulation on areas such as a garage wall and ceiling. 
  • Prevents heat transfer between conditioned house and uninsulated garage. 
  • Foam seals air gaps to prevent infiltration and energy loss. 
  • Provides an affordable way to improve garage comfort and efficiency.

How to overcome challenges with retrofit insulation

In the pursuit of energy efficiency through retrofitting, the installation of foam insulation stands as a key strategy. However, this process comes with its set of challenges. Below, we explore 8 specific challenges related to energy efficiency in foam insulation retrofitting and outline practical solutions. 

1. Inefficient thermal performance

Issue: Some areas of the existing structure may have inadequate thermal performance, leading to energy inefficiencies.
Target these specific areas with precision by utilising specialised techniques. For example, use injection foam insulation to address thermal weak points and ensure a more uniform and efficient thermal barrier.

2. Moisture-related energy loss

Issue: Moisture infiltration and condensation can compromise insulation effectiveness, resulting in energy loss.
Choose moisture-resistant foam insulation materials and conduct thorough moisture tests. You should implement ventilation solutions to manage and prevent moisture-related issues. This ensures the insulation maintains optimal energy efficiency.

3. Compatibility impacting energy efficiency

Issue: Compatibility concerns between foam insulation and existing materials may hinder overall energy efficiency.
Rigorously test the compatibility of foam insulation with existing materials to avoid any adverse effects on energy performance. Select materials that complement and enhance the energy efficiency of the structure.

4. Structural impact on energy consumption

Issue: Additional weight or stress from insulation may impact the energy efficiency of the structure.
Think about seeking help from a structural engineer to assess and minimise any potential structural impact. Also, you can opt for lightweight foam insulation options to maintain or improve energy efficiency without compromising the structural integrity. 

5. Code compliance for energy standards

Issue: Meeting energy efficiency standards may pose challenges during the retrofitting process.
Solution: Keep updated with energy codes and regulations. You should seek advice from others well-versed in energy efficiency standards to ensure compliance, aligning the retrofit with the latest energy standards.

6. Air sealing for enhanced energy efficiency

Issue: Ineffective air sealing can result in energy loss and decreased overall efficiency.
Solution: Prioritise meticulous installation techniques that emphasise foam sealant of gaps and joints. Conduct thorough post-installation inspections to identify and rectify potential sources of energy loss.

7. Minimise thermal bridging

Issue: Thermal bridging can compromise the overall energy efficiency of the structure.
Identify and address thermal bridges through strategic insulation placement and careful planning. Utilise insulation materials with low thermal conductivity to minimise heat transfer. 

8. Managing costs to maximise energy savings

Issue: The challenge of balancing upfront installation costs to maximise long-term energy savings.
Solution: Conduct a detailed cost-benefit analysis to determine the most energy-efficient approach within budget constraints. Explore available funding or incentive programs that align with the intention to achieve optimal energy efficiency and long-term savings.

Top-notch foam available at Building Materials Nationwide

We supply a wide variety of foam sheet and spray foam insulation products to meet any construction or retrofitting need. Our foam boards provide high R-values and moisture resistance for maximum energy efficiency.

We also carry spray foam kits that can air-seal and insulate cavity walls, lofts and other spaces. Foam insulation is the ideal solution whether you are building new or improving the efficiency of an existing structure.

Plus, we welcome our trade customers to open trade accounts. Take advantage of our competitive trade pricing, speedy UK delivery options and seamless ordering process. Whether you are a qualified contractor, builder or HVAC technician, we supply the best tools and materials for all new and retrofit insulation jobs.

Sign up online today to open a trade account or contact us with any questions.

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