Mansard Roof

At the time of writing, 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. In addition, government consultations indicate future changes to the format of EPCs and minimum rating levels required for private rental properties. Approaches to improving EPC ratings will therefore change in the future.

✔ Usually found on older properties

✔ The whole 'roof' is usually of timber-framed construction

✔ The lower part of the roof is actually considered a 'wall'

✔ EPC improvements will depend heavily on retrofitted insulation

✔ Documentary evidence is critical if insulation has been retrofitted

✔ Formal u-value calculations are a perfect source of documentary evidence

A Mansard roof is a particular roof design that is treated differently within EPC assessments compared to a 'room in roof' (aka 'loft conversion').

Understanding how a Mansard Roof is assessed is useful when seeking to improve your EPC rating.

A Mansard roof is essentially a pitched roof where each 'pitch' is split into two levels of differently angled slope, and the structure usually contains the top floor of rooms in the property.

The prominent lower slope is pitched at 70 degrees or more from horizontal, and often includes dormer windows.

The upper slope is at a lower angle, perhaps 35 degrees pitch and is often not as visible from the street, particularly on taller buildings.

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.

This photo is of a Mansard roof, on a 2 bed semi-detached property in Melksham, built in 1937, just before the outbreak of the Second World War. It has cavity walls on the ground floor and the whole of the gable end wall.

The main distinction of a Mansard Roof is the pitch of that lower level of 'roof'.

If that part of the 'roof' is at least 70 degrees in pitch from horizontal, then it's considered in the assessment as a wall, and the whole upper story is also considered a separate storey in it's own right, rather than being a 'room-in-roof'.

The structure of a Mansard roof is normally timber framed, so a wall of this type would be declared in the assessment, in addition to the type of wall on the ground floor.

You'd therefore expect to see two types of wall declared in the EPC features table.

Our example property has cavity wall on the ground floor and a timber framed wall on the first floor, as follows:

The floor area, heat loss perimeter, windows and ceiling height of the first floor are measured as normal for any other storey, along with any insulation present in the remainder of the flat ceiling, if accessible.

Some advice from accreditation bodies is that the upper storey should be recorded as a vertical extension, particularly where there is a hip-roof.

This will be evident by the declaration a first floor 'Floor' with 'Same Dwelling Below' and a ground floor 'Roof' with 'Same Dwelling Above'.

Mansard roofs were built on many pre-fabricated dwellings constructed just after the second world war.

Here is an example of a pair of semi-detached houses with Mansard roofs.

These are 'Cornish Unit Type 1' properties, and in EPC assessments this is considered a form of 'System Build'.

The Mansard storey is assessed in the same way as already described, but the walls are declared as 'System build' walls and appear in the EPC features table as follows:

'System build' is a catch-all term used to represent a range of interesting construction types. The common feature of these is their components are usually pre-manufactured then transported to site before being assembled.

Older buildings such as Cornish Unit Type 1 and 2 are included in this designation, as well as more modern construction techniques such as Insulated Concrete Framework (ICF).

The left hand semi here is in it's original build condition.

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

With Cornish Type 1s, the concrete panels degrade over time, and they were included in a list of property types 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 would be limited.

Many of these properties have undergone works over the years by their owners.

Locally here, the Housing Association that owns (or owned) many of these properties modified 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 leaf built instead to form a cavity wall.

These types of works are classified as a 'repair' and were usually carried out under licence.

Visually, the properties appear from the outside very much like a property of 'standard' construction

I haven't yet found definitive guidance confirming whether 'repaired' walls were required to meet building standards applicable at the time.

Notably, in EPC assessment the walls should still be described as System Build, not cavity wall.

Accreditation scheme advice about recording System Build properties in an EPC assessment includes the following

"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."

In the picture above, we can see the property to the right has has had such work undertaken.

So even though the right-hand semi has had the walls 'repaired' with a brick outer skin, that wall should still be recorded in the EPC assessment as a System build wall.

Also:RdSAP Convention 3.02 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"

This adds the following text to an EPC certificate:

Every wall, roof and floor loses heat. In energy assessment, this heat-loss is represented using a figure called a u-value.

The lower the u-value, the better the building element resists heat loss.

U-values are expressed in W/m2K, representing watts per square meter of element, per degree Kelvin (same units as Centigrade) difference in temperature from one side of it to the other.

With Mansard Roofs we're particularly interested in the u-values for timber-framed walls because that's the type of wall used to represent most Mansard roofs.

The following information comes from Table 6 : Wall U-values – England from the RdSAP 10 Specification document on the BRE website:

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

* = assumed as built

'As Built' means the wall is in the same condition as when it was originally constructed.

'Internal Insulation' means that additional internal insulation has been added since original construction. This is a present-or-not choice. There is no option to specify the type or thickness of any additional insulation.

The u-value for an 'As-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, and modern building regulations apply.

The u-values for 'Internal Insulation follow a similar pattern but are much better (lower).

The other option is for the assessor to manually enter a u-value into the assessment instead. This also requires sufficient documentary evidence of the u-value calculation.

Yes you do.

Energy performance is an area covered by Building Regulations and described in Approved Document Part L.

In particular:

• Renovating more than 50% of a single external thermal element of a property requires building control notification
• Prescribed target u-values are listed which you normally need to meet or exceed

Table 3.3 identifies the target u-value you'd be aiming for subject to feasbility and financial allowances as described in the document:

So for a wall, you may quite possibly be aiming for a u-value of 0.30 W/m2K.

Seek advice on the required u-value and how you would achieve it from a qualified person.

The Building Control completion certificate however is a great place to start in building up the documentary evidence required to include any additional insulation in an EPC assessment.

