Mansard Roof

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.

A Mansard Roof could potentially be confused with 'Room In Roof' in some ways but there is a clear distinction between the two in the methodology for EPC assessments of existing dwellings (known as RDSAP) and the related Conventions. A Mansard Roof is therefore something fairly specific.

Again however, the issues for the property owner in terms improving the EPC rating are around the levels of insulation present, and the documentary evidence that is required to prove energy performance better than the default values used in an assessment based on the original construction date of the property.

A Mansard roof is essentially a pitched roof, identifiable by being split into two sections of different angled slope. This design allows for greater use of the interior volume of space, and usually the top floor of rooms in the property is located within this 'roof' space.

It is said that this design was popularised into the mainstream by François Mansart, but also it's origins are belived to date back to Pierre Lescot 50 years earlier in 1546.

The main element of the design is characterised by the two slopes on each of its sides with the more prominent lower slope, usually at 70 or more degrees from horizontal, and which often includes dormer windows, whilst the upper slope is at a lower angle, perhaps 35 degrees pitch and is often not so visible from the street, particularly on taller buildings.

This first photo is of a Mansard roof, on a 2 bed semi-detatched property in Melksham, built in 1937, just before the outbreak of the Second World War.

The dual slopes of the Mansard Roof are evident on the front and rear of the property, and the construction of this particular property today is the same as it was originally built, back in 1937 (This construction date having being provided by the owner).

The thickness of the front and rear ground floor walls of this property suggest cavity wall construction, although there is no visual evidence to indicate the presence of retrofitted cavity wall insulation, and this property has an end gable wall running to the peak of the roof.

The 'walls' and roof of the upper floor are constructed using timber frame.

This second example is on the same estate as the first one, although on a slightly different style of road. It's likely this one was built just afer the second world war, and it is a little different to the first example:

Firstly, it's an 'end of terrace' property with a hip style roof, so the shape of the Mansard style roof flows around from the front elevation around the end elevation and continues into the rear elevation of the property. The walls and roof of this upper floor are again constructed using timber frame.

This is subtly different to the first example, which had an end gable wall.

In an RDSAP EPC assessment, the two properties are entered slightly differently, and if correctly recorded, this should be evident in the EPC Features table (We'll look at some examples shortly).

Secondly, although the external brickwork would initially indicate cavity wall construction, due to the stretcher bond of the external brickwork, this property is in fact defined within the EPC methodology as a 'System Build' property.

This is because the external brick wall was actually added much later, after the property was originally constructed. Originally, it would have looked like this instead (the left hand semi):

The left hand semi here is in it's original build condition. It's literally just across the road from the second example, and is still in its original state of construction.

This is called a 'Cornish Unit Type 1', and this is a form of 'System Build' in EPC terminology (we'll talk more about this shortly).

The ground floor is formed from pre-cast reinforced concrete panel walls and the second storey constructed with a timber framed mansard roof.

There is a tendancy over time for the concrete panels to degrade, and as a result, the Cornish Units were included in a list of properties that were classified as 'defective' under the Housing Act 1985.

If you were to seek a mortgage on this type of property in its original condition, your choice of available mortgage lenders might be somewhat limited.

Many of these properties have therefore undergone works over the years by their owners. Locally here, the Housing Association that owns (or owned) many of these properties 'repaired' a large number of them to include external wall insulation and additional insulation in the Mansard Roof.

Others, particularly if they are privately owned, have had the external concrete panels removed and an external brick skin built instead to form a cavity wall. This is classified as a 'Repair' and should be carried out under licence. From what I have read, I'm not sure whether the walls needed to be brought up to current building regs when that work was done. I'll try and find out. Anyhow, this work makes them appear externally just like a property of 'standard' construction, even though in an EPC assessment the walls should still be described as System Build.

In the third picture above, we can see that the property to the right has has had such work undertaken, wheras the property on the left is in it's original condition.

I understand from a local estate agent that even with an external brickwork skin added, if the other semi-detatched property that you are attached to is still of its original construction you may still be limited in the number of providers willing to provide a mortgage.

Looking down the whole three roads on this estate, the above example was the only one I could locate in it's original condition. All of the rest had either had renovation that included external wall insulation, or had a more recent brick external skin fitted.

