How important is thermal mass for Passivhaus houses?

June, 2019|

The question of the importance of thermal mass increases as the potential for future overheating of energy-efficient houses becomes a growing concern. Overheating is definitely one of those potential ‘unforeseen’ consequences that gets a risk averse market rather twitchy. As a designer or specifier what is your view?

Some feel that mass is an essential ‘tool’ for managing summer heating, and increasingly so as the climate warms towards 2030 and onward. The houses we build now should have a minimum life of sixty years, taking us up to 2080. Will it be a 3 degree warmer world by then? One professor of climate science advises his students to prepare for a 4 degree warmer world in their lifetime…and in the light of this challenging prediction I offer a few thoughts on the role of thermal mass.

One suggestion from Warm, the Passivhaus certifiers, is to stress-test any new project for overheating by checking project performance using central London climate data using the PHPP software. Given the heat island effect, London will be warmer than most other UK locations (maybe typically by 2 degrees), so perhaps a good initial punt at a 2050 scenario. On a current PH15 project, this raised the overheating percentage (annual hours above 25 degrees) from 1.7% to 5%. That’s a big jump and looking at other projects, we can expect similar significant uplifts. If you had designed your Passivhaus project to perform at 5% – 10% overheating levels based on current climate profiles (and this is within Passivhaus permitted levels), then you really could be looking at very significant overheating periods in the near future – in fact these projects are likely to need retrofitting for cooling at some point in their lifetime.

So, how much help might you get in minimizing overheating from a heavy-weight construction choice (e.g. masonry)? The heat in the interior space could be stored (like a battery) into the material during the day, especially if the air is exposed to its surface, and then, once the interior air starts to cool at night, the heat can be released back out. This achieves a dampening effect on the normal diurnal temperature swings – helpfully reducing peak interior temperatures.

However, the impacts of thermal mass can be more complex than you might initially think (as most things are), especially when you have a building that has already been dampened significantly in its diurnal temperature swings through application of the Passivhaus fabric first approach. In a Passivhaus, the internal air will not cool at night through uncontrolled ventilation (as there is none!), so this process will rely on active night cooling behaviour being in place. A good example would be a window in tilt position, purging to a roof light. Should this purging behaviour not occur (or imagine a two week vacation), then the thermal mass will continue to store and store and store. How long might that heat take to release on your return? In this case, too much thermal mass could even exacerbate your overheating problems.

What might be important to decide is how much mass you actually need to achieve the desired effect. Testing advises a depth of circa 50mm will be where most of the storage transfer takes place, so a lesser depth of the right material can be just as effective as a 150mm concrete slab. In fact these more modest depths might be better suited solutions by not over-sizing your ‘battery’ storage capacity. An ideal material that comes to my mind would be clay plasters. (These are also good for attenuating moisture levels, optimising relative humidity levels – great for comfort and health). Some thermal mass is almost always proven to be helpful, but my sense from the studies done to date, is that a ‘less is more’ attitude could be the better approach, especially in the context of ‘fabric first’ dwellings. In order to provide some thermal mass there is certainly no need to revert to traditional brick and block construction. Brick and block, after all, involves using large amounts of materials with relatively high embodied energy levels.

A related impact on interior temperature is the thermal behaviour of the exterior materials, and the speed in which they transfer heat to the interior. The U-value calculation of thermal performance, does not consider all the behaviour differences of materials in this particular regard. I refer specifically to decrement delay, or the ability of some materials to delay the transfer of heat across a structure – a behaviour characteristic sometimes described as thermal inertia. A material with good decrement delay is wood fibre and the use of wood fibre as the final exterior layer (especially on a roof receiving the brunt of the midday sun) will give you this advantage – slowing down the passage of heat to the interior. With PH15 we add 60 mm depth. The impact of decrement delay is one of the reasons that we do not recommend using oil based insulation materials with timber frame e.g. SIPS panels. A low thermal mass construction like this, may achieve better U-values for less depth, but will not contribute any thermal mass or decrement delay advantages.

In conclusion, my sense is that the role of thermal mass is less significant in a Passivhaus where diurnal swings have already been significantly addressed. The most important design approaches are to avoid the summer heat reaching the interior at all (through appropriate glazing, decrement delay and fixed shading measures) and rigorous application of the fabric first principles. Higher thermal mass will typically lower the overheating percentage, although it is a more modest impact than many perceive, but it could also exacerbate heat retention. Given all these factors, I feel that a medium thermal mass construction solution such as PH15 (a timber frame alongside natural insulation materials like wood fibre and cellulose), is an appropriate specification. I would further consider including some additional thermal mass on the interior, using thinner depth finishes up to 50mm. Clay plaster applied in some areas (its not cheap) would be my current choice. We have also agreed, in-house, that we will set a protocol for stress testing all PH15 projects in order to advise more effectively around future overheating risks. I think this is now essential in the light of higher climate temperatures within the lifetime of our buildings.