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 through anything that will allow radiation to pass.  Managing heat transfer in house – we are really only interested in conduction and convection.

U value measures insulation’s ability to limit conductive heat flow, the lower the U value the better – the slower the rate that heat can transferred through the insulation. As you chase lower and lower U values by adding more and more insulation you get less and less reward for your investment. There comes a stage the cost of reducing U value can outweigh the energy savings that result. At some stage the primary method of heat transfer will not be conductive heat flow but air leakage (convection). Therefore, the best way to increase efficiency is not merely to reduce U value, but rather to combine a low U value 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 you 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 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 cheap. These cavities are likely to be only 50mm – once Building Regulations increased the cavity size 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 condensers 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!
    The less draughts 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!)

The trouble with draughts are:



OK so this leads to the idea of a MVHR system – a Mechanical Ventilation Heat Recovery unit and ventilation ducting. The MVHR unit is a magic box that contains a fan and a heat exchanger. It takes heat from kitchens and bathrooms, warm moist air, and pumps it out of the house through a series of ducts, meanwhile fresh (filtered) air is 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. Details of that job can be found here–balham-retrofit-for-the-future2.html

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.