EPCs Underestimate BISF House Insulation Performance  

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Owen
 Owen
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29/07/2018 12:24 pm  

Hi, first post here but I’ve read many posts over the last few months in preparation for completing on a BISF house in a couple of weeks. Having benefited from the knowledge here and wanting to give something back I noticed a few posts complaining about EPC results and people’s perception that BISF houses are cold and draughty. I have decent knowledge on the calculation methodology used to produce an EPC (RdSAP) so I thought I would share this knowledge and hopefully reduce some misinformation on BISF houses.

 

RdSAP Assumptions

RdSAP (or reduced data SAP) uses assumptions (the reduced data part) when assessing an existing house, to fill in the blanks that would usually be known in a new build from its plans and specifications. Where certainty of the performance of an element is low, the calculation will take a worst case assumption.

BISF houses fall into the catch all category of ‘system build’ which is a very general category containing various different construction types. If your EPC describes the wall construction as ‘system build as built’, the underlying assumption is that the wall U-values are 2.0 W/m2K.

BISF Wall Construction

In the case of BISF houses we have an outer cladding of negligible thermal impact, followed by an air gap, then glass fibre insulation and finally plasterboard. Putting these into a U-value calculator gives U-value of approximately 0.45 W/m2k.

What does this mean?

The U-value is a measure of the rate of heat loss through an element so the lower the better in terms of energy performance. To put into context what the assumed value of 2.0 and the more appropriate value of 0.45, look at the following comparison values:

2.00        EPC assumption for BISF house

1.70        Un-insulated Solid Brick/Stone

1.50        Unfilled Cavity Wall

0.80        Cob wall (as seen in Wales with thatched roofs)

0.70        Filled Cavity Wall (of a similar age to BISF houses)

0.55        Building Regulations recommendations for current retrofit of Cavity Wall Insulation

0.45        BISF house as it was built

0.28        New Build Insulated Cavity Wall (as of 2012)

As you can see, the thermal efficiency of a BISF house was well ahead of it’s time. In fact, cavity walled houses only began to surpass this performance in 2003!

But the insulation could have failed..

It is very likely that the insulation has by now failed. As the initial insulation was effectively like the rolls of glassfibre you’d put in your loft, they are very likely to have slumped and all be bunched up at the base of the walls, leaving the remainder of the wall un-insulated. I’m yet to move into my BISF house but as I need to re-paint the steel frame I will be exposing the frame so I will be able to see the existing condition of the insulation. My plan is to replace this with new blocks of glassfibre which shouldn’t slump in the same way as the rolls. I guess the modern Kingspan and equivalent insulation would avoid this altogether but it is very expensive for the performance it gives.

And a note on EWI in EPCS

If you’re home has been retro-fit with External Wall Insulation there is a good chance that the EPC already takes this into account. When insulation is installed a U-value calculation is carried out and supplied as part of the installation. This U-value calculation can be used by the energy assessor to override the assumed U-value of 2.0. However, to do this the assessor will need a copy of the U-value calculation to be kept as evidence to support the EPC (energy assessors are actually audited and QA controlled far more stringently and frequently than plumbers, electricians, builders etc. and need to keep extensive evidence for 15 years to support their work). If they cannot take a copy of a U-value calculation then rather than attempt to guess at what thickness of insulation is used they are more likely to put ‘system build as built’ which would give the property a wall U-value of 2.0.

So, that may be a bit of a thesis to read but hopefully it all makes sense.


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Admin
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30/07/2018 2:43 pm  

Hello Owen, the warmest of welcomes and thank you for sharing your knowledge and providing a fantastic insight into EPC calculations appertaining to BISF Houses.

I had noticed that during every EPC assessment that I have seen been carried out, the assessor as you say, always classified the property as system built but I had no idea that this would provide an assumptive level of 2.0 to be applied to the property.

In essence, this would mean that in most cases, the EPC certificate for a standard non-improved BISF house, would not provide a true and accurate record of the buildings performance.

I find it quite unsettling that a potential buyer may well compare a traditional construction vs a BISF house of the same period. The traditional property would undoubtedly appear to offer better EPC performance on paper, when in fact this may not be the case at all.

