31 Jul 2014

The Airtightness Test

We set out to achieve a low energy house with the principles of "super-insulate, build tight and ventilate right".  This means that the building needs to be pretty airtight. This ensures that the air that you heat can't just escape through gaps in the building, it is directed through the MVHR to heat/cool the incoming air - minimising additional energy input.  The other thing to worry about when air leaks is that the internal air will be warm and moist, when this moves through a gap in the fabric it can cause interstitial condensation - damp and mold!  No good!

To conduct the test the building is sealed and a calibrated fan is mounted in a doorway.  It sucks air out of the building to depressurise it.  Then they can test the pressure differences between outside and inside and watch what happens as they test at different pressures.  A number of different data points are collected and graphed, creating a final averaged figure.  We have tested twice now and will have one final test when the building is finished.  So the first two tests have been used as aides to discover where in the building the airtightness needs to be improved.  As the inside is depressurised it pulls in air from outside (actually air from outside pushes in and tries to fill the space) and you feel with the back of your hand where air is moving through the building envelope. 

Searching for leaks

Two figures are calculated in the test; air permeability and air changes per hour.  The two figures are very similar and to be honest i'm yet to really understand the difference! (Dimitri says "One is flux and the other is just a measurement that builders and architects can understand"!) However it is the Air Changes per hour that we are really interested in.  Air permeability is measured in cubic metes per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals (m3/(h.m2)@50Pa). Air changes per hour is the number of times all of the air in the house is replaced in an hour at the same pressure difference as above(without MVHR obviously).

Building Regulations state a maximum figure of 10 air changes per hour, although 3 is the minimum standard for AECB (Association Environmentally Conscious Buildings) Silver Standard for environmental building.  Passive Houses have to achieve 0.6. Getting a score of 10 down to 5 is relatively easy, however from 1 to 0.6 is very challenging...

The first airtightness test was conducted once the windows and roof had been installed.  Where the concrete blockwork meets windows and doors and the roof, we have used a lot of very expensive airtightness tapes. We've been told these tapes will last for 50yrs, but to be honest no one has had them in use for that long so nobody really knows as lab conditions are never quite the same thing as reality.

Paul Jennings, the airtightness guru, arrived with all his gear and got to work. We achieved a result of 4 in the first test, we were all disappointed! Leaks were searched for around junctions of the walls and windows and roof and sealed with more tape.  This got the figure down to 3.6.  Not the start we were hoping for.  

Unusually for a passive-style house, we have used traditional concrete blockwork.  Blockwork can vary a lot in air permeability and the more environmentally friendly blocks composed of fuel ash can make the blocks even more porous and very leaky.  So the blockwork could not be relied upon to give us the airtightness.  The redeeming feature of blockwork is that it is a great thermal store, absorbing heat and very slowly releasing it.  It will take a lot of energy to change the temperature of the mass.  At this point in construction we had used blackjack, a paintable bitumen, as a seal when perforating the blocks, so we guess that this had not been as effective as we had hoped either.  

Once our internal render had been completed we conducted our 2nd test.  This time the first score was 0.9 - hooray!  We were under 1, which was the figure we were aiming for when we first began this journey.  A huge accomplishment.  We always said that the aim was to do the best we could without throwing huge amounts of money at it. The costs can increase exponentially as you get down to the final 0.1s.  Just goes to show what a difference a layer of render can do.

But Andrew and Charlie were really hoping they could make it to 0.6. It's boys- they see a target and they want to hit it, goal driven! With more rushing around with smoke pens and backs of hands against junctions they found some more air movement and used an airtightness sealant and mastic to fill the voids.  Apparently this stuff is the business and doesn't fully harden and leak at a later date.  A lot of our issues were around the electrical switch boxes and the wire perforations through the render.

There was a lot of nervous tension as we waited for the final series of tests to be performed.

We managed to get a result of 0.72!  I think that is amazingly good! The photo below shows a result if 0.62, however each result has to be plotted on a graph and it is the average which determines the final score.

I am so delighted.  0.6 would be great but to be honest the difference of 0.12 is nothing that we will notice, our house is going to be super low energy and that is what we wanted.  It remains to be seen whether we'll improve the score by the 3rd test.  Andrew is planning to use some of the mastic inside the switch boxes in an attempt to improve the score.  We've also completed the plaster layer now which may have an impact.  However it has been known for scores to increase in the final test, as during the final finish more perforations are made in the airtight skin when you hang shelves and so on.