Within our context of EPCs for existing dwellings, rather than for new-builds, the following guidance is available:

Building Research Establishment - RDSAP Convension 2.07 (introduced March 2010)

"Rooms within a Mansard Roof - A storey having non-vertical walls of at least 70 degrees pitch constitutes a separate storey; it is not treated as roof rooms. Use 'Alternative Wall' if appropriate"

Elmhurst Energy - All RDSAP Conventions v11.4 Manual

"For RDSAP purposes, the first floor of this type of property would be entered as an alternative wall of timber frame (includes photo similar to a Cornish Type 1 semi)".

Elmhurst Energy RDSAP Methodology Manual v9.9.4

4.2.3 Timber Frame Construction

Mansard Roofs - Non-vertical walls of at least 70 degree pitch from the horizontal constitute walls and should be entered as timber framed construction, unless the property in question has solid/cavity walls which extend to ceiling height on the storey and is covered with tiles hung on the surface. In this case the construction can be entered as such, and the surface mounted tiles are ignored."

"Non-vertical walls of less than 70 degrees from the horizontal should be entered as normal roofs."

"Please note: If an entire storey is timber framed construction above a ground floor of solid/cavity construction (my addition: eg a semi-detatched house with a hip roof, where there is no end gable wall at the Mansard level), the property should be entered as a single storey house with a vertical extension of one storey. The roof type for the ground floor would be entered as 'Same Dwelling Above' and the floor type for the extension would be entered as 'Same Dwelling Below'.

(I reference Elmhurst above because I'm a member of their DEA accreditation scheme, and I have access to their guidance documentation for DEAs).

If you look at the Features table on an existing/previous EPC for a property with a Mansard Roof, you would therefore expect to see information relating to what has been described above.

If the EPC was produced prior to March 2010 (long expired now of course) it might be a bit hit and miss - Convention 2.07 was likely not in place at that time.

For our first example property, the semi with the end gable wall, you might expect a main cavity wall declared as used for the whole of the ground floor. In addition, the first floor will have a wall of timber frame declared for the lower slope of the Mansard roof. This would technically be declared as an 'Alternative Wall' in the assessment, and the 1st floor of the property would be a normal additional storey of the 'Main Building' in the assessment. The end gable wall on the first floor will be the same type of cavity wall as used on the end gable wall on the ground floor. You'd therefore expect to see two types of wall delcared in the EPC Features table - a cavity wall and a timber framed wall.

For the second example, because this is a System Build house (more about that soon), we'd expect to see in the Features table a 'System Build' wall for the ground floor, and because this property has a hip roof, and the entire first floor of the property is timber framed (apart from the party wall), following the guidance in the Methodology Manual we should see evidence of a vertical extension. This will be evident by the declaration of a timber frame wall, and likely declarations of a first floor 'Floor' with 'Same Dwelling Below' and a ground floor 'Roof' with 'Same Dwelling Above'.

You might however be suprised how many properties with mansard Roofs are not represented correctly, with the possibiliity of baked-in disappointment for the owner on the next EPC renewal when the rating changes 'unexpectedly' when the new assessor records things correctly.

In our second example photo, we mentioned that the building was a 'System Build' property. System build is a term used to represent a range of property types, the common feature being that their components are often pre-manufactured and then transported and assembled onsite. It's a generic term which encompasses a variety of different construction techniques, including the Cornish Unit Type 1 and 2.

The Elmhurst RDSAP Methodology Manual v9.9.4 section 4.2.4 (Page 58) makes the following specific observation about recording System Build properties in an EPC assessment:

"Many System-Build properties have had an external brick skin added post-construction due to defects in their construction - these are still entered into RDSAP as System Build even if a cavity exists between the original construction and the brick skin. If the cavity has been filled, details of how to enter this can be found in the Wall Insulation section of this manual."

So, in our second example property, even though the wall has been replaced with a brick outer skin, thus likely forming a cavity wall, that wall should still be recorded in the EPC assessment as a System build wall. Also, (Page 65):

"If a system Built wall has evidence of retro cavity fill, record this as system build with internal insulation depth 'unknown' and select Addendum 1. This also applies if the wall has had a brick outer skin added at a later date".

This originates from the BRE RDSAP Convention 3.02 which advises:

"If there is a System built wall that has evidence of retro cavity fill, record as System build with internal insulation, thickness unknown, and include Addendum 1"

So, related to our second example property, I read Elmhursts note to mean that if a system build wall, either as originally built but with evidence of cavity fill, or with a newer brick outer skin and with evidence of cavity fill, then that wall should be declared in either case as System Build, and the insulation as 'Internal insulation', with depth 'unknown', and Addendum 1 ticked  - which indicates: "Wall type does not correspond to options available in RdSAP".