So in essence, before pen even touches paper, so to speak, a BISF House or indeed any system build, will be disadvantaged from the start?

In cases where the property has been externally or even internally insulated, can we insist the assessor takes this into account, or is it entirely down to the assessor?

 


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DrJohn
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30/07/2018 2:46 pm  

Fascinating read Owen!

Thank you for opening my eyes to this darkened world of energy performance.

Surely though, in the modern world of system built housing and the likes of HUFF and other highly insulated and air tight systems, these properties wouldn’t be automatically assigned a 2.0 would they?


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Owen
 Owen
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Posts: 5
30/07/2018 6:54 pm  
Posted by: Admin

Hello Owen, the warmest of welcomes and thank you for sharing your knowledge and providing a fantastic insight into EPC calculations appertaining to BISF Houses.

I had noticed that during every EPC assessment that I have seen been carried out, the assessor as you say, always classified the property as system built but I had no idea that this would provide an assumptive level of 2.0 to be applied to the property.

In essence, this would mean that in most cases, the EPC certificate for a standard non-improved BISF house, would not provide a true and accurate record of the buildings performance.

I find it quite unsettling that a potential buyer may well compare a traditional construction vs a BISF house of the same period. The traditional property would undoubtedly appear to offer better EPC performance on paper, when in fact this may not be the case at all.

So in essence, before pen even touches paper, so to speak, a BISF House or indeed any system build, will be disadvantaged from the start?

In cases where the property has been externally or even internally insulated, can we insist the assessor takes this into account, or is it entirely down to the assessor?

 

Thanks Admin,

Yes, it’s not possible for the assessment to be accurate because the system build category is too vague and may include very poor performing buildings. EPCs will always take the worst case where there is uncertainty.

Yeah, that’s why I thought it was worth raising. This site has been a great resource for me buying my BISF house so hopefully it helps inform other future buyers. It turns out the design of BISF houses was very advanced for their time from a thermal efficiency perspective.

In cases where EWI or IWI has been installed the assessor is unlikely to take it into account for a BISF house unless he can take a copy of the U-value calculation to be kept as evidence to support overriding the RdSAP default of 2.0. In traditional construction types the assessor could record 50/100/150mm of external insulation and evidence this with a photo at the base of the external wall showing a ruler against the insulation (there’s normally a gap at the bottom with no insulation). However, for system build it can be a bit iffy. Unless the assessor is very familiar with the wall construction they are likely to take the safest option and input ‘as built’.


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Owen
 Owen
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Posts: 5
30/07/2018 7:05 pm  
Posted by: DrJohn

Fascinating read Owen!

Thank you for opening my eyes to this darkened world of energy performance.

Surely though, in the modern world of system built housing and the likes of HUFF and other highly insulated and air tight systems, these properties wouldn’t be automatically assigned a 2.0 would they?

You’re welcome.

The RdSAP assumption lookup tables also take age band into account. This reflects changes in Building Regulations (Approved Document Part L1B) over time. A new build ‘system build as built’ is assumed to have a wall U-value of 0.28.

Also, RdSAP is the methodology used for existing dwellings. For new builds, full SAP is used which is far more granular and contains U-values for all elements. So the EPC for new builds should be the most accurate as this is based on the drawings and specifications of the house.

I used to work in the energy industry and also work on one of the main software calculation tools used by energy assessors. I took the training to become an energy assessor (although I’d be bored stiff doing the job with all the measuring etc.) and I was surprised how highly skilled these assessors actually are. They are very heavily audited too. It’s a little scary how much more strict we are on the quality of our energy assessors than on the plumbers who work with our gas appliances, the electricians who wire our homes or the builders who build structural elements. In comparison they are a law unto themselves.

If you are interested you can find all the RdSAP assumptions here to see what else makes up your EPC:

https://www.bre.co.uk/filelibrary/SAP/2012/RdSAP-9.93/RdSAP_2012_9.93.pdf

P.3 Age Bands

P.19 Wall U-Values

P.23 Roof U-Values


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