Summer 2013 - has the house kept its cool?

I have been asked this question throughout the summer. Last summer was so cold that we were not able to assess how well the building performs in hot weather.

Unfortunately, we don't have data loggers or similar equipment to provide lots of quantitative data. That said, July was our warmest month with temperatures in the high 20s (Centigrade) and from memory hitting 30C on one or two days. The house remained comfortable thoughout nearly all of that period within internal temperatures between 22C and 25C. We did have one day when the temperature in my home office (which has a PC with two monitors, one of which is quite old and gives off significant heat) reached 27.5C and it did feel a bit too warm. Other rooms in the house felt quite comfortable throughout.

Our cooling strategy

Site constraints meant that we needed more east and west glazing than one would have in an "ideal" Passivhaus. This meant needing to use external blinds or similar, not an overhang type shading device which is possible on a south facing window because the sun is lower in the sky in morning and evening. So....

1. We chose to fit Internorm windows with integral blinds - see below - this was a solution specific to our local circumstances, as the window is not a true triple glazed window, consisting of a sealed double glazed unit, and internal venetian blind and a third pane on the outside of the blind: 2 + 1. The blind is in a ventilated space. This means that they have a whole window U-value of only 0.93W/m2K, less than would be needed in many parts of the UK. Internorm do a 3 + 1 window, with a lower U-value but it is considerably more expensive, very heavy and they use Krypton, which is best avoided (more expensive/rare and - when it leaks out as it will after about a decade - results in a more greater drop in performance than Argon-filled units. All the windows on solar gain facades have high g-value glazing (60%), so they will capture 20% more heat than typical 50% g-value triple glazed units. This means that remembering to use the blinds is key to keeping the house cool. This can be solved by using automation but this seems like a step too far in complexity in a domestic situation; although perhaps it is a good idea in an office building.





2. We have to sliding pation doors, one on the east and on the west facade. These are "traditional" triple glazed. We planned two pegulas to provide seasonal shading on these two quite large glazed components. Unfortunately, we haven't got the pergulas in place yet, so we did get more solar gain than is ideal. I don't know how much impact this is having on the overall summer performance of the building. When I modelled it in the PHPP, it does affect the frequency of overheating and, had the pergulas been completed before this summer's not spell, we might have avoided the slight overheating on a day or two in July.
3. We do of course open our windows in the summer and I tried to keep the windows closed in the hot portions of the day (to help keep the heat out) and to make sure the windows were open at night. I think that if we were located in London (where night time temperatures don't drop as much), night time flushing would have been much less effective.

During the warm spell, I experimented with swiching off the MVHR at certain times of the day. Our intake (and exhaust) terminals are on the west facade wall. During the morning, when the west is in shade, I noticed that the MVHR seemed to help to maintain a lower temperature, as it was drawing in cooler air from outside. In the afternoon, it was drawing in warmer air and this possibly caused internal temperatures to warm more than might otherwise been the case. I tried switching off the MVHR in the afternoon but this seemed to make it feel less comfortable in the afternoon. I think that the reduced air change rate may have resulted in higher internal relative humidity levels, adding to thermal discomfort.

More generally, we have noticed that the house is more sensitive to internal heat loads than a typical UK home. Some of this is modelled in the PHPP but, in real world use, occupants may bring older electrical equipment, which will add to overheating risk. In my case, the old computer screen does give off significant heat and, although it is smaller than the newer one, uses more electricity. I don't think, however, that it makes economic or ecological sense to replace a perfectly good computer monitor.

Lessons learnt from this summer

1. Keep the MVHR running. Although it provides no cooling function (a common misconception amongst some Passivhaus sceptics), the regular air changes it delivers moderates internal relative humidity.
2. Use the blinds.
3. Consider the impact of internal gains - heat emitting devices will have a surprising impact.

We get our Passivhaus Certification

This week we had a little ceremony to mark getting our formal Passivhaus Certification. Rob Hopkins, of the Transition Network, kindly came to handover our newly gained certificate.
 

We have now been in the house just over two months and we are "looking forward" to a decent winter, to give the house a bit of a test. We are probably going to want to make some adjustments to our ventilation, once we have started using it to transport the small amounts of heat around the house. More on this in a future post.

From left to right: Janet Cotterell - Passivhaus architect, me - Passivhaus energy modelling (and client), Jonathan Williams - Passivhaus builder, Joe Bellows - one of the Passivhaus contractor team, Peter Warm - Passivhaus Certifier.

