Chances are if you’ve built new, your house can become uncomfortably warm in summer; here we look at why Irish homes are overheating and what to do about it.
In this article we cover:
- What is overheating
- Why Irish homes are overheating
- How to prevent overheating
- Best strategies to prevent it with tips for each
- List of less effective strategies
- Can MVHR provide cooling?
The growing issue of overheating in Irish homes, i.e. uncomfortably warm indoor temperatures, is largely a result of modern design approaches.
Ireland’s cool temperate climate typically wouldn’t suggest overheating as a concern. Pretty much every methodology for building design will describe Ireland and the UK as located in a cool temperate climate.
But even in Ireland, homes with large areas of glazing and unclear ventilation strategies are prone to heat buildup – particularly during summer months.
Passive house principles, when properly applied using accurate thermal modelling through the German Passivhaus Institute’s design software PHPP, offer a clear pathway to mitigate overheating.
By focusing on optimal building orientation, strategic use of shading, and ensuring effective natural ventilation, homeowners can minimise their reliance on active cooling systems. Internal heat gains, often overlooked, also play a role, and careful design of systems like domestic hot water can further reduce overheating risks.
Ultimately, prevention is key, and with thoughtful, climate responsive design that avoids energy-intensive cooling systems, the comfort of Irish homes can be preserved even as summer temperatures rise.
The problem
My hypothesis is that our regulations are now approaching metrics akin to the passive house standard, which we all want.
However, the issue is that most but not all five passive house principles are being implemented and PHPP, is not. PHPP is designed to provide accurate thermal modelling – for both space heating and overheating.
The passive house standards require that dwellings do not overheat (25degC or above) any more than 10 per cent of the year which is only 876 hours. Indeed, best practice within the passive house community is below 3 per cent, which is 263 hours or 11 days.
Having attended two of the last Selfbuild Extend & Renovate Live events with Southwest College and the Passive House Institute Ireland, and having conducted 1-2-1 clinics there, our experience is that most designs are presenting with significant glazing proportions coupled with an unclear ventilation strategy.
This equation is at the heart of this issue. I would be of a firm standpoint that active cooling systems here are only a last resort and with good quality design and best practice, the overheating potential can be eliminated.
The best way to prevent overheating is to adopt specific design strategies, in the following order:
- Optimise solar gains to reduce overheating
- Reduce internal gains
- Passive cooling
- As a last resort, other strategies including active cooling
Best strategies
Building orientation and wall to glazing ratio
Consider overheating from the beginning of the design process by optimising the building’s orientation. You will want to maximise winter solar gains and minimise summer heat. Combined with a reasonable wall-to-glazing ratio, this is the single most effective means of minimising overheating.
Ideally, orient the building north south with daylight-optimised glazing on the north façade and 15 to 25 per cent glazing on the south façade.
Because windows are much less efficient insulators than walls, at least four times less, best practice for residential buildings is to only have 15 to 25 per cent glazing, excluding frames, relative to the Treated Floor Area (which is the floor area of the rooms within the building that are heated); rooflights 10 per cent or less.
If north-south orientation isn’t possible, extra care is needed for east-west orientations to manage low summer sun angles in the morning and evening.
External Shading
External shading can reduce solar gain by 80 to 100 per cent. For this reason, in France for example, it is common to see external blinds as a feature of vernacular architecture. Throughout Europe, modern roller shutters on contemporary family dwellings are also ubiquitous.
Indeed, if you look at our own heritage buildings it’s not uncommon to find external shading. But it is definitely not a mainstay of modern construction, despite the fact that external shading systems are highly effective in reducing solar gain.
Fixed systems like brise soleil and overhangs are preferred as they require no movement or occupant action. When fixed shading is not suitable, deployable options like shutters, blinds, or awnings are effective, but less so.
Choose the most robust solution available. Consider automatic deployment for hard-to-reach areas but be aware that maintenance contracts for these systems can be difficult to arrange.
Maximise inherent cooling potential
Moving cooler outside air through the building mainly relies on opening windows, making them crucial for passive cooling. Key design considerations include:
- Window design and opening limitations: First off, maximise the openable area of the windows. Tilted windows have a smaller effective opening area compared to side-hung windows. Top-hung windows also don’t open as far as side-hung ones. Inward-opening windows allow for external shutters and insect mesh, while outward-opening windows can be affected by wind. The way a window opens, and its area, determine the actual free-flow openable area. Consider security and safety constraints, such as restrictors on higher windows, and remember that insect meshes, while useful, can restrict airflow.
- Cross-ventilation: Airflow from cross-ventilation (air moving from a window on one façade through the building to another window on a different façade) is much higher than airflow through multiple windows on a single façade. Design for cross-ventilation where possible, ensuring a clear path for air within the building. If this isn’t possible, or if privacy concerns reduce airflow, apply reduction factors in the PHPP modelling.
- Overnight ventilation: Most effective for cooling since outside air is coldest at night. However,
there are significant limitations to window ventilation at night, such as noise, security (especially if unoccupied), drafts, insects, and closed internal doors. Identify the windows and openable areas that can realistically be used for overnight ventilation as part of the design. Apply reduction factors where cross-ventilation may be compromised.
Minimise or reduce internal heat gains
One of the primary contributors to overheating is often the domestic hot water system, especially where there are long pipe runs. Appliances can also contribute to overheating. When in doubt, assume the worst-case scenario in the PHPP model. More on this topic in the Spring 2025 edition, including ways to reduce this source of overheating and heat loss.
Less effective strategies
Overshading from adjoining buildings
Using nearby buildings or structures for shading might seem smart, but it can be counterproductive. These structures often block low winter sun while allowing more high summer sun through. As a general guideline, buildings should be spaced at least 1 to 1.5 meters apart for every meter of height.
Internal Shading
At best, internal shading will reduce overheating by 40 per cent. The figure is usually much less. Internal shading also reduces cross ventilation through the window and relies on occupant behaviour,
meaning it is ineffective if not properly deployed. Internal shades work by reflecting solar radiation back out through the glass, so for best results use reflective or bright white blinds close to the window.
G-value
A lower G-value on the glazing reduces solar gain but this affects both winter and summer gains, so the reduction in overheating must be balanced against lower winter gains. Specialist solar control glass with G-values below 0.3 might seem appealing, but it is costly and usually unnecessary unless there is excessive glazing. It also results in poor daylighting and can still lead to overheating. Therefore, very low G-values aren’t usually an effective way to compensate for too much glazing.
What about MVHR?
A Mechanical Ventilation with Heat Recovery (MVHR) system can provide some cooling through air circulation, but it will be significantly less than what open windows can achieve, making it a secondary mechanism for passive cooling.
Even in boost mode, a typical MVHR system in a house designed to the passive house standard achieves only around 0.5 ACH (Air Changes per Hour). Without summer bypass, the MVHR system provides almost no night cooling as the cooler incoming air is warmed by the outgoing air.
In a passive house building, MVHR can still however help reduce overheating risk. The summer bypass mode ensures that when the outside air is cooler than the indoor setpoint and the internal temperature is too high, cooler external air is brought directly into the building.
When window opening is not possible due to noise, pollution, or security, the MVHR system may need to provide all passive cooling.
If more cooling is needed, it might be feasible to increase the MVHR size to deliver more airflow during summer. However, this must be balanced against the impact of larger duct diameters and a potentially larger and more expensive MVHR unit, while still meeting Passivhaus noise criteria. Other ventilation systems which I would advocate are demand controlled ventilation systems.