I assume that in the absence of any evidence of retrofit cavity fill then it would just be entered as a System Build wall, As Built'.

As far as the dateband that should be applied to the building part where an outer brick skin has been added to a System Buid property, I expect that this should still be the original build date of the property rather than the date of the 'Repair'. I'm having that confirmed at the time or writing.

Finally, just to advise that the ticking of Addendum 1 results in the following information being added to the final EPC certificate:

Let's look at the EPC Features tables of the three example Mansard Roof properties we have looked at above.

Here is the EPC Features table from the first example property:

The EPC Features table for the first example property seems to be quite good in that what we would expect to see, is in fact what is present in the table.

Firstly, there are two declarations for Wall, one is the cavity wall for the ground floor and end gable wall, and the other is for the tmber framed construction of the 70+ degree Mansard roof components.

There is an entry indicating access was available to the central roof section and that 200mm of loft insulation was identified.

Finally the floor is described as a suspended floor. From visiting the property once on a viewing day I remember noticing the timber ground floor.

Just lastly though, I'd like to mention the Secondary Heating row where it lists 'Room heaters, electric' as being present in the property (note the word 'assumed' is not present). Because the main heating system is a gas boiler, if the electric heaters were removed and all habitable rooms still had heat provided by the main gas boiler, the EPC rating would almost certainly be improved. Just a passing observation - see our Heating Systems page for more information about this.

Overall though, the Features table seems to be representative of the property.

The EPC was conducted by a member of staff of the Housing Association that was selling the property, and I've noticed that EPCs they have conducted on other properties in the past seem to have been good as well.

Next, here is the EPC Features table from the second example property, which at the time of writing is being marketed for sale:

We notice that there is only one type of wall declared - Cavity wall, filled cavity. This represents the ground floor wall, and from the research we covered earlier, assuming evidence of cavity fill is present, I believe this should be 'System Build' with Internal insulation, depth unknown (the depth won't appear on the Features table though). A note should also be present on the EPC about the wall type not corresponding to options in RDSAP - that's not present.

There is also no mention of the timber framed wall for the 1st floor, and I'm certain the property has not had that replaced with a 'cavity wall'.

We also notice that the Roof is declared as 'Another dwelling above'. This appears to be incorrect as from our research earlier, assuming a vertical extension has been declared, we believe it should say 'Same dwelling above'.

'Another dwelling above' is used for example where you might have a block of flats and there is another separate flat above the flat being assessed. That's not the case here, and there doesn't appear to be any neighbouring property with a building part on top of this one either.

We are also missing a Floor entry for 'Same dwelling below'. it's possible that might not show up on the Features table anyway, but it's absence adds to the picture.

In summary, this EPC raises a number of questions about it's representation of the actual property.

This EPC was issued in September 2024 and the contact details are for a someone working for a property services company - one that provides photos, floorplans and EPC certificates as an all-in-one service for estate agents.

A bit more research reveals that a previous EPC also exists on the national register for this property, issued in February 2023, and this was therefore replaced after only one year and seven months, even though EPCs are valid for 10 years.

That previous EPC had a lower rating of 62D versus the new one of 73C. It had a cavity wall and a timber wall declared in the Features table.

The newer EPC was likely requested due to a new boiler having been fitted in May 2024, and the owners quite reasonably seeking to have the rating updated to reflect the property. This replacement boiler was listed in the Building Control applications on the Wiltshire Council portal.

The older EPC indicated that a newer boiler would only add 4 SAP points to the EPC, assuming some other recomendations had also been conducted.

Finally, here is the Features table for the third example property above:

This time, the walls are declared as we are expecting them to be: System Built, as built, and also a Timber frame wall, as built.

Being a property with a 'hip' roof (no end gable wall), I was expecting to see evidence of a vertical extension declared with a ground floor roof of: 'Same dwelling above', and a first floor Floor of: 'Same dwelling below'. These would be present if the direction requested in the Elmhurst Methodlogy manual had been followed.

Otherwise however, the EPC is reasonably representative of the property.