Living in a Passivhaus, the first few weeks

We have been living in the completed Passivhaus Home since 19 August and are really looking to seeing how the house will perform this winter. So far, the temperature internally has been pretty constant. The only time we had some overheating was over the weekend of 10th and 11th September, when we took part in the Open House event. We had over 150 people visiting and there were a lot of people in the house adding to the heat gain noticeably. It was warm, humid and windy outside (unusual weather) and we found that the tilt and turn windows, some of which we had tilted open to provide extra ventilation and cooling kept blowing closed in the wind.

We are still tweaking the settings on the MVHR unit (heat recovery ventilation unit). The MVHR is designed to recover the heat from the old, outgoing air and give it to the new, incoming air. Of course, during warmer summer weather, this is the last thing you want. So MVHR units are designed not to recover any heat when this is not needed.

The heat recovery was kicking in when it was not needed. We have tweaked the settings so that it only comes in at 18C (the minimum temperatures allowed) and turns off if the temperature reaches 20C. The picture above, which was taken a few minutes ago, shows that the summer bypass is enabled, which means that it isn't recovering heat - a bit counter intuitive.I must say that the user interface needs a little improvement. There's definitely a little product development work for Paul here. I feel that this is an often ignored part of a product's design. An impatient user might have decided just to switch the unit into standby until the weather got cold enough for the heat recovery to be needed, which would have defeated the purpose of the system. That said, once set up correctly, we shouldn't have to fiddle with this again.

Our hot water system has an even less user-friendly interface. Although this is not a Passivhaus specific issue. I think Passivhaus buildings need as much thought to go into how easy they are to use as goes into the design and construction of the fabric. Although not a problem for us and probably for other early adopters in the UK, it will have an impact on the way Passivhaus is perceived.

Certification airtightness test

Note: since posting this, the completed air test documents show a result of 0.2 air changes per hour.

It's been a while since I've posted during the always-longer-than-the-client-expects period where all the finishing work is completed.

We have started the process to get Certification as a Passivhaus, using the UK's top certifier and general guru on all things building physics related, Warm Low Energy Practice. Yesterday, Paul Jennings ("doorfanman"), our airtightness consultant completed the official airtightness test and we got:

0.19 air changes per hour!

This is a really big improvement on April's test. Partly because I calculated the internal volume more accurately, using Bluebeam, which has a great tool for measuring dimensions, areas etc of PDF plans. Our internal ventilation volume is 442.38m³ and the April test had been based on a rougher calculation of 400m³. Mostly though, the information from the April test helped us to to improve the weak areas.

It will be interesting to see how durable our result is. We believe we have created an airtightness layer which lasts.

I had already put up a DIY shelf in one of the rooms before yesterday's test was conducted, which required drilling a few holes into the plaster than forms the airtightness layer in the external walls of the refurbished part of the house. I put some sealant into the holes before inserting raw-plugs. Obviously, I will be aware of the need not to drill holes in the original external walls without thinking about airtightness. In the new build section, where the airtightness layer is beneath a 50mm service void, I will not have to worry nearly as much about affecting airtightness. For this reason, in any new build, I would always want to design in a service void. I think that, in the real world, it stands a much better chance of remaining intact over the design life of the building.

Next week, our MVHR (heat recovery ventilation unit) will be commissioned. This will involve, amongst other things, calibrating the airflow to each supplied room (and from each extracted room) to:

a) ensure that there is an appropriate air change rate for the use of the room
b) ensure that sufficient heat is delivered to the rooms (the tiny trickle of heat needed in a Passivhaus is delivered via the supplied air in the ventilation system)
c) ensure that the total rate of air supplied equals the total rate of air extracted. This is important because if the two are not in balance, the MVHR wastes electrical energy and because it results in the house being slightly over or under pressure, it will increase the flow of air through the fabric of the building.

April update

We have spent the last month continuing to tweak and refine the design. It seems to be what we were doing last time I posted but the process is iterative and a few days ago we got to the point where we felt we could submit our planning application. Window placing and sizing, both hard to change after planning, have a big effect on the performance of a Passivhaus, so we had to be sure this was right before submitting planning.

Next, we are working out where all the ductwork for the heat recovery ventilation (MVHR) will go. The MVHR is a small unit, typically a metre and a bit square by half a metre. It needs to be located very near an external wall for maximum energy efficiency and, although barely audible, away from bedrooms and living areas. In our refurb, it will be going into the loft. The principle of MVHR is to extract stale air from kitchens, bathrooms and WCs through extraction ducts back to the MVHR unit where it passes through the incoming air whilst remaining physically separate. In this way, most of the heat (at least 75% in a Passivhaus) is transferred to the fresh incoming air. This new air then supplies the living areas and bedrooms through a separate network of supply ducts. Air is moved very slowly through large, smooth sided ducts, with silencers between rooms to eliminate perceptible noise. Choosing a good MVHR unit, designing the ducts and the room valves, and configuring the whole system once installed are vital parts of achieving a Passivhaus. The MVHR ensures that the internal air is always fresh, even when the windows are closed during the heating season. It means that the building is continually being aired - something we normally have to do briefly as a daily ritual. It gives a much better indoor air quality than virtually all other modern and many older buildings without sacrificing the building's energy efficiency.

