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11 hours ago, ADLIan said:

Read the SAP document carefully as most answers are in there. Possible errors are air infiltration rate, treatment of linear thermal bridges, thermal mass, boiler efficiency, heating controls...

 

Having a look at the calcs there as an assumed air permeability rate of 15 m3/hm2, which is to high. I have reduced this down to 5 m3/hm2, which is a conservative estimate as looking to get it lower than this. It may need to be is i use a MVHR.

 

DER is now 20 with a couple of other changes, EPC of 81 and 80 (Efficiency / Impact), so i need to have further look and the SAP document more and see if its the building design or the calcs.

 

 

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Ventilation has a big impact.  5m3 or lower is easily achieved.  To get as poor as 15m3 you would need to forget to install the windows.  It is easier to remediate a failed air test than a sound test.  If there is any room for solar PV it would help lots?

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Do you have PV? From memory, that makes a significant difference.

 

I remember discussion aeons ago about other heat recovery gadgets that could add a point or two  - separate issue as to whether they were a good return on investment.

 

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17 minutes ago, Mr Punter said:

If there is any room for solar PV it would help lots?

 

The roof is all 'flat / 1:40 fall' which i could put PV on, but i am pretty concerned about putting PV up there due the potential for leaks and costs.

 

I have put in a s 1Kwp solar system in the calcs, which gets the building to a SAP of 85, though really not wanting to go down the route of solar.

 

So it looks like the solar does have a big effect.

 

I am considering using a ASHP, which doing the calcs swapping out the boiler actually raises the TER, to 25 and lowers the DER to 18, so all good on that front

 

however the SAP result is still at 81 / 83, still some tinkering to go. 

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8 minutes ago, Moonshine said:

 

The roof is all 'flat / 1:40 fall' which i could put PV on, but i am pretty concerned about putting PV up there due the potential for leaks and costs.

 

I have put in a s 1Kwp solar system in the calcs, which gets the building to a SAP of 85, though really not wanting to go down the route of solar.

 

So it looks like the solar does have a big effect.

 

I am considering using a ASHP, which doing the calcs swapping out the boiler actually raises the TER, to 25 and lowers the DER to 18, so all good on that front

 

however the SAP result is still at 81 / 83, still some tinkering to go. 

@Moonshine

Check that you are using Accredited Construction Details (to do with thermal bridges). They will make a big difference to your SAP score.

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Posted (edited)
31 minutes ago, Ian said:

Check that you are using Accredited Construction Details (to do with thermal bridges). They will make a big difference to your SAP score.

 

Ah now this is something that i have been missing as i wasn't calculating HTB manually, and left FSAP to do this automatically.

 

Time to put in some actual accredited values for the thermal bridges, anyone able to provide guidance on this?

Edited by Moonshine

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23 minutes ago, Moonshine said:

 

Ah now this is something that i have been missing as i wasn't calculating HTB manually, and left FSAP to do this automatically.

 

Time to put in some actual accredited values for the thermal bridges, anyone able to provide guidance on this?

some examples for timber frame construction here: https://webarchive.nationalarchives.gov.uk/20141202161423/http://www.planningportal.gov.uk/uploads/br/wood_frame_illustrations.pdf

There are other examples for different forms of construction

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Posted (edited)

@Moonshine That document (SAP OCDEA nher) is out of date but does give good info on TBs. Download latest SAP conventions (Sept 19) from the BRE SAP website for current advice.

Edited by ADLIan
clarify reference
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Posted (edited)
23 minutes ago, ADLIan said:

@Moonshine That document is out of date but does give good info on TBs. Download latest SAP conventions (Sept 19) from the BRE SAP website for current advice.

 

Nice, these are available here

 

https://www.bregroup.com/wp-content/uploads/2019/08/SAP-Conventions-version-8_1-from-01-September-2019.pdf

 

One of things i was struggling with as the value of the thermal bridge of a flat roof with a parapet wall, as the default Ψ-value in SAP 2012 is 0.56, with no approved value.

 

However i found the BRE Certified Ψ-value (W/mK) of 0.046, for the construction i am wanting to use.

 

https://www.bregroup.com/certifiedthermalproducts/product.jsp?id=3594&thermalId=817

 

A significant reduction as there is 40 linear metres of this junction.

 

With use of BRE Certified Ψ-value's in key areas rather than the default  / approved values of SAP 2012, the HTB is down to 22.2 W/m2K. It can probably go lower.

 

If i can get the HTB is down to 10 W/m2K using BRE Certified Ψ-value more, that gives me a SAP of 84.4.

 

Nearly there......

Edited by Moonshine

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27 minutes ago, ADLIan said:

Download latest SAP conventions (Sept 19) from the BRE SAP website for current advice.

 

Good job you showed me this as it clarified the dimension measurement, with the floor to ceiling height used for volume calcs (e.g. 2.4m), where i was using the floor to floor measurements (e.g. 2.7m), that was worth half a SAP point! :)

 

 

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The parapet roof/wall junction you refer to is from the Scottish ACDs. (It is not very detail in one sense in that it is showing a hybrid warm roof construction which is generally not recommended). Note it has a layer of insulation internal to the TF (not just between). If you have this construction then download the Scottish ACDs as all the psi-values will be much better. Note also limitations/constraints on rest of construction.