(I have to confess that I failed to write down the scores at the time, believing that they would be indelibly etched into my memory, however that has turned out not to be so!  Paul may correct me if I've got any of these numbers wrong.)

10 Jul 2014

Render and Plaster

Those experienced in the building trade know the difference between render and plaster.

The render, sand based cement with waterproofing additives, went off really quickly.  We think it must be the thermal mass releasing the heat from the last couple of weeks of good weather. It's this layer of render together with the plaster which forms the majority of our airtightness layer.  The plastering was started a couple of days earlier than expected. 

In order to keep things simple visually we have chosen a shadow gap detail on the door.  The stop bead itself is not expensive, but it was the fact that they only sell them in boxes of 50 which put the costs up.  So we now have about 30 pieces unused in each box, as we needed two different sizes of bead - anyone interested in buying them?  I'm becoming a supplier now!

There was a break in the work after all exterior walls had been rendered but not yet plastered, when we performed the critical air-tightness test (next post).  

WOW!  Mike and his team, R. Churchill Plastering, have done a fantastic job.  A local firm who were able to fit us in on pretty short notice.  It's a beautiful trade to watch - it's like ballet!  It's really looking like a proper house now.

6 Jul 2014

1st Fix Completion & MVHR

Alongside the 1st fix of plumbing and electrics, there was plenty of 1st fix carpentry to get done.  All openings and junctions got taped up with the very, very pricey airtightness tape.

Andrew and Paul also cracked on with the MVHR (Mechanical Ventilation with Heat Recovery) installation.  We are using an Airflow system.  Large red oval ducts are running through the ceiling.

As the house will hopefully be incredibly airtight, we need a mechanical system to circulate air. The MVHR will take out stale air and condensation (from the kitchen, bathrooms and utility areas) and bring in fresh air (to the bedrooms, sitting room etc).  It will use the heat of the extracted air, via a heat exchanger, to warm the supply air.  Equally if the air inside the house is cooler than outside then it will cool the incoming air.  The efficiency is more than 90%, so will we have to use very little energy to maintain internal temperatures.  I'm really looking forward to the air quality - we will have fresh air constantly flowing into the house.  Everyone who has installed it is evangelical about it, the comfort levels are supposed to be fantastic and they say they notice the difference when they go into normal houses, especially in the winter when people keep the windows closed.  Allergens such as mould, dust and pollen should be hugely reduced too.

The sliding door running tracks were fitted.

Preparing the steels for render and plaster.

Andrew and Paul worked flat out in order to get the place ready for the plasterers to get started while they had a short break in the schedule (Paul was off to enjoy Glastonbury).

Starting to look like real rooms now!

4 Jul 2014

Lighting Re-re-design...

The lighting design has been a lot of fun and games. Mat, my electrician (Aspey Electrical),  has shown a lot of patience and support as I have had to very quickly change almost everything related to the lighting design.  We definitely wanted to use LED lights (low energy house!) and we wanted to keep costs down so I had asked for loads of LED strip lights to be concealed behind wooden plank pelmets which would wash the walls with light.  I wanted to avoid spot/down lights as much as possible.  However once we got to site everyone in the business was quick to tell me that LED strips just wouldn't provide enough light!  Very quickly I had to change the designs and specify exactly what would replace it and where it would be located.  

Lighting is a nightmare as there is so much choice out there.  It is very technical and definitely user unfriendly. Unlike bathrooms there aren't really any showrooms you can go to, to get your eye in and see what the choices are. I browse the internet and find a fitting I like, but then there will be 4 different versions and I don't really understand what the differences are, and I still am none the wiser as to whether it will be suitable.  Also, lighting is where, if you are looking for non-standard items, the budget can be blown - it's silly money mostly.  The difficulty is finding what there is out there.  At the moment I'm just browsing the internet, finding what I like and want to do and then passing it over for technical support.  

In the living area I'm having a pendant light over the kitchen island.  I love this light, Plank,  and think something inspired by this would look amazing in our birch ply kitchen.  As it is £600 it's just not within our budget.

Plank, Northern Lighting

Over the dining table there are a triplet of pendants.  The exact light fitting hasn't been chosen yet.  I've been quite taken with some reclaimed glass funnels from Retrouvius.  I like the way they are small and elegant with a bit of extra quirkyness and, for £65, affordable.

Small glass funnel light, Retrouvius.com