Preventing summer overheating
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Vicki Cowan
Preventing summer overheating
Blog, Cooling, Design, Internal Enviromental Quality, Ventilation
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It’s an irony that as we make advances in insulating the thermal envelope to keep homes warm over winter, the need to keep homes cooler in summer is also growing. Changing climate is meaning hotter summers and combined with today’s airtight and well-insulated new homes, summer overheating is becoming an increasing problem.

Modern lifestyles, where homes are often shut up during the day, mean that coming home to a hot, overheated house is common.  Once the house has become too hot, it’s much harder to cool down without using electricity.

There are two simple rules to prevent summer overheating, yet they often prove harder to put into practice:

  1. keep the sun out in the first place; and,
  2. create air movement through good cross and stack ventilation.

Harder to achieve in practice

Two Beacon homes are good examples: both performed excellently in every other way but struggled with summer overheating.  Both homes were well insulated, double glazed and designed for passive solar heating.

At the height of summer, the Waitakere NOW Home became too hot, especially in the evenings, with a significant amount of time spent above 25°C. The HomeSmart Home also overheated in summer, particularly upstairs where it was difficult to sleep at night at the hottest times.  Both had some elements for summer cooling: opening windows, passive venting, solar powered stack ventilation, overhanging eaves; yet these elements were not enough to keep the homes cool.  So what could we have done differently?

The Waitakere NOW Home had limited cross ventilation and the windows generally remained closed during the day.  Trickle vents would have been ideal in this house, together with security stays to allow the windows to be left open at night, and better shading on the northern face.

The layout of the HomeSmart Home did not address how to best pull cool air into the house and the location of the stack vent meant it had little or no impact on hot upstairs bedrooms.  Northern shading, passive vents, and layout to encourage both cross and stack ventilation, would improve this home’s summertime performance.

Rating systems need to address overheating

Both the Waitakere NOW Home and the HomeSmart Home received 8 Star HERS ratings.  It appears that in awarding points for well-insulated thermal envelopes, our rating systems place less emphasis on factors which would enable summer cooling.  Neither HERS, ALF or Homestar deal successfully with the potential to overheat.  Yet the alternative is mechanical cooling which brings its own problems of summer energy draw.


Tips for dealing with Overheating

Keep the sun out

Prevent overheating in the first place by keeping the sun out during the hot parts of the summer day – over the middle of the day in north-facing rooms and later in the day in west-facing rooms.  Try:

  • deep eaves on the northern side (calculate these here
  • deep verandahs or shades on western sides
  • minimising west-facing glazing.
  • adding shades to northern and western windows
  • using thick lined curtains or blinds, drawn to keep the heat moving into the room, and with an open window behind so the heat can escape – better than nothing for existing homes.

Create air movement

Air movement increases cooling by raising evaporation rates. Passive ventilation creates air movement even when the house is closed up, through cross and stack ventilation. Cross ventilation happens when cool air from outside enters a building and forces warm air out through an opening such as a window or door. Stack ventilation happens when inside air warms, rises and escapes through any gaps at the top of the house, drawing in cool air from gaps at the bottom of the house.  This pull is called the stack effect because it is the same process that draws smoke up a chimney.  Using the stack effect to ventilate homes is the most effective way of keeping a house cool in summer.  Try:

  • small high windows (e.g. clerestory windows) which can be left open
  • side opening windows – better at pulling breezes into the house
  • frames with in-window passive vent systems
  • fitting security stays to ensure windows can be left open on all sides of the house
  • solar or wind powered ventilation systems –located in a high point in a home
  • ensuring windows can be opened on all sides of the house and that there are no barriers to airflow within the house – this could mean including an air grill above internal doors so that cross ventilation can still occur even if doors are shut.