With a Mansard Roof specifically, the two main elements you'd look to improve are:

• The First floor 'walls' (timber frame)
• The Top, flatter, roof section

Everything else including windows, heating etc. would be considered in the same way as any other dwelling.

The First Floor Walls (Timber Frame)

The first floor walls are the 70 degree+ angled part of the Mansard roof. These are recorded as 'As Built' unless there is sufficient evidence of additional insulation.

The properties we looked at above were constructed in the late 1930s or late 1940s, when formal building regulations weren't in place.

The default u-value for a timber framed wall in that dateband is 1.9 W/m2K.

With a timber framed wall, if additional insulation has been fitted, it is recorded as 'Internally Insulated'. External insulation is not available as an option. If external insulation is present then 'Internal' has to be recorded instead. Also, for timber framed walls the thickness of additional insulation cannot be recorded.

With the wall declared as Internally Insulated, the u-value in the EPC calculation for our example properties would then fall to 0.55 which is a decent improvement.

If you were targeting a u-value of 0.30 W/m2K due to the requirement in Part L table 3.3 then to get the full benefit of this value in an EPC assessment you'd have to revert to arranging a formal u-value calculation to be carried out for you.

This would enable the energy assessor to manually override the u-value of the wall in the EPC assessment to your calculated value.

Sufficient documentary evidence is required however.

The overiding of any building part's u-value results in a Smart-audit being triggered for the EPC. This makes it highly likely it will be audited by the assessors accreditation scheme. Assessors will therefore be very careful to ensure they are happy that sufficient documentary evidence is available before they overide the 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.

There are a couple of examples on the Internet of these approaches:

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 example 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.

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

Within the rooms in the Mansard roof there will be an area of flat ceiling with a rather shallow roof void above.

This part of the Mansard roof is recorded in an EPC assessment as a standard pitched roof.

The level of insulation in that roof space is recorded during an assessment.

If access is available via a hatch for example, an energy assessor could measure the insulation, in which case you can top up the level beforehand.

If access is not available, consider making access available if the insulation is just standard mineral wool and at a depth of more than 100mm. Without access, RdSAP now assumes a minimum of 100mm, depending on construction ageband of the property.

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 has much better thermal performance than mineral wool for the same thickness of material.

The measured depth of PIR board would ordinarily be doubled before being entered into an RdSAP EPC assessment.

You could though consider again having a formal u-value calculated by a suitably qualified person and providing that calculation to your energy assessor.

To understand the potential benefit of insulating a Mansard roof I'm really intrigued to estimate how much Jamie's EPC rating might have improved by.

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 similar 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
• Changing the construction dateband for the whole building to 1930-1949 (E&W)
• Setting the pitched roof insulation depth to 100mm

Here are the results:

Taking a look, we see:

• The baseline EPC rating worked out at 69C

• By changing the insulation status of the timber framed wall to 'Internal', the EPC rating goes up to 74C. This is 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 0.30 W/m2K the EPC rating is 75C. This is the u-value likely to be required by Approved Document Part L.

• By overriding the u-value of the timber framed wall to a value of 0.25 W/m2K the EPC rating is 75C (albeit 0.18 SAP points higher than 0.30 W/m2K). This is the u-value that Jamie thinks is representative of his walls. 

So with enough documentary evidence to support the entry of timber framed walls with internal insulation we get a rating increase of 5 SAP points.

With further documentary evidence of a formal u-value calculation of 0.3 or 0.25 W/m2K we would get an additional increase of 1 SAP point, making a total of 6 points.

If you have a similar property please do not read anything into these figures. Please consult a local Domestic Energy Assessor near you to model your own property and get them to model the improvement 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.

Formal conventions define what is required in terms of documentary evidence to support:

• Manually overriding the u-value of a building element such as a wall
• Declaring a timber framed wall as 'Internally Insulated'

At the time of writing the assessment methodology for existing dwellings is RdSAP 10.0.1 and the corresponding Conventions document is publicly available on the BRE SAP10 page: 

https://bregroup.com/expertise/energy/sap/sap-10

All EPCs have a chance of being audited by the assessor's accreditation scheme, but in particular when manually overiding a u-value for a wall the EPC will triggers a Smart Audit flag, and is much more likely to be audited.

If audited, any documentary evidence will be checked to make sure it fulfills the requirement of the conventions.

Your energy assessor 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 status.

Here are the Conventions related to documentary evidence and overiding u-values manually.

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"

In addition, a u-value calculation provided by a suitably qualified person needs to have the property address on it, the full calculation of the u-value and the name of the suitably qualified person. Contact your energy assessor to find out what they are willing to accept.

Convention 9.02 (England, Wales & NI) 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."

This convention is more related to the assessor being able to confidently mark the timber-framed wall as internally insulated. Again, contact your energy assessor to check what evidence they would are happy to accept.

Consider invoices for materials specifying the exact material brand and model type (so that specifications can be identified). Delivery addresses on any invoices need to be the property address, and photographic evidence can further support this if context is clear, but don't simply rely on photographs alone.

In both cases, your building control completion certificate is an excellent starting point for building up sufficient documentary evidence.

• A Mansard roof is usually assessed as a storey in its own right with a timber framed wall, not a 'room-in-roof'.
• Improving the timber-framed walls can improve the EPC rating.
• Building Regulations Part L apply when carrying out insulation work.
• Keep all documentary evidence, including Building Control records, U-value calculations and receipts etc.
• Ask a Domestic Energy Assessor to model the likely EPC improvement before starting any work.
• Most of the potential gain may be achieved by simply being able to declare the timber wall as Internally Insulated.
• Further gains will require a formal u-value calculation as documentary evidence of an even lower u-value.

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