The other great thing about using MVHR in a Passivhaus is that you can use it to distribute heat around the house; but only if your building needs very little heat. The air can only carry a very small amount of heat because the air moves through the ducts quite slowly (to avoid wasting energy or creating air noise by running the fans at a higher speed) and because the air can only be heated to about 30C above room temperature. These physical constraints translate (once you do the physics) into a maximum of 10W/m2 maximum heat load that can be conveyed by the ventilation system. For a typical UK home of around 100m2, 10W/m2 translates to 1000W (or 1kW). Compare this with the output of many UK gas boilers: 25kW or more. If your building can be heated with such a small heat input, you can use the MVHR as your heating system as well as your fresh air system, saving upfront capital costs and reducing on-going maintenance. In fact, an MVHR is a very simple system, not to be confused with heat pump systems or air conditioning systems. The only regular attention it needs is a change of air filters, something you can do yourself - there's no need for any servicing. The units use very little energy, many times less than they save.

Floor wall thermal bridge, the plot thickens
We have revisited the floor-wall thermal bridge problem. In a new build it is possible to design out thermal bridges but in our refurb, there is no economic way to do so. When you get a building as energy efficient as a Passivhaus, any remaining weaknesses, such as thermal bridges, can become quite significant sources of heat losses. In the PHPP, it is possible to include figures for the thermal bridge losses, however the calculations you need to get the figures are complicated. One good reason to design them out if you can.

This time I used another free, if infuriating, piece of software called Therm, which is intended for window designers but which can be adapted for other purposes. It allowed me to calculate a "U-factor" (as distinct from a U-value) for the external floor-wall junction, something which Heat 2, which I mentioned in an earlier post, doesn't do.

Once you have successfully calculated your "U-factor" in Therm, a further manual calculation is needed to gives you a psi-value for the junction: the psi-value is the linear equivalent of the U-value. Multiplying the psi-value by the length of the junction gives you the number of Watts the junction will lose for each degree centigrade difference between inside and outside.

The convention in Passivhaus is to use the U-value and the external dimensions to calculate heat losses through a wall or other area, rather than the internal dimensions normally used here in the UK. The Passivhaus convention of using external dimensions means that, at corners, the area is double counted, resulting in a slightly over conservative figure for the building's overall heat loss. In a well designed new build Passivhaus, this allows for any minimal thermal bridging that might remain. In our refurb, the thermal bridge looks like it will be significant and I don't know yet whether it will stop us achieving the Passivhaus standard. The Passivhaus Institute are planning to relax the standard for refurbishments because in part, I suspect, of the extra heat loss caused by these unavoidable thermal bridges.

External insulation
We have managed to reduce the thickness of the external from 300mm to 180mm. Originally, I'd hoped to use an insulator made from a tongue-and-groove wood fibre board product, like Pavatherm. The insulation would have had to be more than 300mm thick to achieve a wall U-value of 0.1. As this would have been applied to an existing wall of 300mm, the external walls would have been excessively thick. The weight of the insulation would probably have posed structural issues too, possibly requiring reinforcement of the footings. Instead, we are using Phenolic foam, which will deliver the same performance for half the thickness as wood fibre. Even though Phenolic foam is made from fossil fuels and has a higher embodied energy, our solution is overall less energy intensive than the wood fibre alternative. At 180mm, Phenolic foam costs £40/m2 or £100/m2 including installation, so not cheap. However, the thickness of the insulation has turned out to be more than just an aesthetic matter. The deeper the insulation, the deeper the windows are inset into the walls. This cuts down the amount of solar energy the windows can capture and negates some of the additional benefit of the thicker insulation.

Part of our refurb is a new build extension. Here we are using wooden I-beams in-filled with a cellulose insulation like Warmcel. I-beams are so called because they have the shape of a capital letter I when looked at end on. This design, using engineered wood, gives high strength with less thermal bridging because of the thinner central section. We will also be using I-beams in-filled with cellulose in the roof. An I-beam construction can become a source of thermal bridging unless the wall is carefully designed so that the "I" of the beam does not run through the entire thickness of the wall.

As all Passivhaus projects have to manage thermal bridging between and within walls, floors and roofs, a very helpful group of people have produced a compendium of building drawing and materials details that architects and Passivhaus Designers can copy, rather than always trying to re-invent the wheel. The "Passivhaus Bauteilkatalog" is in English and German. It costs about £81, so not cheap, but cheaper than doing all the work yourself.