Sustainability

ICF: Sustainable Construction Industry

When planning an eco-friendly home, understanding the true sustainability of construction materials on a whole-life basis is crucial. While some might initially question Insulating Concrete Forms (ICF) due to its concrete and steel components, a deeper dive reveals its significant environmental advantages. In fact, as advanced tools like the new Home Energy Model (HEM) come to the fore, ICF’s superiority in creating truly eco-friendly homes is set to become undeniably clear. Any lingering doubts about ICF’s role in a more sustainable construction industry will likely evaporate as traditional methods, such as Timber Frame and Cavity Wall construction, struggle to meet the stringent Dwelling Primary Energy Rate (DPER) requirements set by the HEM.

Cavity Wall Construction Issues

Cavity walls invariably incorporate a high Thermal Mass wall of either brick or dense rendered concrete blocks on the outside of the cavity to provide weather protection. However, the insulation is always placed in the cavity and the inner leaf is always of aerated concrete blocks or lightweight aggregate blocks that are essential for the thermal performance (adding to U-value), speed of construction, and easy services installation. As a result the internal thermal mass is relatively low, as the main dense mass is external to the thermal envelope.

Cavity Wall or ICF?

Heat pumps, particularly when aiming for high efficiency Seasonal Coefficient of Performance (SCOP) and comfort, need a stable internal environment and to take maximum advantage from their capacity to distribute heat at low flow temperatures over extended periods. The thermal mass inside the heated space is key to this.

With the primary thermal mass (brickwork) outside the insulation, the inner lightweight blockwork doesn’t offer much thermal buffering capacity within the conditioned space. This means the internal air temperature in such a cavity wall construction will be more susceptible to rapid fluctuations from:

Intermittent heating cycles (temperature drops quickly when heating is off).
Solar gains (temperature rises quickly when sun hits windows).
Internal gains (temperature rises quickly from people/appliances).

The external brickwork does absorb and release heat from the outside environment. This can sometimes mildly buffer the external surface temperature of the insulation, but its effect on the internal thermal comfort and heat pump efficiency is minimal because it’s outside the thermal envelope. It doesn’t contribute to storing heat from the internal heating system or internal gains.

This rapid fluctuation can lead to less effective load shifting in the heat pump than in a building with high thermal mass where the building fabric can be charged with heat during off-peak electricity hours or when ambient temperatures are more favourable for the heat pump operation (e.g., mid-afternoon). This stored heat would then slowly release when prices are higher or ambient temperatures drop.

With lightweight internal blockwork, there’s very little mass to “charge”. This limits the ability to perform load shifting strategies effectively, potentially leading to higher running costs if energy is consumed during peak tariff periods.

Because the internal environment is more volatile, the heating emitters themselves need to be very responsive. This means:

Oversized radiators: To compensate for lower flow temperatures and quicker response times.
Underfloor heating (UFH) with Screed: If the inner blockwork walls lack mass, a concrete floor screed (with UFH embedded) becomes the critical internal thermal mass element. The UFH can then warm this screed, which acts as the thermal battery for the space, moderating temperatures. This combination is often recommended for heat pump systems.
Careful Controls: More sophisticated controls are needed to manage the heat pump’s operation to match the building’s rapid thermal response.

HEM will be based on BS EN ISO 52016-1 and will identify cavity wall conctruction as follows:

Mass Distribution Class: A typical cavity wall with external brick and internal lightweight blockwork should be classified as a “construction with external thermal insulation” or “negligible mass” on the internal side, depending on the specific block density and internal finishes. This tells the model that there’s little to no internal thermal mass contributing to heat storage.
Areal Heat Capacity: The software will calculate the low areal heat capacity of the inner leaf (lightweight block, plasterboard, etc.).
Dynamic Response: The half-hourly simulation will then accurately show:
- Faster internal temperature swings: Rapid rise with solar/internal gains, rapid fall when heating turns off.
- Higher peak heating/cooling loads: Because there’s no mass to absorb peaks.
- Increased operating cycles for the heat pump: As it tries to maintain a tighter temperature band in a responsive building.

