Summer comfort

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With the recent heat wave in UK, it’s been interesting to experience internal comfort in our house. Although we haven’t fully moved in yet, we’ve been spending considerable time inside doing various finishing tasks. Over the last few days the external daytime temperatures have been gradually rising, peaking at a scorching 30 degrees C today. Night time external temperatures have also been going up, last night levelling at 17.5 degrees C. This pattern is likely to carry on over the next few days.

We’re really impressed with how our house has been coping with the hot weather. Internal temperatures have been fairly steady, gradually rising from 18.5 to 21.5 degrees C. We’ve been keeping the windows & doors shut and the ventilation unit on. The MVHR unit has a built-in automatic summer bypass, which kicks in at 21 degrees C. We can contribute the comfortable internal temperatures to a number of design features:

  1. exposed concrete slab with its high thermal mass acting as a giant temperature regulator (warning: this can work both ways!)
  2. 1.5m roof overhang along the south verandah means no direct sun hits the big glass = zero unwanted heat gains
  3. external roller blinds to main bedroom (south facing), and two windows to east & west gables
  4. high levels of insulation & airtightness in walls & roof (thermos flask effect in reverse)
  5. fully vented facades & roofing. It is worth noting the facades and roofing are black colour, but the vented cavities help isolate the hot surfaces from the thermal envelope, slowing the heat flux down

We don’t have the hot water system installed yet and have not been cooking/ living in the house, so internal heat gains have been minimal (bar the dehumidifier which we’ve temporarily turned off). We’ll keep an eye on the temperatures & relative humidity and will start proper monitoring once we get connected to broadband. The initial impressions are very positive though and we can’t wait to move in!

Summer comfort


Every passivhaus home has a controlled mechanical ventilation system with heat heat recovery (MVHR), and ours is no exception. With expert advice from Alan Clarke and Green Building Store we have designed a cascade system based around Zehnder Comfoair 160 VV Luxe ventilation unit. This is the smallest unit from Zehnder and can supply up to 160m3 of fresh air per hour – plenty enough for the two of us and a medium sized dog. Another benefits are built-in frost heater and automatic summer bypass when internal temperature gets over 21 degrees C. We have kept the ventilation ducts dust-free during the clay plastering and only installed the terminals just before commissioning.

The ventilation system works best when properly commissioned, so it was  great to have Alan and Nick coming over to help. Alan hired a nifty anemometer with self-balancing fan for accurate reading of flow rates, whilst Nick used manometer as an alternative way of measuring. We also installed a condensate drain, which wasn’t in place by default. A few hours later (including pub lunch & a game of ping pong) our system was set up. The first thing that struck me was how quiet the whole system is. The MVHR unit itself is just audible, but as it’s located in the Utility room it won’t be an issue. Supply & extract terminals throughout the house are totally quiet – a sign of good design and installation.

The next big step will be installation of hot water system and then we’re just about ready to move in – can not wait!