Loft Insulation

At the time of writing this page, the methodology for EPC assessments of existing dwellings is RDSAP 10. A significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2027 and changes are proposed to the format of EPCs and minimum rating levels required for private rental properties. Methods for improving EPC ratings will therefore change in the future.

✔ One of the easier EPC improvements

✔ Biggest gains occur when starting with no existing insulation

✔ Further improvements reduce rapidly often beyond around 200mm

✔ EPC recommendation usually appears when existing insulation is 150 mm or less

✔ Always confirm the likely EPC improvement with an energy assessor before spending money

Loft insulation is often described as one of the easiest ways to improve an EPC rating, but how much difference does it actually make?

The answer depends largely on how much insulation you already have.

For a typical property with a pitched roof and access to the loft, your energy assessor measures the depth of any loft insulation.

Mineral wool insulation is usually laid between and then over the loft joists, and greater depths result in lower heat loss through the top of the building.

The general approach therefore is to increase the depth of your loft insulation, but the EPC rating will quickly plateau, and beyond that depth it's not worthwhile adding further insulation as a single measure if your goal is simply to increase your EPC rating.

An energy assessor will look at your property, including any extensions, and select the type of roof(s) from this list:

• Pitched (slates/tiles), access to loft
• Pitched (slates/tiles), no access
• Pitched, sloping ceiling
• Pitched (thatch)
• Flat
• Same dwelling above
• Another dwelling above

For what we might call 'traditional' loft insulation we're interested in the first two types. We'll touch on the other types in another page.

Building regulations have changed significantly over the years. Back in the early 1960s, before the introduction of formal building regulations, loft insulation wasn't fitted as standard.

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

Retrofitting loft insulation in older properties has therefore become a very common measure for improving the energy performance of existing dwellings.

Fitting additional insulation in the loft is not too difficult or costly and with some research, due diligence and care (maintaining ventilation for example) can be achieved as a DIY project, so this is a reasonably attainable step.

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 and quickly plateau 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.

Always check with a local Domestic Energy Assessor to see what improvement could be achieved with your property before committing yourself to any expenditure.

Loft insulation depth is measured using a ruler or tape measure. I use an 'Early Learning' type ruler because the figures are easy to see on a photo we take as evidence for audit purposes.

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

If the insulation material is a type of PIR board then generally the depth recorded is twice that as measured, although a convention dictates the exact thermal U values required of the material in order to do so.

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. This was more of an issue under the previous methodology (since updated to RdSAP 10 on 15th June 2025) as three of the following levels were not available.

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

Prior to RdSAP 10's release on 15th June 2025, the options for 125mm, 175mm and 225mm were not available.

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.

During an EPC assessment, energy assessors are expected to put their head and shoulders into the loft area to measure the depth of any insulation present.

This works fine when a loft hatch is available and is usable, and in this case the Features table on an EPC certificate will contain a line of text describing the roof (for example) as:

Roof - Pitched, 250mm loft insulation

The presence of a figure there and the absence of the word 'assumed' indicates that access was available and the assessor actually measured the depth of insulation.

There are situations however, where access to the loft is not possible on the day of the assessment. One example could be when:

• There is no loft hatch

In this case the assessor has to chose the option: 'Pitched (slates/tiles), no access', insulation: 'Unknown'

If this is the roof of the main part of a building (rather than an extension), then this will trigger a Smart Audit once the EPC is lodged on the national register, and the assessor will want to take additional photos to support this choice to satisfy audit of the EPC.

The Smart Audit rules are found on the EASOB website, and are currently at version 1.8: http://www.easob.co.uk. The rule referred to above is Rule 12, Priority 19.

The EPC methodology then assumes the presence of insulation depending on the construction dateband of the building part. We discuss this a little later on this page under it's own section.

Other scenarios where a loft hatch is present but access is still an issue include:

• The loft hatch is sealed shut - perhaps painted over or locked shut
• The loft hatch is inaccessible due to obstructions on the landing
• The loft area has been fully boarded out
• The loft area is highly obstructed with belongings

In these scenarios where measuring the depth of any insulation isn't possible, the assessor selects roof type: 'Pitched (slates/tiles), access' and insulation depth: 'Unknown'.

The EPC methodology will again assume the presence of insulation with a depth that's dependant on the construction dateband of the building part.

In both scenarios, the Features table on the EPC will indicate something like:

Roof: Pitched, insulated (assumed)

Roof: Pitched, no insulation (assumed)

So we see the word 'assumed' in brackets. The insulation status (ie insulated or not insulated) will depend on the construction dateband assigned to the building part and the methodology version.

Note that where the insulation depth is recorded as 'Unknown', the EPC will not contain a Recommendation to increase the depth of loft insulation.

The EPC methodology can add a recommendation to the EPC certificate to increase the depth of loft insulation.

This will be considered when:

• The Roof Type is Pitched Roof (slates or tiles), with access to loft
• Roof Type is not: Thatched Roof
• If insulation is already present, it is between the joists, not the rafters

This recommendation is triggered if:

• Insulation already present between the joists is 150mm or less, or a manually entered U value by an assessor of 0.35 W/m2K or more.

The recommendation is to:

• Increase insulation between the joists to 270mm or use a U-value of 0.16 W/m2K. (For park home insulated loft 220 mm insulation or U-value 0.20 W/m2K)

From the information above we can see that if the roof type is declared as 'Pitched Roof, no access' then a recommendation for loft insulation will not appear on a resulting EPC.