If the owner of a property such as these was interested in improving their EPC rating, then assuming all other aspects of the property were fairly decent, such as the main heating system being a relatively modern, efficient and cheap one, such as a condensing gas boiler, the main avenue they might look down is to improve the fabric of the building.

If I had a property with a Mansard Roof of this age band, I'd look to see what I could do to improve the walls and roof.

Splitting the property into three parts:

• The First floor 'walls' (timber frame)
• The Top, flatter, roof section
• The Ground floor walls (System Build in examples 2 and 3, Cavity in example 1)

The First Floor Walls (Timber Frame)

With the first floor walls, which are the 70 degree or more angled part of the Mansard roof, normally, in the absence of any evidence to prove otherwise, these are entered in the EPC as a timber frame wall, with insulation declared as 'As Built'.

With the example properties we have seen above, they were all constructed either in the late 1930s or late 1940s, when energy efficiency wasn't as important as it it today, and the building regs weren't anywhere near as demanding.

As we will see in the section below, the default U value for a timber framed wall in this dateband is 1.9 W/m2K. This is an indication of how fast energy is lost through the wall, and that figure is pretty bad.

It is however posible to retrofit insulation to the timber frame walls, in order to reduce the rate at which energy is lost, and have this represented in the EPC assessment.

There are two ways this can be indicated in the EPC assessment of an existing dwelling.

Firstly, with a timber framed wall it is only possible to manually specify Internal insulation as being present during an assessment. External insulation is not available as an option - if external insulation is present then 'Internal' has to be indicated instead. Additionally, the system does not allow an assessor to indicate the depth of the insulation that is present. A single lower U value is then used by the EPC methodology dependant on the dateband applied for that building part.

The other option is to get the U-Value of the final insulated wall to be calculated by a qualified person, and have the energy assessor manually overide the U-Value of that wall in the EPC assessment.

The latter option would clearly be better because you should be able to fully realise the true improvement of any works carried out. This is paticularly the case if your calculated U value is better than the one used by the EPC methodology in the paragraph above. The U value calculation will also be able to accomodate the use of a combination of different materials if needed.

Sufficient evidence will need to be gathered for both cases however. For the second case in particular, the overiding of any building part's U-value will result in a Smart-audit being triggered for the EPC, and it being double checked by the accreditation scheme that the assessor belongs to. Assessors will be very careful to ensure they are happy that sufficient documentary evidence is available before they overide a U value.

Retrofitting insulation to the timber framed walls would involve significant work either fitting the insulation internally after having removed all the internal plasterboard/boarding, or it could be achieved externally after removing the tiles and battons and sarking. The Local Authority Building Control would need to be involved either way.

There are a couple of examples on the Internet of these approches that I located from a quick search:

Firstly, here is an example where 500 homes owned by a housing association were retroftted with SuperFoil insulation by removing the external tiles to carry out the works:

https://psbnews.co.uk/features/housing/500-cornish-type-housing-association-homes-get-high-performance-insulation-in-herefordshire/

Secondly, here is an example of a private owner carrying out his own retrofit work, all above board through Building Control, and getting the job done properly himself. I'm particularly interested in this one because all the steps are described, and he has put really good detail into the Building Control application. This is a shining example of what a Domestic Energy Assessor needs in terms of documentary evidence when you have retroftted insulation that cannot be visibly seen:

https://www.jameswhanlon.com/cornish-unit-house-retrofit-insulation.html

Jamie has kindly given permission for me to share content from his webpage so I'm re-presenting one of his photos here:

In this photo you can see all the internal plasterboarding has been removed from the external wall in this bedroom, and the old limited quantity of mineral wool insulation has been removed from between the 'rafters'.

Jamie has added some additional wood battons (the light blue/grey coloured ones) to create greater depth to the 'sarking' fabric, beyond which are the external tiles.

He is fitting PIR board inbetween the wood battons, making sure a suitable gap exists behind them for ventilation, and in front of this eventually he fits a vapour control layer (VCL), and then some insulated plasterboard.

In particular, he has submitted an application to Building Control which lists the specific brands and grade of materials he is using and how he is intending to fit them.

He then takes significant care to make sure the insulation is fitted tightly without any gaps.

Further photos are available on his website, linked above, and I'd thoroughly recommend taking a look.

Jamie estimates from an online U value calculator that his first floor walls now have a U value of 0.25 which is a significantly good value. This compares to a value of 1.9 which an EPC assessment would otherwise use for a timber framed wall built in 1930-1949 (in England & Wales).

