Solar Thermal cost effective?

[AliG]

PV beats solar water heating, but the SAP calculation is the real issue here.

 

I don't quite understand what you mean by failed the SAP calls, what is the minimum target?

 

We have gas for heating and hot water and our expected SAP was 88 I think, we have added solar PV since then and will be borderline an A.

 

The problem is maybe the assumed 5m3/h/m2. With very modest attempts at air tightness you will be well below this, but you need an air tightness test to put that lower number in the SAP calc.

 

The other problem could be your walls. What is the exact build up, full fill 100mm Rockwool with brick and block has a U-Value of around 0.3? This is the absolute minimum to meet building regs in England. The easiest way to improve this without changing the design would probably be insulated plasterboard on the inner face.

 

The best way to spend your money is probably on improving the insulation, not PV or an ASHP.

Edited August 19, 2020 by AliG

[PeterW]

5 hours ago, DerbyLad said:

The external walls are full fill with 100mm, the roof has 400mm and the floor has 150mm insulation

150mm of what ..?? PIR would give you 0.12, EPS is about 0.18

 

Don’t agree with the SAP guy about adding wall insulation - you will reap the benefits of living in a house with decent insulation through lower bills so why not do it ..? It’s definitely cheaper to do it now. 

[Nickfromwales]

@DerbyLad

 

Have a read of this too;

 

 

[PeterW]

5 hours ago, DerbyLad said:

The external walls are full fill with 100mm, the roof has 400mm and the floor has 150mm insulation

150mm of what ..?? PIR would give you 0.12, EPS is about 0.18

 

Don’t agree with the SAP guy about adding wall insulation - you will reap the benefits of living in a house with decent insulation through lower bills so why not do it ..? It’s definitely cheaper to do it now. 

[Nickfromwales]

37 minutes ago, PeterW said:

150mm of what ..?? PIR would give you 0.12, EPS is about 0.18

 

Don’t agree with the SAP guy about adding wall insulation - you will reap the benefits of living in a house with decent insulation through lower bills so why not do it ..? It’s definitely cheaper to do it now. 

+1

[DerbyLad]

19 hours ago, joth said:

Unless you're in a conservation area it can go on the front of the building and still qualify for permitted development, no Planning application needed

https://www.planningportal.co.uk/info/200130/common_projects/27/heat_pumps/2

 

(These might be rights you only gain after the property is signed off as complete?)

 

Unfortunately, you are correct. Permitted development rights are only gained once the property is signed off built as planning and occupied.  Crazy UK planning laws!

[DerbyLad]

Thank you every one for your input, it's great to read so many suggestions.

 

I would, if possible, prefer not to change the construction.  The drawings have been passed by building control and I've had the structural calcs done.  I would assume I would have to have these all done again if I went for 150mm wall insulation!? If needs must then I well.  Just feel like I'm going backwards with every step forward at the moment

 

Ok, Information taken from the building reg drawings

The roof has 100mm mineral wool laid between joists with 2 layers of 150mm laid over

The walls are 100mm Knauf Dritherm 32

Inner walls are 100mm plasmor fibolite 3.6n block

Flooring is 150mm Kingspan or similar grade insulation

 

I did wonder if the wall insulation could be a material with a lower u value - better insulation-  which would help?  Or perhaps insulated plasterboard as has been suggested. My rooms are quite generous so could stand it.

 

[joth]

57 minutes ago, DerbyLad said:

 I would assume I would have to have these all done again if I went for 150mm wall insulation!? If needs must then I well.  Just feel like I'm going backwards with every step forward at the moment

If it's any consolation, it's still a heck of a lot cheaper to change this now rather than after the building is built and you (or future owner) decides a need to retrofit more insulation.

 

Edited August 20, 2020 by joth

[PeterW]

1 hour ago, DerbyLad said:

The walls are 100mm Knauf Dritherm 32

Inner walls are 100mm plasmor fibolite 3.6n block

That is your SAP issue - that’s barely inside regs. Wall is 0.27 based on that build up, limiting factor is 0.30. You can get to 0.18 just by using a 50mm additional cavity which is far more cost effective than adding insulated plasterboard. Reducing the cavity really doesn’t affect all of your structural calcs as 100mm is negligible in terms of spans etc. Lintels are £5-10 more expensive for the wider cavity so that adds nothing. 
 

I would speak to Building Control and ask their advice if they want it completely redrawn - standard founds can cope with a 50mm variation as can most other structural elements. Floors and roof components should always be as built measurements anyway as you will find the brickies will adjust your layouts to meet the standard block and brick sizings so they aren’t cutting slips to make sizes work. 