Also, if the roof type is declared as 'Pitched Roof, access to loft ' but the insulation type is declared as 'Unknown' then a recommendation will also not appear on a resulting EPC.

Note that a recommendation is made only if it increases the SAP rating by at least 1 (one) SAP point, or 0.5 SAP point in the case of C (cylinder insulation), D (draughtproofing) and E (Low Energy Lighting). These thresholds may be overridden by amended values in the PCDB.

Let's take the example of the house in our Case Study 2. This is a 1950s 3-bed mid-terraced house with unfilled cavity walls and a gas condensing combi boiler.

As the level of loft insulation at the joists is progressively increased, we'll record the EPC rating at each level.

Although EPC ratings are normally displayed as whole numbers such as 68 D, the calculation is actually carried out to four decimal places and then rounded.

• Decimals from 0.0001 to 0.4999 are rounded to the integer below.
• Decimals from 0.5000 to 0.9999 are rounded to the integer above.

In this table we display the rating as both an integer value and also to 4 decimal places so we can see what's happening in more detail:

We can display this graphically as well with 100mm tall joists:

The data clearly shows the EPC rating improves significantly as insulation is initially added, starting from 'none' (0mm), but then it reaches a plateau as we continue to increase the depth as a single improvement measure.

This property has 200mm at the moment and we won't increase this any further as a single improvement measure because the integer EPC rating won't increase.

You can see in the table that each increase above 150mm for this property does make a fractional SAP point increase to the EPC rating, but it's not enough on its own to flip the integer value up to the next number.

There are two contributing reasons for this:

Diminishing reduction in u-value

The u-value of the loft insulation improves significantly as you start to add depth from 0mm but then follows an exponential curve. Beyond a certain point, adding even further insulation, even significant amounts then only improves the u-value by a much smaller proportion.

Here is a chart of loft insulation u-value displayed against increasing depth. This is based on data in Table 16 on pages 42/43 of the RdSAP 10 Specification Document:

As insulation is initially increased from zero the u-value rapidly improves.

However, beyond the elbow in the graph, adding more insulation only improves the u-value by smaller proportions.

Notice for example the improvement in u-value from 0mm to 200mm to 400mm:

• u=2.30 at 0mm
• u=0.21 at 200mm
• u=0.11 at 400mm

Other elements contribution to overall heat loss

As you increase the depth of your loft insulation, this part of the building no longer contributes anywhere so much to the overall heat loss of the building.

Other elements such as walls, floors and windows become more significant as a proportion.

Further improvement in roof heat loss makes only a diminishingly smaller reduction to the overall heat loss

Expenditure at this point might be better invested in the other elements.

If loft insulation depth is recorded as 'Unknown', assessment methodology assumes a certain depth is actually present, based on the property’s age.

These assumed levels were adjusted slightly for older properties when the methodology was updated to RdSAP 10, in June 2025.

Here's how the assumed levels changed for 'unknown' insulation depth:

For the 1950s property in our case study here, this would now be 100 mm, but previously would have been 0mm, ie none.

In the EPC rating table above, our property gets a 62D with 0mm depth (whether it was assumed under the previous methodology or actually kmeasured'), but a 67D if the insulation depth was 'Unknown' and assumed to be 100mm. That's an increase of 5 SAP points.

So this is something to bear in mind. If you have an older EPC where the loft insulation was genuinely recorded as unknown and the insulation depth assumed, if you were to have the property reassessed today and the insulation is still recorded as 'unknown' you may get an uplift in rating.

I'd just like to fully qualify the information in the table above:

The information is from Table S10 from RDSAP 9.94 Appendix S, and from Table 18 of the RDSAP 10 Specification document. Both documents are available on the BRE website.

* - Under RDSAP 9.94, dateband L was known as '2012 onwards'. Dateband M (for 2023+) was introduced in the update to RDSAP 10, so L was then renamed '2012-2022'.

The numerical values shown in columns 2 and 3 are u-values and the numbers in brackets are depths of mineral wool insulation.

The datebands in column 1 are normally referred to using their letters, indicated in brackets. The years shown are for England & Wales, where I am based. The years vary slightly for Scotland and NI.

Be mindful. If you place any items or boards down flat, directly on top of the loft insulation, this will flatten it down possibly as low as the top of the loft joists thus reducing the depth to 100mm (the depth of the joist).

Energy assessors do have the ability to calculate a weighted average across the whole of the loft when assessing overall depth of insulation, but in this case the recorded depth will always be lower than when the insulation was 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 the full depth of the insulation can be recorded, so long as it's possible to ascertain that 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.

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 in every respect:

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.

I've purchased a 3-bed semi at auction in this condition and re-sold it after having the roof completely renewed as part of a full refurbishment. The roof alone cost £12,000 to replace.

Loft insulation is one of the improvement measures that can be tackled on a DIY basis.

The potential for rating improvement depends on the level of insulation you start with.

If your loft has little or no insulation, adding more could be a cost-effective improvement measure.

However, beyond around 200–270mm further increases are unlikely to improve the EPC rating on their own.

If your goal is to move up an EPC band, it may be better to combine loft insulation with other improvements such as cavity wall insulation, heating upgrades or renewable technologies.

As always, consult a local Domestic Energy Assessor to model your property so you know the rating outcome before proceeding with any works. This enables you to work from a position of certainty when implementing improvement measures.

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