The Top, flatter, roof section

With the roof, you could simply top up with mineral wool insulation as much as you can.

Alternatively you could remove any mineral wool, and renovate similar to Jamie's work on the walls as described above - fitting PIR board, VCL,and insulated plasterboard to the ceilings as well.

PIR board is better than mineral wool in terms of performance/depth, and the depth of PIR board would ordinarily be doubled before being entered into an RDSAP EPC assessment.

You could though consider again having a U value calculated and providing sufficient evidence for an energy assessor.

The Ground Floor Walls

Informaitotn to follow...

A U-value is an indication of the rate of heat transfer (ie loss) through a building component. This could be a window a door or a roof. These items are often made up of more than one type of material, and a U value can be calculated that takes them all into account. A lower U-value means the material is not so good at transferring heat, and therefore better at keeping it within the property (ie insulating). U-values are expressed in W/m2K.

I've copied the information in the table below from Table S6 : Wall U-values – England and Wales from the Appendix S document: Reduced Data SAP for existing dwellings from the Building Research Estabishment's website.

If you wish to look at the whole document, it can be found here:

https://bregroup.com/expertise/energy/sap/standard-assessment-procedure-2012

Once there, click on the link for the document: RdSAP 2012 v9.94 (unless a more recent version is available).

These are the default U values used by the software for EPC assessments of existing dwellings, when timber framed walls are indicated in England and Wales:

* = assumed as built

So, the value for a standard built timber framed wall starts off pretty poor at 2.5 prior to 1900, and it gets better over the decades as we approach modern times. Today there are plenty of modern timber frame new-build properties, and the modern buiding regs for them are quite demanding.

Notice though that in the second column, the U values are much better for older properties if the assessor is able to simply, and manually, declare that additional insulation (declared as internal insulation) has been retrofitted to the wall. Evidence is required to back this up of course. No further drop down box is availble to specify how much insulation is present via this route, so it's a 'present or not' type of option. If sufficient evidence is available then this should improve the EPC rating.

However, if you are going to put in some significant effort to insulate the timber frame wall of your older mansard roof, likely to medern building regs, it would probably be worth getting a U value calculated for your final wall by a qualified person. You could then get your energy assessor to use the calculated value to overide the default value, and this should hopefully resut in an even better EPC rating. Further evidene will be required by the energy assessor for this though.

As always it's worth employing the services of a local energy assessor to calculate in advance what difference your updated U value will make to your EPC rating before proceeding with any works because you will then be operating from a position of certainty.

I'm really intrigued to try and estimate how much Jamie's EPC rating might have improved following all the insulation that he retrofitted to the first floor of his own property.

I'm interested because he's taken all the right measures to get the job done correctly and has put specific details of all the materials in the Building Control application. That last thing will serve him really well when it comes to proving the existance of that insulation to a visiting Domestic Energy Assessor.

The only real way to calculate accurate EPC figures would be to assess Jamie's property directly. Without that option though I have taken the details of my own house, including floor area (86m2), perimeter lengths, heating systems, loft insulation depth etc, and made the following changes to make the building fabric as similar as I can to his, by:

• Defining the house as semi-detached (rather than my mid-terraced) and moving my second party wall length figure into the Heat Loss Perimeter (HLP) figures instead
• Defining the house as a single storey building
• Defining the first floor as a vertical extension
• Setting the ground floor walls to System Build, Internal insulation present, unknown depth (simulating a brick built external skin with evidence of cavity fill)
• Setting the first floor walls to timber framed, and 'as built' as the insulation status
• Setting the dateband for the whole building to 1930-1949

The estimate (guess!) base EPC rating worked out at 67D

By changing the insulation status of the timber framed wall to 'Internal', the EPC rating goes up to 73C. I expect this could be possible if you can prove you have retrofitted internal insulation by simply adding it to the existing wall - see section below re documentary evidence.

By overriding the U value of the timber framed wall to a value of 0.25 W/m2K raises the EPC rating to 74C. This is possible when the additional proof is made available of a calculated U value of 0.25 W/M2K - see documentary evidence section below.

So I suspect it's likely that if Jamie were to get a formal U-value calculation carried out to support the 0.25 W/m2K figure, then he might get an EPC uplift of around 7 SAP points.

