How low a U value is good enough?

[Russell griffiths]

2 hours ago, Thorfun said:

it would be interesting to know the thickness of the ICF walls to get you to 0.12 W/m2K as that's my target U-value in our timber frame and our walls are 140mm studs with mineral wool between the studs and 80mm PIR on top so 220mm (not including VCL or service void which the ICF won't have either). I would reckon that ICF walls would need to be nearer 400mm thick to get similar U-values, but I might be wrong. But I do think TF will get you better U-values with thinner walls simply due to the fact you can put the insulation within the wall whereas ICF is on either side.

 

But, as has been said before, U-values aren't everything and every one needs to make their own decisions on their requirements and ideals.

I suppose it depends if you believe what your frame co are telling you regarding u values, have you done the calcs yourself, I’m not convinced by your figures. 

[Thorfun]

1 minute ago, Russell griffiths said:

I suppose it depends if you believe what your frame co are telling you regarding u values, have you done the calcs yourself, I’m not convinced by your figures. 

I did the calculations myself using changeplan.co.uk. that's Frametherm32 between the studs and 0.22W/mK PIR on top of that. works out to 0.12W/m2K

 

[Russell griffiths]

I’m baffled then, I have a similar build up in a roof pir under rafters, Frametherm in between the  rafters, I have more insulation than you and rockwool did my calcs and came out at 0.14. 

[Thorfun]

5 minutes ago, Russell griffiths said:

I’m baffled then, I have a similar build up in a roof pir under rafters, Frametherm in between the  rafters, I have more insulation than you and rockwool did my calcs and came out at 0.14. 

dunno then. I can only go on what the computer says! I think standard PIR on changeplan is 0.023W/mK but I changed that to 0.022W/mK as you can find stuff that has that thermal conductivity but if I change it to 0.023W/mK I still get a 0.126W/m2K U-value.

 

maybe it's changeplan's calculations? or maybe it was Rockwool being conservative? or maybe they didn't include all the other bits of the wall makeup like the external battening and cladding and internal service void etc?

 

I know there are much cleverer people than me at U-value calculations on here so maybe one of them can tell us who's right. ?

[A_L]

20 minutes ago, Thorfun said:

dunno then. I can only go on what the computer says!

 

because of the ventilated cavity you should remove the thermal resistances for the associated softwood/air space/wood cladding, but you can insert an enhanced exterior surface resistance, add a layer of 0.09m² .K/W to correct.

 

does the VCL have a shiny surface? it needs one to get the enhanced thermal resistance for the unventilated cavity

[ADLIan]

Insulation manufacturers are normally exact with their U-values. Looking at the above calcs;

• the service void airspace is unlikely to be low emissivity (only if AVCL is reflective/foil)
• anything external to a vent airspace is ignored and external surface resistance set to same as internal

See BR443 on U-value conventions and BS EN ISO 6946

[Thorfun]

18 minutes ago, A_L said:

 

because of the ventilated cavity you should remove the thermal resistances for the associated softwood/air space/wood cladding, but you can insert an enhanced exterior surface resistance, add a layer of 0.09m² .K/W to correct.

 

does the VCL have a shiny surface? it needs one to get the enhanced thermal resistance for the unventilated cavity

I know we've had these conversations before but my memory is so poor that I forget everything! did that but it didn't really make much difference.

 

again, bad memory, but I thought that the unventilated cavity was low emissivity if it wasn't a shiny surface? but again, how much of a difference does it really make?

[Thorfun]

19 minutes ago, ADLIan said:

Insulation manufacturers are normally exact with their U-values. Looking at the above calcs;

• the service void airspace is unlikely to be low emissivity (only if AVCL is reflective/foil)
• anything external to a vent airspace is ignored and external surface resistance set to same as internal

See BR443 on U-value conventions and BS EN ISO 6946

ok. my bad. again, I'm sure I've had these conversations before with you guys so thanks for setting me straight again. I guess my U-values aren't as good as I think they're going to be! if I remove all the external void and make the unvented one high emissivity then I get 0.137W/m2K which would correlate with @Russell griffiths calculations. looks like I'll need to get my SAP re-done then. the thermal modelling company didn't pick up on it, but I'm not surprised, they were a bit s**t.

[A_L]

2 minutes ago, Thorfun said:

I thought that the unventilated cavity was low emissivity if it wasn't a shiny surface?

 

no, that makes it high emissivity. It is deemed to have a thermal resistance of 0.18m²K/W 

 

page 11 attached pdf

 

BR_443_(2006_Edition).pdf

[epsilonGreedy]

3 hours ago, PeterStarck said:

Snowing here .

 

 

It is snowing in Bude as well but @joe90has not noticed because his wife is chasing him around the houses demanding to know why he opened the bi-fold doors to let in an arctic blast of 27 Fahrenheit air.

[Thorfun]

47 minutes ago, A_L said:

 

no, that makes it high emissivity. It is deemed to have a thermal resistance of 0.18m²K/W 

 

page 11 attached pdf

 

BR_443_(2006_Edition).pdf 153.24 kB · 0 downloads

yeah, cheers. updated calculations attached.

 

[joe90]

1 hour ago, epsilonGreedy said:

27 Fahrenheit

Nah, Centigrade, toasty !

