Conservatories

At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update has not happened yet and a significant methodology overhaul from RDSAP to the Hone Energy Model (HEM) is proposed for 2025.

For EPC assessment purposes, RDSAP defines a conservatory as a structure where at least 75% of the surface area of the roof and at least 50% of the area of its external walls are glazed or of some other translucent material eg. polycarbonate roofing sheets.

Otherwise, it is considered as an extension.

Even if considered as an extension rather than a Conservatory, the information on the rest of this page will likely still apply. Check with a local Domestic Energy Assessor to make sure either way before making changes to your property.

If your conservatory fulfils the above definition, the key distinction about a conservatory is whether it is thermally separated from the building to which it is attached, or not.

If there is an external quality door separating the conservatory from the building then the conservatory is ‘Thermally Separated’.

If there is only an internal quality door, or an open archway without any door, or no separating wall at all, then the conservatory is not thermally separated.

In the case of a thermally separated conservatory, one further question is asked:

• Is the conservatory heated (with fixed heaters)? Yes/No

Regardless of whether the conservatory is heated or not, a thermally separated conservatory is ignored from the EPC assessment.

The Heat Loss Perimeter (HLP) of the building part onto which the conservatory is attached, is considered continuous, as if the conservatory was not there.

The floor area of the conservatory is not recorded for the assessment.

In the case of a non thermally-separated conservatory, further information is requested as follows:

• Is the conservatory double/triple glazed (Yes/No)
• Floor Area (m2)
• Glazed Perimeter Length (m)
• Room Height (1/1.5/2/2.5/3 storeys)

A non thermally separated conservatory is included in the habitable room count.

If the conservatory is heated, then it is also included in the heated habitable room count.

However, where a conservatory is open plan to another room, count them as one room.

The floor area of the conservatory is recorded in the conservatory section of the assessment. It is not included in the floor area calculation of the main building.

The section of wall of the main building onto which the conservatory abuts, is not included in the Heat Loss Perimeter of that floor (usually the ground floor).

I think the most important thing to notice here is that non thermally-separated conservatories are included in the count of habitable rooms.

Consider the situation where a non thermally-separated conservatory is separated from the main bulding by an internal quality door, and is not heated. In this case, a count of 1 is added to the count of habitable rooms in the property, but not to the count of heated habitable rooms. This creates a difference between these two counts.

As we discussed in the section about Heating Systems, where there are un-heated habitable rooms, RDSAP will normally revert to using a form of Secondary heating to fulfil the perceived heating shortfall in it’s model of the property.

If you do not have any other forms of Secondary heating already declared for the property (such as wood burners etc) then RDSAP will assume occupiers will use the worst case example of peak-rate electric heaters to fulfil the heating shortfall.

This will then affect your EPC rating by an amount dependant on your main heating system. If you have a main heating system that operates on a much cheaper fuel (such as main gas) compared to that of the secondary heating - peak rate electric is the worst case we are looking at here (or it operates on a more efficient basis in the case of a Heat Pump for example), then the effect could be dramatic.

If a form of secondary heating has already been declared in the property, say because there is a log burner in the lounge, RDSAP will use that form of secondary heating instead to fulfil the shortfall. In our example of a gas boiler for the Main Heating system, the difference in the fuel coss is smaller and therefore the differenece to the EPC rating will be smaller.

Let’s look at some examples by simulating a conservatory attached to my own home and see what the effects are:

The baseline for my house is currently a 68D, and I don't have a conservtory. I’ve detailed my house in Case Study 2 – cick on that link for more information about this particular house. In summary, it’s a 1950s 3-bed mid terrace property with un-filled cavity walls and a modern condensing gas combi boiler fitted in 2015/16. I have 5 habitable rooms and all 5 of them are heated by the gas boiler. Importantly, there is no Secondary Heating in the property.