  1. This is a standard text the Fred and I drafted on ventilation – very similar to Lois’ advice, but perhaps directed to those who are considering installing mechanical ventilation:

    Ventilation and heat transfer systems
    These are two topics and they are interconnected. Ventilation can be passive or active. A well-designed house with bathroom and kitchen extractor fans ventilating outside achieves adequate ventilation passively, i.e. without the need for mechanical assistance. Active (mechanical) ventilation involves energy consumption and the maintenance and eventual replacement of filters and moving parts. A well-designed house should have no need for extra measures to reduce condensation – the temperature and humidity should never reach the point where condensation will occur. Passive ventilation costs nothing to run but needs some management by the occupants. Older houses often have uncontrolled ventilation through cracks and gaps. But in a modern airtight house ventilation needs to be planned for.
    Options for passive ventilation include:
    o trickle vents in window frames,
    o security stays that allow windows to be safely left open when the house is unoccupied,
    o tilt-and-turn windows, much used in Europe, that can be left open in a secure ventilation position (top tilted inwards) or fully opened like a casement window,
    o planning for each room to have openable windows in more than one wall so that summer breezes can provide free cross-ventilation.
    o flyscreens where flies are a problem
    You may need to mechanically duct some of the warm air from the heated areas into other areas as required. This would require a mechanical heat transfer system. These are cheap to run and can also be configured to provide active ventilation of stale air to the outside. For energy efficiency such a system needs to include a heat exchanger to extract the heat contained in the exhaust air, which would otherwise be wasted, and reuse it within the house.
    For a discussion of the relationships between climate, temperature, insulation, thermal mass, moisture, humidity, and condensation, see

    • Hi Eion I largely agree with your advice, but I think the “mechanically duct warm air from the heat areas into other areas as required…” is a little confusing – I’m not sure how this will prevent overheating. I have found that to retrofit for overheating – or to ensure overheating doesn’t occur in new houses, then you need to do quite a lot of specific design for summer – which is not well catered for in our modelling problems. The Waitakere NOW Home and the HomeSmart home meet all the standard suggestions for preventing overheating – yet they overheat badly – and in the HomeSmart Home’s case, very badly. We know also that generally highly insulated homes – even in places like the UK and Norway (!) overheat and there is now quite a body of research on the problem – and less on the effective solutions. So this is something we need to get a lot better at managing. Probably I should talk about fans a bit – a seriously under-rated device – fans reduce the felt temperature by about 3 degrees – and use very little energy compared to a heat pump on cool (or I suspect a whole house mechanical ventilation system).

      • Hi Lois. Very much enjoyed your article.

        I’d like to extend two issues touched upon – fans and thermal assessment/rating tools. Firstly, I also think (ceiling) fans are hugely under-rated in NZ as effective cooling devices too. I suspect this is largely due to many people still having those old-style, ugly looking ones in their minds when making a mental picture. However, the modern ones are a far cry from this in aesthetics and performance. They are:
        – extremely quiet in operation, while still providing sufficient air movement for significant cooling (up to 3 degrees)
        – can be linked to sensors so that they are only on when the room is occupied, therefore ensuring minimal energy use
        – can be effectively used in conjunction with open windows/doors, unlike heat pump units
        – are far cheaper to purchase, maintain and run than heat pumps
        – can operate effectively using as little as 18W (yes, I haven’t dropped a zero off the end of that figure).

        However, they do require a little more thinking in their placement and use, than heat pumps, as they are limited to effecting the occupants within their immediate vicinity (i.e. under their ‘wash’). This means that if the room in question overheats severely, then even a well placed ceiling fan will not be able to provide the comfort probably desired. However, for a well (i.e. all-year-round) designed home, having a ceiling fan in the worst effected rooms should suffice – even in Auckland/Northland. Also, because ceiling fans work by increasing the skins ability to evaporate perspiration rather than cooling the air, their location within the room is everything. Taking a large bedroom as an example, the fan should be placed directly above the bed for maximum effectiveness, rather than at the center of the room itself. One last point – their airflow efficiency should high – there are still many low quality products out there.

        Regarding the inability of thermal and rating tools (such as HERS, ALF or Homestar) to deal successfully with overheating issues… This is a topic that has been discussed at length in the development of each of these tools and is not easy to fix. Some issues include, that:
        – overheating is almost always a ‘room’ issue, rather than a ‘whole house’ issue, so is outside the range of whole-of-house thermal assessment tools such as ALF and its derivative used within Homestar (which, as you know, only focuses on the winter season anyway).
        – Homestar always saw winter-time underheating as a more important issue for the existing NZ housing stock initially, so that is where emphasis went
        – over-heating is very dependent on occupant behaviour, even more so than under-heating. Standardizing any behaviour issue (i.e. the opening of windows for cross-flow ventilation, the drawing blinds etc) into a tool is always going to be problematic due the wide operational differences between say, a ‘stay at home mother with two kids’ and a ‘young couple who are out at work all day’
        – there are external environmental issues which are hard to quantify within even sophisticated rating tools, but may be significantly impacting. Examples include: nearby foliage which changes every year or nearby swimming pools which cool incoming air on less humid days.