[Nick Laslett]

3 hours ago, cbk said:

ICF Wall thickness is 440 if you go for the woodcrete type to get to 0.12 and 375 for EPS type and this achieves 0.11.  Planners have forced me to use multiple finishes externally - Brick, Stone, Timber and render all of which can go directly onto the ICF wall surface so no need to batten etc. which also saves time.   

I’m using the Thermohouse ICF system, their block U values are:

 

300mm Standard Block
0.20W/m2K
400mm Passive Gold Block
0.12W/m2K
 450mm Passive Platinum Block
 0.10W/m2K

 

They use a rebated block for the reveals, which makes window air tightness much simpler. They also have an EPS based flooring system and roofing system. They use the BASF Neopor EPS which has some advantages over regular EPS. 

[bradders3109]

On 10/02/2021 at 15:03, Thorfun said:

SWMBO doesn't really care and just wants the ****ing house built!

 

I know that feeling : (

[cbk]

On 10/02/2021 at 21:00, Nick Laslett said:

I’m using the Thermohouse ICF system, their block U values are:

 

300mm Standard Block
0.20W/m2K
400mm Passive Gold Block
0.12W/m2K
 450mm Passive Platinum Block
 0.10W/m2K

 

They use a rebated block for the reveals, which makes window air tightness much simpler. They also have an EPS based flooring system and roofing system. They use the BASF Neopor EPS which has some advantages over regular EPS. 

 

Which type of block do you decide to use?  Also do you plan to use the ThermoRoof option?

[Conor]

We've decided to add another 100mm EPS to the standard Amvic block, bringing us to 0.12.

@cbk we're about to have our thermohouse roof panels installed. Roof will be on within a week. Cost is coming in less than a cut roof. Not sure now it would compare to a standard truss roof tho.

 

[cbk]

So adding 100mm EPS to a standard block is a cheaper option than choosing a thicker Amvik block?

[Nick Laslett]

13 hours ago, cbk said:

 

Which type of block do you decide to use?  Also do you plan to use the ThermoRoof option?

I went with the 300mm Standard Block (0.20W/m2K) and with the ThermoRoof and the Thermofloor. 
 

I am not particularly motivated by wall U-values, especially once you factor in windows and doors. Airtightness is probably more important. From memory the U-value wall test is pretty dumb and bears very little correlation to real life use, e.g. the junctions between elements are what really matter. You need an holistic approach, make sure the walls, floor, roof, windows and doors are adequately insulated and in proportion. 
 

If you really want to understand the thermal properties of your design, then I believe the Passive House Planning Package (PHPP) xls does a better job of the modelling. You can find some example spreadsheet outputs online to see the details of this modelling.

 

The Stroma SAP tool also allows you to play about with the thermal modelling, this is free to down load. 

Edited February 25, 2021 by Nick Laslett

[Conor]

12 hours ago, cbk said:

So adding 100mm EPS to a standard block is a cheaper option than choosing a thicker Amvik block?

Significantly. The EPS and fittings are costing me about £2k. Just my time then to fit them.

[Conor]

10 minutes ago, Nick Laslett said:

I went with the 300mm Standard Block (0.20W/m2K) and with the ThermoRoof and the Thermofloor. 
 

I am not particularly motivated by wall U-values, especially once you factor in windows and doors. Airtightness is probably more important.

The builder and another ICF guy have both told me not to bother with the extra as they say all their customers with standard block have very low heating bills and are perfectly happy. I'm just doing belt and braces.

Edited February 25, 2021 by Conor

[Nick Laslett]

2 minutes ago, Conor said:

The builder and another ICF guy have both told me not to bother with the extra as they say all their customers with standard block have very low heating bills and are perfectly happy. I'm just doing belt and braces.

I thought your approach of adding extra EPS was actually really clever and an elegant way to increase the U-value. I spent some time thinking about it for my project. 

[SteamyTea]

17 minutes ago, Conor said:

The builder and another ICF guy have both told me not to bother with the extra as they say all their customers with standard block have very low heating bills and are perfectly happy.

Get back to them and ask what the w.m^-2 for external wall figure is.

'very low' is meaningless, and misleading.

  

17 minutes ago, Conor said:

I'm just doing belt and braces.

Unlike trousers, you only have to attach them once

Edited February 25, 2021 by SteamyTea

[cbk]

41 minutes ago, Conor said:

Significantly. The EPS and fittings are costing me about £2k. Just my time then to fit them.

What fixings are you using to attach the 100mm EPS panel to the ICF block?  Is it just adhesive or is there a mechanical fixing as well?

[Conor]

8 minutes ago, cbk said:

What fixings are you using to attach the 100mm EPS panel to the ICF block?  Is it just adhesive or is there a mechanical fixing as well?

Expanding foam all the way round each sheet, then 140mm zinc plated screws with 36mm countersunk washers. 600mm spacing, so three rows of 5 screws. I'll probably put a few more in to be safe. Then a bead of foam on the edges of the block before placing the adjacent block.

 So a couple dozen per sheet. Screws are driven in about 30mm or so and filled in with more foam. This is to prevent moisture reaching the screws and causing rust issues. With a waterproof silicon render, moisture shouldn't be an issue anyway. Alternative would be stainless steel screws and washers, but these are significantly more expensive. This is all from advice from an ICF contractor that does it all the time. EPS arriving next week so will post up a blog entry.

Edited February 25, 2021 by Conor