[AliG]

@PeterW is correct. A wider cavity is probably the cheapest solution, if you used 37.5mm insulated plasterboard on the inside you would also get the U-Value down to 0.2 or thereabouts. This is simple if you already plan to use plasterboard but more expensive.

 

I don't know why most houses in England still seem to be built this way, it would fail the minimum regs in Scotland

[joe90]

I have a 200mm cavity with dritherm 32, 250mm underfloor polyurethane foam, 400mm dritherm in the warm roof. Quite toasty.

[DerbyLad]

Still looking at options to insulate the 100mm gap.  Is dritherm 32 the best to use or are there any better products with higher r values?

[joe90]

dritherm is not the best for 100mm, I would consider blown in poly beads.

[PeterW]

Beads don’t do much better - seriously change the cavity now whilst you can. 

[AliG]

I found this on the Plasmor website -

 

A couple of things -

 

1. It looks like your build up has a U-Value of 0.25, I would check the SAP calculation has this data and not generic data.

 

2. You can get that down to 0.18 using Xtratherm CavityTherm but it costs £26ish a square metre compared to about £6 for the Dritherm. The uplift for using insulated plasterboard would be more like £8 a square metre compared to £20 here.

 

 

 

 

[DerbyLad]

Here is my SAP report, just to make sure we are all looking at the correct problem before I discuss further.  Changing the walls seems the only realistic option.

SAP Report - Priory Way, Ripley.pdf

[A_L]

@DerbyLad, The flat roof skylight? (Lantern) at 5.06m² and U=2.7 would have a significant effect if U could be reduced to 1.5, about half the effect of reducing the wall from U=0.25 to 0.15. Also why is door 3 (D3) U=3.0, reducing to same as other doors would be about 1/4 the effect of reducing wall U value. Judicially reducing the design air infiltration rate would also help, but not to 3.0 or less which wood require a ventilation system, but you would have to achieve it come the 'as built' SAP assessment.

[Onoff]

Apologies if slightly off topic but is there an option to fit solar thermal tubes as an "in roof system"? maybe sat on sarking boards or similar?

[Nickfromwales]

1 hour ago, Onoff said:

Apologies if slightly off topic but is there an option to fit solar thermal tubes as an "in roof system"? maybe sat on sarking boards or similar?

An example here

https://www.smallsolar.co.uk/installed-systems/

[Onoff]

13 minutes ago, Nickfromwales said:

An example here

https://www.smallsolar.co.uk/installed-systems/

 

Ta. But you know me...tight.

 

Mulling how to go about "dropping" ST evacuated tube panels into a tiled and battened roof (not counter battened). Half thinking to leave the tiles and battens off of the area under the ST panels but put cloaking type, waterproof cement boards atop the felt. This for both waterproofing and heat protection.

 

Not sure if I'd gain much depth wise...

 

 

YES, I know this SW facing, unshaded roof screams out for PV but it ain't happening!

 

Anyway, @scottishjohn is going to answer all my ST questions! ?

[DerbyLad]

19 hours ago, A_L said:

@DerbyLad, The flat roof skylight? (Lantern) at 5.06m² and U=2.7 would have a significant effect if U could be reduced to 1.5, about half the effect of reducing the wall from U=0.25 to 0.15. Also why is door 3 (D3) U=3.0, reducing to same as other doors would be about 1/4 the effect of reducing wall U value. Judicially reducing the design air infiltration rate would also help, but not to 3.0 or less which wood require a ventilation system, but you would have to achieve it come the 'as built' SAP assessment.

Hi,

D3 is an internal door into the garage.

I'm double checking with my window people on the u-value of the Lantern 

[AliG]

These people can do a double glazed roof-light with around a 1.2 U-Value or triple glazed at around 0.9.

 

https://roof-maker.co.uk/rooflights/lanterns/slimline-rooflights/

 

they give you a price instantly on the web-site. They can make a 5m2 lantern light.

 

I would be worried that as well as having a poor U-Value that rooflight would be very noisy.

 

The 3.0 U-Value on door D3 seems to be an assumed value. It is covered up by the word Draft, but if it is a steel fire door it may have a U-Value more like 2.2.

[DerbyLad]

Hi All,

 

Had a thermal mass calculation done, changed the roof lantern to 1.2 u-value, increased loft insulation to 500mm and I now pass the SAP with a gas boiler!

Yes, door D3 has an assumed u-value. It's a fire door.  Getting an insulated fire door is quite tricky.