        These factors plus others mean that even the most sophisticated thermal assessment tools used today (such as HERS AccuRATE, which refers to the human psychrometric chart to establish comfort boundaries) only are able to provide indicative overheating-related information.

        Sometimes, the KISS ideal is best – and I support the first of your two simple rules Lois by employing SketchUp. This program can easily and quickly ensure that by simple visualisation during the warmer months, direct sunlight is kept out of the internal spaces through the use of smart external shading devices and foliage.

        BTW: all views expressed here are my own, and may not reflect my employer, BRANZ Ltd.

        • Hi Roman, just read this. I totally agree re fans. As you know, I live in a very hot location and summer overheating issues are complicated by us also having a hot north west foehn wind which blows during summer (it’s really roaring now!) which means opening windows can at times make it hotter inside as the hot wind blows in. But fans are our salvation. I guess my criticism of models is a result of incredible frustration – constantly seeing stupidly designed houses (ie lots of west facing glass) that do quite well in the thermal models – or in the case of the HomeSmart Home it did spectacularly well (got a HERS 8) but was so hot in summer, with all the windows open, that the family had to sleep downstairs – and they came from Fiji, so might have a predisposition to being able to cope with warmer temperatures. I don’t think overheating in that house was a “room issue” – it was a whole house issue and one that I see a lot with modern two storey houses. I know overheating in highly insulated houses is a global problem (I’ve been to presentations on the problems of overheating in Passiv Haus properties in Denmark!) but I think for the models to have any credibility in a warming world this can’t be put in the too hard basket.

  2. Hello Lois

    You said that in the HomeSmart house the location of the stack vent meant it had little or no impact on hot upstairs bedrooms. Are plans of the house (on Beacon’s website, for example) where I can see where the stack vent is located in relation to the bedrooms in order to understand what happened?

    With regards to your shading recommendations, I would add vertical louvres, preferably on the exterior side (not easy if the windows open outwards as they normally do in NZ.

    In my experience, with deep verandas (as you suggest) you may have to add pull-down or fixed vertical shading perpendicular to the windows to the ends of the veranda, more so if there are windows close to the ends of the verandah.

    With regards to Eion’s comment about a heat transfer system, it may be beneficial to transfer heat from upstairs to downstairs in winter in two storey houses. The upper floor is unlikely to have thermal mass to absorb excess heat from solar gains and most two storey houses do not have any doors to stop heat from naturally flowing upstairs.

    A difficult case I came across was a west facing upstairs bedroom facing what the homeowner considered to be a noisy street so windows remained shut.

    • Hi Adriana I think the plans are in one of the reports, but not easy to read. Essentially the stack vent was located at the top of the stairs. The front door was at the bottom of the stairs. So they would leave the front door open and the air would move upstairs – but not into the bedrooms which were off the hallway next to the stairs. Also the window openings in the upstairs bedroom were quite small because there are security stays on the windows – the windows could only be opened about 15 cm max (awning windows).

      Re the heat transfer system I’m clear on their role in winter – I’m just struggling to understand how they can be used to cool a house in summer.

  3. Its been suggested to me, and it seems logical, that recessed lights are not only heat loss chimneys in winter but the source of considerable heat gain in hot weather. I’m still trying to find time to post details on my current learning curve about developments re LEDs etc driven by the fact my daughter has just bought a modest house in Auckland with 14 rececessed lights. That’s unless there has been the definitive document produced on this topic. Depending on how they experience this summer, excess heat could be the third driver for replacing these with IC LEDs.
    Cheers, Norman Smith

  4. Can you please clarify what ‘awaiting moderation’ means via-a-vis comments I made earlier on other topics which appeared to have been immediately posted and available. Thankyou, Norman

    • Hi Norman
      Comments on blogs are moderated (at this point). I.e. they require approval. The comments in the forum are not (as it was anticipated this would hold discussion up too much). These settings are our defaults and can be reviewed if need be. What are your thoughts thus far?

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