Desk with Laptop, Calculator, Files and small model of house with epc colours
Desk with Laptop, Calculator, Files and small model of house with epc colours
EPC Works
EPC Works

Loft Insulation

At the time of writing the methodology for EPC assessments of existing dwellings is RDSAP 9.94. The much anticipated RDSAP 10 update is due in Q1 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026.

'Standard' Pitched Roofs

For a typical property with a pitched roof, and access to the loft, we measure the depth of the loft insulation as part of the EPC assessment.

 

Mineral wool insulation is usually laid between and over the loft joists and generally speaking, the greater the depth of this insulation, the better the EPC rating can be.

 

The aim therefore would be to increase the level of your loft insulation but the gains will eventually plateau, and beyond a certain insulation depth will not be worthwhile implementing if your single goal is an improvement in EPC rating.

 

Building regulations have changed significantly over the years. Back in the early 1960s, loft insulation wasn't originally fitted, but more recently this is seen as a standard insulation measure required to improve the energy performance of existing dwellings.

 

Modern building regulations are now quite tight and new properties have significant insulation installed by default.

 

Fitting additional insulation in the loft of older properties is not too difficult or costly and with some research, due diligence and care could be achieved as a DIY project, so this is a reasonably attainable step (Care should be taken to maintain ventilation in pitched roofs for example).

 

We have a worked example below that indicates the effect on EPC rating for an example mid-terraced property using different levels of loft insulation. The results are quite interesting, but the gains become smaller as the insulation depth is further increased.

 

As we mentioned previously when considering Low Energy Lighting however, any small increase could be enough in some cases to take you up a band if you are near a threshold, particularly when combined with other improvements.

 

The design of the property can make a difference to the potential gains, so as I've mentioned elsewhere, do check with a local Domestic Energy Assessor to see what improvement would be achieved with your property before committing yourself to any expenditure.

Measuring Insulation Depth

The depth of loft insulation is measured using a ruler or tape measure. I use an 'Early Learning' type ruler as the figures are easy to see on a photo that we take as evidence to support the EPC assessment.

Measuring Loft Insulation Depth Measuring Loft Insulation Depth

If the insulation material is the standard mineral wool type, then the depth is recorded as meaured.

 

If the insulation material is a type of PIR board then the depth recorded is twice that as measured.

 

When recording the depth, an absolute depth measurement cannot currently be entered into the assessment. Instead, a value is chosen from a drop down list.

 

If an exact matching value is not available in the list then the next lower value must be selected. This can result in a slightly lower value being recorded than is actually present.

 

Currently the list of available depths for insulation between the joists is:

Loft Insulation Depth
None
12 mm
25 mm
50 mm
75 mm
100 mm
150 mm
200 mm
250 mm
270 mm
300 mm
350 mm
400+ mm

When the EPC methodology is updated to RDSAP 10 in Q1 2025, additional insulation depths will be made available of: 125mm, 175mm, 225mm.

 

A weighted average can be calculated if there are areas of differing depth within the loft space, although if there is an area that has no insulation at all then a separate building part needs to be split off in the assessment and declared with no insulation.

EPC Recommendation for Increased Loft Insulation

The EPC methodology will generally add a recommendation to your EPC certificate to increase the depth of loft insulation under the following conditions:

  • Roof Type: Pitched Roof, with access to loft
  • Roof Type is not: Thatched Roof
  • If insulation is already present, it is between the joists, not the rafters
  • Insulation already present between the joists is 150mm or less, or a manually entered U value by an assessor of 0.35 or more.

the recommendation is to increase the insulation between the joists to 270mm.

Loft Insulation (Joists) - An Example Property

Let's take the example of my own house, which is a 1950s 3-bed terraced house without cavity wall insulation. I've covered this building in a bit more detail in our Case Study 2 page.

 

The property had an upgrade to 250mm of mineral wool insulation fitted some time ago, from the 50mm that it had previously. A certificate in the loft space, stapled to one of the rafters indicates this nicely.

 

Due to various works on the property and having to re-arrange the insulation, and using a weighted average calculation to consider a small area that only had 100mm depth, my calculation was for 230mm average over the whole loft.

 

We have to choose the next lower figure in the list, so I have to declare 200mm depth from the list. Bear this in mind when checking your own insulation depth.

 

My EPC rating is currently a 68D which is on the top of band D, just one point from moving up to a band C.

 

I'm therefore interested in any measure that could give me a one point improvement in EPC rating:

 

Here is a table of increasing loft insulation depths and the EPC rating that I would achieve in each case with my property:

Loft Insulation Depth My EPC Rating
None 62D
12 mm 64D
25 mm 66D
50 mm 67D
75 mm 67D
100 mm 67D
150 mm 68D
200 mm 68D
250 mm 68D
270 mm 68D
300 mm 68D
350 mm 68D
400+ mm 68D

The EPC rating on your property will vary differently to the way mine does, so don't necessarily read improvements in value from the table above and try to overlay them on yours.

 

Your property might be a bungalow for example which has a greater ground footprint versus volume ratio compared to a house. This might expand the range of rating increase, but at the cost of needing to cover a greater loft area.

 

However, what's interesting to see is how the EPC rating improves significantly as insulation is initially added, but then the gains reach a plateau as the depth is increased further.

 

I have 200mm at the moment, and I probably won't increase it further because my EPC rating won't increase with this one measure on it's own.

 

The increase in insulation level above 200mm will make a fractional difference to the EPC rating but its not enough on its own for the rounded integer value to increase beyond my existing rating of 68.

