Building Air Pressure Testing and MVHR Fitting

Energy Efficient House Renovation

A friend of mine who lives in Baffin Island told me about her friend who moved from Baffin Island to Ireland to live with her Irish partner. Reporting back to her friends in Baffin Island, she commented on how cold houses were in Ireland compared to Baffin Island houses. Houses in Baffin Island are built insulated, West Coast of Ireland houses are not. An extreme example but the big trouble with renovating old houses to make them energy efficient is they weren’t built to be energy efficient. To make houses energy efficient and so save money on energy, you have to stop heat moving.

Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation. When trying to manage heat transfer in houses, we are really only interested in conduction and convection.

U-value measures insulation’s ability to limit heat flow, the lower the U-value the better – the slower the rate that heat can be transferred through the insulation. However as you chase lower and lower U-values by adding more and more insulation you will get less and less reward for your investment. This means there comes a stage where the cost of reducing the U-value can outweigh the energy savings that result. At some stage the primary method of heat transfer will not be conduction but air leakage (convection). Therefore, once you have reached this level of insulation, the best way to increase efficiency is to start reducing leaks with an air barrier.

    This is by far the easiest and also the best return on your investment. 270mm of “fibreglass” insulation is the minimum on new build houses. Fibreglass traps air which has a reasonable u-value and also slows convection. Make sure when you insulate your roof  you keep the space above the insulation ventilated – this may require fitting spacers to prevent the insulation blocking the airflow. If you are fitting Kingspan or Celotex between your rafters make sure it is fitted; gaps are holes where the heat will escape. In the same way as a bucket with a hole in it is not really a bucket, a layer of insulation with a gap is not a layer of insulation. See REDUCE DRAUGHTS.

    If you have a cavity wall, then cavity wall insulation is simple and reasonably cheap, but take advice if you live in an exposed area with high rainfall as there have been problems with the insulation soaking up water from the exterior wall and wick-ing the moisture through to the internal wall . These cavities are likely to be only 50mm – once Building Regulations increased the cavity size to above a 50mm cavity, insulation was specified too but 50mm of insulation between two layers of brick will help.
    If you have solid walls then internal insulation is possible but it is not that easy – you have to beware of DEW POINTS. Dew points are where the warm, moist air from inside the house condenses on its way out of the house as it cools. Where it condenses is the dew point. (Warm air can “hold” more moisture than cold air). So sticking insulated plasterboard on a cold, north-facing, bathroom wall is not a good idea. The water vapour will pass through the plasterboard and the insulation and then condense when it hits the external cold walls. This will trap the damp and you are likely to get dry rot. (I can see internal insulation fitted by well meaning DIYers keeping builders busy for years in the future as a result of badly thought through internal insulation.)
    External wall insulation is by far the best solution – the dew point is outside the fabric of the house and the bricks become a thermal store.

    Usually this is only possible if you have a cellar or are prepared for major disruption especially if you have solid floors! There are some new products on the market now that have amazing u-values but are very thin, e.g Thermablok which can be put on an uninsulated concrete floor under engineered and laminate floors to reduce your cold concrete floor.

    The less draughts in your house, the less of your expensively-heated hot air escapes! However, to prevent mould, damp and rot it is important that a house is ventilated. This is why bathrooms have extractor fans. (It is also important for humans that there is fresh air!)

    All new build houses have to be pressure tested to pass their SAP test. However if you were to have your house pressure tested you can find where your house is leaking. The normal places are: under window boards, loft hatches, letter boxes, where the kitchen waste goes out through the wall – same with the toilet bath and shower waste pipes. Note: sealing the outside where the pipes appear will do very little to seal the leak if you have a cavity wall.  Also if your soil vent pipe goes through the roof internally you can bet your bottom dollar that the boxing in around the pipe is not sealed and you basically have warm air disappearing up this chimney.




OK so this leads to the idea of a MVHR.

A Mechanical Ventilation Heat Recovery unit and ventilation ducting provides ventilation to a house but also retains a large % of the heat . The MVHR unit is a magic box that contains a fan and a heat exchanger. Ducts extract warm moist air from kitchens and bathrooms and blow it out of the house through a series of ducts through a heat exchanger, meanwhile fresh air is sucked in through the heat exchanger and pumped back into the other rooms to replace this air. The wizardry happens in the magic box/heat exchanger where the incoming air is heated by the outgoing air clawing back “up to 98%” of the heat of the outgoing air. More information here.


OK you can fit energy efficient boilers but it only really makes sense to do this as your old one wears out. This will reduce the amount of energy you buy but will not change the amount of energy needed to heat the house. (The new boiler is just more efficient at converting the gas or oil to heat.) To reduce this “heat load” INSULATE. Insulate your water cylinder too if you have one. Also you can change your light bulbs to LEDs, have a 2 minute shower instead of a bath, turn off TV computers, phone chargers etc. when not using them. It all helps and if you get an energy monitor you can see how much it helps if you want, but bang for buck insulation is the best way forward.


As I said I started real energy efficient work with my sister’s job 89 Culford Road – you can read about that job here.

This lead to my moving on to work for Rob Prewett again on another refurbishment job in Balham. This was one of the houses in the government’s Technology Strategy Board sponsored program.

I also finally got around to renovating my house in North Shropshire (delayed by the my sister’s job and the Balham refurbishment). More about this house here.

I was commissioned to build a bungalow in Oswestry “to as near Passivhaus” standards as possible. Details here:-  (Ty Gwyrdd = Green House in Welsh).