If you have a similar property please do not read anything into this estimate. Please consult a local Domestic Energy Assessor near you to model your own property and get them to estimate any improvement that your planned works might provide. Many factors combine to create the final rating.

When energy assessors issue Energy Performance Certificates, they have to follow a number of rules.

In relation to our subject of Mansard Roofs, lets look first at what's required when overiding the U-value for a wall. This would most likely be the timber framed wall on the first floor where perhaps we have retrofitted some significant insulation and had a U value calculated for us.

The first thing to be aware of is that if a U value is overidden in an assessment of an existing dwelling, that will result in the EPC being put forward for a manadatory audit.

There are a number of triggers for such 'Smart audits', and the list is publicly available to view if you are interested - direct link here:

http://www.easob.co.uk/DEA%20%20Smart%20Rules%20v1.7.pdf

Looking down the list, rule 4 dictates that a Smart audit is triggered upon the overiding of the U values of any building part (apart from windows and doors).

This means that when the EPC is finally lodged, it will automatically be put forward to be audited by the accreditation scheme through which the energy assessor lodged it.

The auditor will check carefully that the rules have been followed in the creation of that EPC, and issue a decision.

Your energy assessor will be aware of this and will therefore be very careful in making sure they have sufficient evidence before lodging the EPC because they will want to protect and maintain their accreditation.

Let's look at what's required as 'documentary evidence' for this, so you can provide this to your energy assessor and make the process as smooth as possible.

The current list of official rules, or 'Conventions' as they are known, are also publicly available, and can be downloaded from the Building Research Establishment (BRE) website.

At the time of writing (Jan 2025), for assessments on existing dwellings, currently the methodology in use is RDSAP 9.94 and the Conventions are at Version 11.4. The document is available here:

https://bregroup.com/documents/d/bre-group/rdsap-conventions-11_4-from-01-jul-2024-

The overiding of U values is covered in Convention 3.08...

"The U-values of existing elements (walls/roofs/floors, etc.) must be the RdSAP default values (e.g. entered “as built”) and must not be overwritten unless specific documentary evidence of the thermal conductivity of individual materials of the building element of the property being assessed is provided and was undertaken in accordance with BR 443 “Conventions for U-value calculations” (BRE, 2006).

The U-value is that of the whole element, including any added insulation.

Documentary evidence applicable to the property being assessed (see convention 9.02) must be provided and recorded if overwriting any default U-value. This evidence shall be either:

- relevant building control approval, which both correctly defines the construction in question and states the calculated U-value; or
- a U-value calculation produced or verified by a person with suitable expertise and experience.

Evidence of suitable expertise and experience can be demonstrated by, but is not limited to, membership of a recognised U-value calculation competency scheme or OCDEA or Level 4 non-domestic energy assessor membership, or any other process recognised by Accreditation Schemes/Approved Organisations and Government.

Where it is known that only part of an element has been insulated use the alternative wall if possible for the insulated part, or use extensions"

I have further seen information that a U value calculation provided by a suitable person needs to have the property address on it, the calculation resulting in the final U value and the name of the suitably qualified person.

Convention 9.02 reads:

"Acceptable documentary evidence includes, but is not limited to, official correspondence from the Registered Social Landlord (RSL) or certificates, warranties or guarantees or any documents verifying that work has been carried out. The assessor must be confident, and able to demonstrate, that any documentation relates to the actual property being assessed, and/or the work
has been carried out, and that there is no physical or other documentary evidence to the contrary.

Evidence of intent to install does not on its own qualify as acceptable documentary evidence.

Evidence of intent to install, supported by evidence of subsequent Building Control oversight, or visual evidence that such an upgrade has been undertaken does qualify as acceptable documentary evidence.
a) If it can be demonstrated that there was both an intention to upgrade the element (in planning documents for example) and Building Control oversight of the work, the element can be treated as having been upgraded as indicated in the planning documents.
b) If it can only be demonstrated that either there was an intention to upgrade the element (in planning documents for example) or that there was Building Control oversight but it can also be seen that an upgrade has occurred, the element can be treated as upgraded using the minimum selectable upgrade of the relevant type defined in RdSAP. (If upgraded from ‘as-built’ to ‘insulated’
using the lowest selectable insulation thickness* that is better than the as-built assumption).

*For roof insulation between joists use 100 mm as the lowest selectable insulation thickness."

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