Passive Retro Fitting

Passive Homes Ltd are experts in retrofitting houses and commercial units to achieve the best possible outcome in reducing energy costs and increasing comfort levels. (see comfort in passive homes)


Reducing energy costs and protecting yourself against increasing energy bills is very important.

Retrofitting the Passive way is the integration of insulation air tightness heat recovery ventilation and renewable energy into a refit programme.

If you wish to look at insulation retrofitting the passive way or passive standard building follow the links.

The passive standards are very onerous even on new build's so the task of getting a retrofit to meet the passive standard depends on available budgets and the previous construction layout and junction details.

Here is the Passive standard

Insulation Walls U < 0.175 W/m2K

Insulation Roof U < 0.15 W/m2K

Insulation Floor U < 0.15 W/m2K

Window Frames, Doors U < 0.8 W/m2K

Window Glazing U < 0.8 W/m2K

Thermal Bridges Linear heat Coefficient < 0.01 W/mK

Structural Air Tightness n50 < 0.6/ air changes per hour

These are the major standards and don't take into account elements such as renewables lighting and appliances.

As I said it is possible but a more pragmatic way to look at passive retro fitting is to refit the passive way and achieve as much as possible with careful, clever planning and design.

See Passive house standard construction

Older houses have inherent defects some of which are repairable and some not so repairable.

For example a two storey three bed roomed semi detached house build with a standard 100mm cavity wall construction.

The floor is concrete and the roof tiled.

Windows are old double glazed units.

Our example house has problems on all fronts but with careful design and clever thinking it can be raised to a standard which will ensure the comfort of its occupants and substantially reduce the heating costs.

To achieve the passive standard we require a u value of 0.175w/m2/k

Our test house has a wall U value of 0.52w/m2/K

Let’s look at the improvements step by step

1. Lets fill the cavity with cavity bead: revised U value = 0.32w/m2/k

This is a big improvement but is far off the passive value. In fact its way off the regulation value of 0.27w/m2k for the BER and SAP standard. insulation:

2. Now we apply external wall

We apply 100mm of Phenolic Insulation and the U value falls to 0.14w/m2k

This exceeds the Passive standard.

The passive standard is 0.15w/m2/k

Our test house has a u value of 2.4w/m2/k

By adding 300mm of rock wool the U value is reduced to 0.11w/m2/k

This is within the Passive standard

The Passive is 0.15w/m2/k

Our test house is slab on grade with a u value of 1.6w/m2/k

This is a real problem in retro fits.

How do we reduce the floor U value or increase the R value.

Well this isn’t easy and there is no magic bullet.

There are two ways to increase the R value of the floor.

1. Remove the floor and add insulation. 200mm of Phenolic insulation on a 200mm slab will give a U value of 0.12w/m2/k which meets the passive standard.

2. Insulate on top of the existing floor. (this is more than likely not feasable)

Our test house has windows with U values of 2.4w/m2/k

The passive standard is o.8w/m2/k.

If we install these passive windows in the house retrofit and address the thermal bridge the effects will be enormous in terms of comfort and energy efficiency.

Triple glazing has other benefits as well.

They reduce noise levels

They increased solar gain and in a passive refit are essential for solar gain.

The thermal bridge issue is very important in window installations.

Modelling windows can help but in small cavities they best way to minimise thermal bridging is by installing insulated cavity closers.

To achieve the standard the window will be triple glazed.

Now triple glazing is expensive but if you are doing a retrofit it is the best investment you make after insulation.

Retrofitting of houses usually fail because the builder or architect fails to take into account the heat losses from thermal bridging. The heart of an excellent retrofit is the elimination of thermal bridging.

Sometimes this is not possible to totally eliminate thermal bridges in some retro fits but if the thermal bridge is minimised it will result in major savings as well as eliminating condensation.

All of the element junctions i.e. floors to walls etc should be modelled to see where the heat flows occur and to improve same.

Older buildings never thought about air tightness and so to retrofit to passive standards the air tightness will become a major issue.

The Passive standard is 0.6 air changes per hour.

This is a very difficult standard to achieve.

There are numerous ways for air to leak out of buildings from opes to chimneys to wiring routes to pipe work to ceiling joists etc.

The sealing of the house to an acceptable standard is essential so that the mechanical heat recovery system works efficiency.

Installing a heat recovery system to a retrofit recovers up to 95% of the energy from the house by using a heat exchanger.

Air being extracted from wet areas and from kitchen areas are used to heat incoming air. The air does not mix but the extracted air heats up the incoming air by up to 95% depending on the model used.

The installation of air recovery systems to existing houses is complicated and difficult.

The routes of ductwork will require careful planning to meet the balance requirements of the system.

In any retro fit the installation of renewables is essential to augment the gains from insulation, air tightness, thermal bridging and heat recovery.

The use of solar water heating and photovoltaics will decrease the house owner’s dependence on energy requirements such as oil, gas, or electricity.

However this is only the start of the retro fit story and it in many ways highlights the problems with simplistic solutions which are offered to increase thermal efficiency to home owners without looking at the overall picture.

Say if the owner of the test house decided to upgrade the elements to this level is he getting value for money or is the retrofit a waste of time.

The answer lies somewhere in the middle.

By doing all this work the owner will get a sizeable benefit but in many ways it will be by default.

If the owner applies the insulation by retrofitting the Passive way he will greatly benefit.

When increasing the insulation values of each element such as walls floors and ceilings the opportunity arises to eliminate or greatly reduce the effects of thermal bridging.

The perverse thing about increasing the insulation in elements such as walls or floors is that the heat loss from thermal bridging increases substantially so if a wall has its U value increased by 50% the thermal bridges in that element increases.

Thus a wall with a much better u value will lose more heat from thermal bridging sometimes up to 50% more losses at the thermal bridge.

This applies equally to attic and floors.

So if you are insulating an element such as external walls you will not reduce thermal losses substantially unless you insulate and tackle the thermal bridge problem at the same time.

If a piecemeal approach is taken then the sum of the parts will be less than the whole.

If however you look at all the parts of the retrofit together then you get a much improved house which is energy efficient and comfortable.

Energy retrofitting fits nicely with other green efficient ways of reducing energy and waste. Whether its recycling or other green initiatives the idea is to reduce waste.

Contact Passive Homes Ltd for a free consultation


Back to Home Page