Loft Storage

Be mindful. There are many properties where owners have placed items, or even doors or boards down flat, directly on top of the loft insulation, flattening it down on top of the loft joists and reducing the depth to 100mm (the depth of the joist).

 

We do have the ability to calculate a weighted average across the whole of the loft when assessing the overall depth of the insulation, but the recorded depth will always be lower than when originally fitted, and will likely reduce the EPC rating.

 

If you want to use your loft for storage, it is much better to install plastic loft legs and screw boards flat on top of the legs. Items can then be placed on top of the boards, the insulation will not be compressed, and we can record the full depth of the insulation, so long as we can ascertain the depth is consistent across the whole of the loft - this may require the owner to lift boards as necessary for the assessor to be convinced.

Avoid Spray Foam Insulation

There are many companies that actively advertise the installation of spray foam insulation in pitched roofs.

 

This foam is sprayed into place between the rafters in pitched roofs (Rafters are the angled lengths of wood that follow the pitch of the roof and meet at the peak. In contrast, joists are the horizontal wood beams on the 'floor' of the pitched roof).

 

There are many reasons why spray foam should not be installed and here is a link to a really good video explaining why this type of 'insulation' is a bad idea:

 

https://youtu.be/4gfc2l7Se5w?si=RqLT8j-XvxNeqhE9

(What's the fuss about spray foam? - West One Surveyors)

 

If you have spray foam installed, then even if you ignore the physical issues that can arise from it, you'll eventually find it very difficult to sell your property because mortgage lenders really do not like this material installed. A valuation survey instructed by a buyer's lender will usually report a property's value as zero.

 

You will probably find yourself needing to have the spray foam removed before being able to sell your property in the normal way, or having to sell the property at auction instead.

Other Roof Types

For flat roofs and sloping/vaulted roofs (ie pitched roofs where the ceiling follows the slope of the roof rather than being horizontal), access is not normally available to confirm the presence of any insulation.

 

In these instances, unless documentary evidence is availble for those roof types, the level of insulation will be declared as 'As Built', and the EPC methodology will select an assumed level of insulation/U-value from the following table.

 

Also, if access is not available to a pitched roof to measure the level of loft insulation present between the joists then the insulation level is declared as 'Unknown' and a value used from this table also.

 

The information here has been taken from Table S10 from the RDSAP Appendix S document from the BRE's website:

Dateband

Pitched Roof

(Slates/Tiles)

'Unknown'

Flat Roof or

Sloping Ceiling

Thatched

Roof

<= 1966 2.3 (none) 2.3 0.35
1967 - 1975 1.5 (12mm) 1.5 0.35
1976 - 1982 0.68 (50mm) 0.68 0.35
1983 - 1990 0.40 (100mm) 0.40 0.35
1991 - 1995 0.30 (150mm) 0.35 0.35
1996 - 2002 0.26 (150mm) 0.35 0.35
2003 - 2006 0.16 (270mm) 0.25 0.30
2007 - 2011 0.16 (270mm) 0.25 0.25
2012 + 0.16 (270mm) 0.18 0.18

The datebands are normally given a letter from A to L, and they vary slightly from England & Wales to Scotland to NI. For ease of reading I've populated the table with the actual England & Wales datebands (rather than the letters A to L), as this is the region in which I live. Note that the first row in the table, '<=1966' is a combination of datebands A, B, C and D. The table also had some additional notes which I have not reproduced here, including some differences of U values for Scotland. Please refer to the BRE document for the full information.

 

Firstly, notice how good the U values used for thatched roofs are, right back and including the earliest dateband of pre 1900 (Band A). Thatched properties often return an EPC rating that's impressive considering their age and wall types.

 

Notice in comparison that the assumed U values for Flat roofs and Vaulted ceilings prior to 1966 are terrible. They are equivalent to there being no insulation present at all.

 

It was in 1966 that modern building regulations came into force for new applications, and that was the greatest driving force in the tightening of the assumed U values from there onwards (The EPC datebands are offset by one year from significant changes).

 

My property is a 1950s mid-terraced house. I don't have a vaulted or flat ceiling in my property, and I do have access to the main loft through a hatch so I can measure the level of loft insulation in my pitched roof easily.

 

However, had this not been the case, say if I had a flat ceiling which could not be dated later than 1966 (such as on a newer extension), or likewise a vaulted ceiling, then without any documentary evidence to the contrary, that U value of 2.3 would have been used in the EPC rating calculation for that roof element.

 

If I was in that situation, and looking to improve the EPC rating of my property then I would be very interested in getting insulation retrofitted to those types of ceiling and gathering documentary evidence to support it's presence. I would research first with a local Domestic Energy Assessor to see what the rating gain could be and choose the insulation type and level based on that knowledge. I might even go to the extent of getting a U-value calculation completed by a qualified person to suppliment the documentary evidence I gathered whilst carrying out the work - see our separate page on documentary evidence requirements.

 

Similarly, if access to the main loft was not available, such as in the case of a hatch fixed in position or locked, or the loft had been boarded out fully so the energy assessor could not identify insulation as being present, then with an insulation level declared of 'Unknown' then that U value of 2.3 would also have been used (equivalent to no insulation present). From the table earlier on this page, we can see that if I had no insulation present, my EPC rating would drop from 68D to 62D - ie 6 points. I'd definitely want to get those access issues resolved so that the loft insulation could be represented in the EPC accurately.

 

Similar considerations might be made for 'Room in Roof' and 'Mandard Roofs'. See our pages on those roof types for further specific information.

 

 

As always, consult with a Domestic Energy Assessor before deciding what improvements to make to your property to improve the EPC rating.

 

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.


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