In Summary

Cavity wall construction, while thermally efficient in terms of U-value (with cavity insulation), is less inherently suited to optimize the dynamic performance benefits of heat pumps compared to constructions with significant internal thermal mass such as heavyweight blockwork inner leaves that will require a substantial thickness of External Wall Insulation to achieve the required U values in an eco friendly house plan.

As a result Architects will have to focus on:

U-values and Airtightness to minimize overall heat loss.
Providing Mass in Floors when using a lightweight wall construction probably combined with underfloor heating.
Overheating Mitigation using external shading and ventilation to manage internal heat gains.
Heat pump output and smart controls to manage the more dynamic internal environment.

Timber Frame Concerns

There is far less thermal mass in timber frame construction than in cavity walls and so exactly the same issues will arise because the Home Energy Model (HEM) and its advanced, half-hourly accounting will make it far more difficult to demonstrate compliance because their inherent low thermal mass means they respond differently to the dynamic thermal environment, and HEM will be much better at capturing these dynamics. This is going to shock a lot of people because a material made of foam, steel and concrete will provide a far more sustainable construction industry than timber!

Timber Frame or ICF?

timber-frame-construciton-compared-to-icf

Implications of HEM on Timber Frame

Under SAP, the monthly averaging and simplified thermal mass models often underestimated the overheating risk in lightweight (low thermal mass) buildings. Solar gains or internal heat gains that occur rapidly during the day in a timber frame home have nowhere to go without significant thermal mass to absorb them.

The half-hourly simulation in HEM will accurately show the rapid and significant internal temperature spikes that can occur in timber frame homes during sunny periods, even in winter, and especially in summer. So a factory made product, as ICF is, will result in a more sustainable construction industry.

If a timber frame building quickly overheats, it will show as an unmet cooling demand in HEM, or simply higher internal temperatures that impact comfort. This will mean that, to achieve comfort and compliance, timber frame homes will need to rely more heavily on:

Effective external shading: To block solar gains.
Enhanced natural or mechanical ventilation: To purge excess heat quickly.
Consideration of active cooling: Which increases energy consumption and reduces the DPER benefits.
Adding internal thermal mass: Designers may be forced to incorporate internal elements with higher thermal mass (e.g., concrete floor slabs on upper floors, dense internal blockwork partitions, or even phase change materials) into timber frame designs to help buffer temperatures, which adds cost and complexity.

So as summers get hotter, more and more thermal mass in the way of cement products will be needed for timber frame construction to survive. Thus timber frame construction is not going to contribute to a more sustainable construction industry as many imagined it would.

The effect on heat pump operation will be the same type of impact as for Cavity Wall construction, but even more so, because cavity walls do have some thermal mass in the lightweight concrete inner leaf.

In Summary

HEM will expose the inherent thermal characteristics of Timber Frame far more accurately with the result that:

Designers will need to be much more proactive in addressing overheating risk in timber frame homes.
There will be a need to incorporate some level of internal thermal mass into timber frame designs, or at least use heavyweight floor finishes.
Highly effective external shading, controlled ventilation, and meticulous airtightness will become even more critical for timber frame homes to meet the stringent DPER and DFEE targets of the Future Homes Standard and ensure occupant comfort.

HEM will essentially level the playing field in terms of thermal performance modelling, and low-mass construction will need to actively demonstrate how they manage the dynamic energy flows that high-mass constructions facilitates without a problem.

If timber frame is going to compete as a more sustainable construction industry the lack of thermal mass will need to be resolved in order to provide eco friendly house plans.

Positive Sustainability of ICF

ICF construction inherently addresses the very challenges that low-mass timber frame and externally-insulated cavity walls face with dynamic thermal modeling and heat pump synergy and is inevitably going to be part of a sustainable construction industry in the UK, providing eco friendly home design.