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8 minutes ago, ADLIan said:

The parapet roof/wall junction you refer to is from the Scottish ACDs. (It is not very detail in one sense in that it is showing a hybrid warm roof construction which is generally not recommended). Note it has a layer of insulation internal to the TF (not just between). If you have this construction then download the Scottish ACDs as all the psi-values will be much better. Note also limitations/constraints on rest of construction.

 

Thanks, my understanding of the hybrid system, is as long as dew point occurs in the top layer if the thermal insulation (e.g the warm roof PIR), there isn't an issue, though very happy to be corrected though.

 

I am planning an internal wall leaf of 140mm timber stud with mineral batts, with 25-50mm PIR internal to the studs.

 

 

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I doubt you'd get the insulation manufacturers to support the use of hybrid flat roof. Not covered in Appr Docs or BS on condensation risk. BS on flat roofing mentions hybrid roofs but warns against their use requiring full CRA, under specific climatic conditions, and very careful detailing of the air & vapour control layer, it must be 'fully sealed' (not sure how you achieve that in practice!)

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6 minutes ago, ADLIan said:

I doubt you'd get the insulation manufacturers to support the use of hybrid flat roof. Not covered in Appr Docs or BS on condensation risk. BS on flat roofing mentions hybrid roofs but warns against their use requiring full CRA, under specific climatic conditions, and very careful detailing of the air & vapour control layer, it must be 'fully sealed' (not sure how you achieve that in practice!)

 

Thanks, what i am proposing is the insulation above the deck is there to achieve the required U-value (150mm PIR), and 100mm mineral wool insulation between the joists is for acoustic reasons (aircraft), as below

image.png.f9db84dee2ab7dc90e23fa25ea32b74d.png

 

I am not trying to 'double' up the thermal performance as seems to be main rational for the hybrid roof, however this is more a by-product of the insulation being in there for acoustic reasons, to absorb some sound energy.

 

Are you saying that for a true warm roof the cavity has to be completely void / no significant difference in temperature across its depth?

 

If the acoustic insulation in the cavity had very poor thermal properties, would there be an issue?

 

An alternative could be the rolls of mineral wool insulation between the joists could be laid out to cover 75-80% of the ceiling area, so that in the 20-25% area gaps there was enough thermal weakness that the heat from the room below would heat both sides of the mineral wool insulation negating the risk for a temperature difference and condensation?

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Normally there would not be any additional insulation between the roof joists. If adding something for acoustics keep it low density/higher conductivity mineral wool and ask the flat roof insulation manufacturer to run the CRA with specific reference to BS 6229. Ensure the AVCL above the plasterboard is as 'fully sealed' as possible, it will never be perfect!

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Posted (edited)
3 hours ago, ADLIan said:

If adding something for acoustics keep it low density/higher conductivity mineral wool and ask the flat roof insulation manufacturer to run the CRA with specific reference to BS 6229. Ensure the AVCL above the plasterboard is as 'fully sealed' as possible, it will never be perfect!

 

Cheers, i was thinking something up to 24 kg/m3, so something like 100mm rockwool roll, density 22 kg/m3, which has a thermal conductivity of 0.044 W/mK, i have sent off to a few manufacturers to see what their thoughts are.

Edited by Moonshine

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Calculations from a PIR manufacturer of 150mm PIR, indicates no condensation and U value of 0.11

image.thumb.png.ee3f96651607b37cc26039c17359f0f6.png

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Good news. However that calculation is not in line with BS 6229 on flat roofing which uses more extreme external conditions (as my message above). Internal humidity class may need stepping up (dwelling with high occupancy) to cover worst case scenario

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4 minutes ago, ADLIan said:

However that calculation is not in line with BS 6229 on flat roofing which uses more extreme external conditions (as my message above). Internal humidity class may need stepping up (dwelling with high occupancy) to cover worst case scenario

 

That is a bit over my head, if its not too much to ask, could you break it down of what the extreme external condition of BS 6229 is, and what the internal humidity class may need to be upped to? this is so i can back to them?

 

btw Its a 5-6 person house.

 

Thanks

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Posted (edited)

The Standard uses -5 Deg C as external winter temp. Not sure what the other conditions are as I'm not directly involved in this sector. Normally the internal classes are low occupancy dwellings and high occupancy dwellings (there are others but not relevant here). A good PUR  manufacturer with experience in flat roofing should have this info and work to this Standard.

On the positive side I would imagine the construction would be relatively safe looking at the relative thermal performance of the PUR and mineral wool but proceed with caution!

 

Was the calculation done by the manufacturer? I would expect a product name and reference not just ‘PIR’.

Edited by ADLIan
Additional question

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Posted (edited)
23 hours ago, ADLIan said:

Was the calculation done by the manufacturer? I would expect a product name and reference not just ‘PIR’.

 

yes it was with a specific product, i just covered the name of the manufacturer up just in case there was any issue in me posting it up.

 

I think that i need to get a copy of BS 6229 and read the bit about condensation (if i can get a copy of it)

Edited by Moonshine

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