We'll consider a conservatory that has the following characteristics:

• Size: 3m x 3m – ie a square conservatory connected straight to the back of the property with windows all round and a fully glazed conservatory door (which is treated as a window)
• Double Glazed: Yes
• Floor Area: 9m2
• Glazed Perimeter Length: 9m
• Length in contact with the rear wall of the main property: 3m
• Room Height 1.5 storey (assuming a peaked conservatory roof)

We'll add this conservatory onto my house in a few different ways, and see what difference it makes to the EPC rating.

Here are the results:

Scenario 1

If I add a conservatory to my house, make it thermally separated by the presence of a good external quality door, and it's not heated, the EPC rating stays the same. This is to be expected because the conservatory is effectively ignored in the assessement.

Scenario 2
This is the same as Scenario 1, except that we declare the conservatory is heated, with fixed heater(s). The EPC rating also stays the same. The Conservatory is ignored in the assessment.

Scenario 3

In this scenario, the conservatory is declared as non-thermally separated, due to there being an internal quality door separating it from the main building. It is also declared as being heated by a radiator on the main central heating system, and I increase both the count of habitable rooms and heated habitable rooms by 1 each. Because the conservatory is now part of the assessment, the Heat Loss Perimiter (HLP) of the ground floor is reduced by 3m. This is the length of the wall that the conservatory abuts onto on the main building on the ground floor. The EPC rating goes down to 66D. That’s a drop of two points from our baseline of 68D.

Scenario 4
If we continue using Scenario 3 but declare the conservatory as not heated, so we therefore have to reduce the count of heated habitable rooms by 1 (back to our baseline count), but keep the count of habitable rooms the same (baseline plus one), the EPC rating goes down to 59D. That’s a drop of 9 points from our baseline. That's a huge amount. It happens because RDSAP assumes the use of portable electric heaters to fulfil the heating shortfall in the absence of any other form of Secondary Heating (There's no Secondary Heating otherwise declared for this house). There are two contributing factors to this - Firstly, the cost of heating using peak rate electricity is much higher than the cost of heating from mains gas which is the fuel used by my combi boiler. Also, when a form of Electric Secondary Heating is included, RDSAP assumes it is used for 20% of the property floor area. See the next scenario where I manually add Secondary Heating to the assessment, delcaring that I have a modern gas fire in the house somewhere, with a balanced flue

Scenario 5

This is the same as Scenario 4 but here I manually declare the presence of Secondary Heating in the form of a modern gas fire with balanced flue somewhere in the house. The EPC rating rises to 64D. That's a change of +5 SAP points from Scenario 4, simply becuase RDSAP assumes the gas fire will be used to fulfil the heating shortfall rather than assuming the introduction of peak rate portable electric heaters. The gas fire, running on mains gas, uses the same type of fuel as the boiler, so the fuel costs are roughly comprarable, hence the increase in EPC rating over Scenario 4, although the boiler is probably more efficient than the gas fire.

Scenario 6
If I declare the conservatory as being open plan to the lounge, and therefore included with the lounge as a single habitable room the EPC rating becomes 65D. (that’s a drop of 3 points from our baseline).

Note that there won’t be any recommendations on the EPC about thermally separating the conservatory to improve the rating.

Take caution in that the changes in EPC rating for a conservatory on the back of your property won’t match the ones presented here because there will be many differences between your property and mine that affect the calculation.

The main observation is: If you have a conservatory, and it is connected to your property by an internal quality door, and it is not heated, and you have no forms of 'Secondary Heating' in the property, your EPC rating may well be being dragged down significantly.

You might consider changing that door to an external quality one so that the conservatory can be ignored in the assessment.

Alternatively, for a smaller gain, you could consider providing a fixed heater in the conservatory, fed by the main heating system of the property, even if you have that radiator for example with a TRV valve on it turned all the way down so it never turns on.

Before making any changes in this area, contact a domestic energy assessor to simulate the changes for your property to confirm whether the EPC rating will improve, and by how much. Use the figures from your assessment to make an informed decision about whether to proceed with any changes.

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