Furthermore, the new UK Home Energy Model (HEM), set to launch in 2025 alongside the Future Homes Standard, will significantly change how new homes are assessed for energy performance. Insulated Concrete Formwork (ICF) construction is exceptionally well-positioned to thrive under this new model for several key reasons.

Understanding HEM

Core Concrete Mass is Internal: ICF walls consist of a solid concrete core sandwiched between two layers of continuous insulation (typically expanded polystyrene, EPS). This means the significant thermal mass (the concrete) is located within the thermal envelope, on the internal side of the primary insulation to provide eco friendly home design.

Maximised “Thermal Battery” Effect: HEM’s half-hourly simulation, which explicitly models areal heat capacity and mass distribution classes, will accurately capture how this internal concrete mass:

Absorbs internal gains: Soaks up heat from the heating system, occupants, appliances, and especially solar gains, preventing rapid internal air temperature spikes.
Stores and Releases Heat Slowly: Releases this stored heat gradually back into the room as temperatures drop or the heat pump cycles off. This creates a much more stable internal temperature.

This stable thermal environment is ideal for heat pumps as it allows them to:

Facilitate load shifting: The concrete core acts as a substantial thermal battery, enabling the heat pump to operate during off-peak electricity times to store heat, which is then released later. This is a significant advantage for DPER calculations.
Run for longer durations at optimal efficiency: Reducing short-cycling.
Operate at lower, more efficient flow temperatures: As the thermal mass helps to distribute and retain heat.

As we wll know heat pumps are absolutely essential for eco friendly home design.

ICF walls, by their very nature (monolithic concrete core, continuous formwork), create an exceptionally airtight envelope. Air leakage paths are inherently minimized.

Benefit for HEM & Heat Pumps: HEM rigorously penalizes air leakage (as will the Future Homes Standard). The inherent airtightness of ICF will contribute significantly to a very low DFEE and DPER, as there are minimal uncontrolled heat losses/gains through air infiltration. This reduces the overall heating/cooling load, further optimizing heat pump performance and potentially allowing for smaller, more efficient heat pumps.

The EPS forms in ICF provide continuous insulation across the wall, largely eliminating the thermal bridging issues common in timber frame (through studs) and some cavity wall systems (through wall ties or mortar inclusions).

Benefit for HEM & Heat Pumps: This continuous, unbroken thermal envelope translates to lower U-values and reduces localized cold spots, leading to lower energy demands calculated by HEM. Lower energy demands mean less work for the heat pump, enhancing its efficiency and reducing running costs. The use of heat pumps is essential for eco friendly home design.

While highly insulated, ICF’s internal thermal mass is a key defense against overheating. The concrete absorbs excess heat, preventing the rapid temperature spikes seen in low-mass structures.

Benefit for HEM: HEM will show that ICF homes are less prone to overheating, leading to lower or no unmet cooling demands, contributing to a better DPER and enhanced occupant comfort and more eco friendly home design.

In Summary

In essence, ICF construction directly provides the “fabric first” benefits that HEM is designed to accurately measure and that heat pumps thrive on:

Significant internal thermal mass for buffering temperatures and load shifting.
Exceptional airtightness to minimize uncontrolled heat loss/gain.
Continuous insulation to achieve very low U-values and eliminate thermal bridges.

Therefore, as the Home Energy Model becomes the standard for compliance, ICF dwellings are very well-positioned to demonstrate superior energy performance, achieve excellent DER, DFEE, and DPER ratings, and provide an optimal environment for highly efficient heat pump operation.

Timber frame or conventionally constructed cavity wall buildings are going to find it very difficult to demonstrate that they are part of a sustainable construction industry.

Ready to design your project?

Let’s get your Home Design Plans approved and ICF ready!

Review our different design services in more detail and see how we can help get your project off the ground and built to the highest standard.

Our Design Services