What U value in walls is it worth paying for and having? My calcs must be wrong! Please help.

[Mike]

7 hours ago, Norbert said:

Let's just say I decide to go ahead with U .11, while my identical brother decides to build an identical house next door, but with just one exception, he uses .14.

 

Some time later both houses are finished. ASHP, UFH, MVHR, air tightness, windows, all other insulation, .... all identical. We are both very pleased with our comfortable houses.

 

It seems to me that the only difference is that I am spending ca. £50 per annum less on electricity, and he paid £6K less on his build. Thus it seems perfectly reasonable to focus on payback in this instance as there is no other difference. Or am I missing something?

 

It may not help in your circumstances, but Passivhaus design is one way of overcoming this economic dilemma. That is, if you increase insulation, and all the other factors, 'enough' (to require no more than 15 kWh/m² of floor area per year for heating and cooling, as calculated using the Passivhaus Planning Package), then you won't need to pay for a traditional central heating system. And that saving then pays, in theory, for the extra insulation and other measures required to achieve the standard.

 

In other words, there could be a world where you and your identical brother spend identical amounts on the build, but you save on your fuel bill each year and he doesn't.

 

Edited January 21 by Mike

[Norbert]

21 hours ago, Kelvin said:

Can you show us the exact wall build up?

External Wall Thermal Upgrade to U-Value 0.14W/m²K

External Reflective Thermo Breather Membrane

9mm OSB3 external sheathing

140mm x 38mm CLS C16 timber studs

140mm Mineral Wool Insulation (fitted between timber studs)

50mm PIR Rigid Insulation

Reflective Thermo VCL / Airtight Membrane

35mm Service Cavity Batten

 

External Wall Thermal Upgrade to U-Value 0.11W/m²K
External Reflective Thermo Breather Membrane
9mm OSB3 external sheathing
140mm x 38mm CLS C16 treated timber studs
140mm PIR Rigid Insulation (fitted between timber studs)
50mm PIR Rigid Insulation
Reflective Thermo VCL / Airtight Membrane
35mm Service Cavity Batten

 

These are the examples I quoted. I am also considering the blown cellulose option U .12

[AppleDown]

7 minutes ago, Norbert said:

External Wall Thermal Upgrade to U-Value 0.14W/m²K

External Reflective Thermo Breather Membrane

9mm OSB3 external sheathing

140mm x 38mm CLS C16 timber studs

140mm Mineral Wool Insulation (fitted between timber studs)

50mm PIR Rigid Insulation

Reflective Thermo VCL / Airtight Membrane

35mm Service Cavity Batten

 

External Wall Thermal Upgrade to U-Value 0.11W/m²K
External Reflective Thermo Breather Membrane
9mm OSB3 external sheathing
140mm x 38mm CLS C16 treated timber studs
140mm PIR Rigid Insulation (fitted between timber studs)
50mm PIR Rigid Insulation
Reflective Thermo VCL / Airtight Membrane
35mm Service Cavity Batten

 

These are the examples I quoted. I am also considering the blown cellulose option U .12

I'm going through the exact same decision route as you right now. Also remember that the Passive option for MBC will mean a much thicker wall, so could end up eating into your floor space.

[SteamyTea]

Which way is it reflecting, back into what what it is in contact with?

 

[Iceverge]

I did a payback on cost vs paybacktime. I picked 25 years for all the insulation improvements. I picked this as at the time it coincided nicely with the passivhaus targets, plus a bit for mum. 

 

Hence the best spend for us was 200mm eps in the floor, 250mm eps beads in the wall and 450mm cellulose in the attic. 

 

Thermally 275mm everywhere might have had the same heating load but the cellulose was cheaper than the eps boards so I put more in the roof. 

 

In your case, if you can afford it, get the twinwall with cellulose. It'll be much quieter and more airtight. You might even save money if you don't bother with a central heating system like we did. 

 

[joe90]

2 hours ago, AppleDown said:

MBC will mean a much thicker wall, so could end up eating into your floor space.

Not necessarily, with my build I designed the rooms then added the wall around them, only adding IWI to an existing build reduces room dimensions.

[AppleDown]

3 minutes ago, joe90 said:

Not necessarily, with my build I designed the rooms then added the wall around them, only adding IWI to an existing build reduces room dimensions.

True - my assumption was that it may be going through/been approved by planning without that in mind though...

[SteamyTea]

26 minutes ago, Iceverge said:

You might even save money if you don't bother with a central heating system like we did. 

I am not sure which bit of Ireland you at at, but a lot of your climate is similar to the SW of England.

250/300 miles East of me and the climate is really quite different.

40⁰C was recorded last year, so while full time, central heating systems may not be needed, cooling almost certainly will be wanted.

As an ASHP can do both heating and cooling, apart from the system size, a system will almost certainly be needed (not everyone likes the high temperatures that I do).

So I am not sure there is much saving to be made really.

 

Initial better thermal design if the building is really the answer, but it is impossible to convince people that they can live with small windows, and still get a good view.

It is even harder to convince people that reducing energy usage is the right thing to do as most think they are immune to the effects of climate change, and anyway, the severe weather we have had in the last decade is just that, weather, and (expletive deleted) all to do with climate.

[joe90]

15 minutes ago, AppleDown said:

True - my assumption was that it may be going through/been approved by planning without that in mind though...

If you added 100mm I doubt it would be noticed, I have yet to hear of any build being measured after completion (my roof was 100mm over the designed height but no one noticed 🤷‍♂️)

[Kelvin]

Given those three options the PIR might offer the best U value but overall it won’t be the best performing. Of the three the 0.12 blown cellulose is the best option. 

[AppleDown]

1 hour ago, joe90 said:

If you added 100mm I doubt it would be noticed, I have yet to hear of any build being measured after completion (my roof was 100mm over the designed height but no one noticed 🤷‍♂️)

After a chat we had recently with someone who had built a lovely chalet bungalow, I checked on their planning app as I wanted to copy their materials statement. Let's just say what we saw was a teeny bit different size-wise.

[Nick1c]

It’s worth bearing in mind that it is common for push fit insulation to be fitted less than perfectly, this can result in a dramatic reduction in performance. Blown insulation is less likely to suffer from this.
We went for cellulose partly for this reason, partly for the noise attenuation (which is fantastic) & partly for decrement decay. 

[Kelvin]

Keeping the outside noise, outside, is part of the overall equation. I overheard the plasterers chatting today about how quiet the house was. It was blowing a gale at the time. 

Edited January 22 by Kelvin

[Iceverge]

15 hours ago, SteamyTea said:

I am not sure which bit of Ireland you at at, but a lot of your climate is similar to the SW of England.

 

Cork.

 

15 hours ago, SteamyTea said:

40⁰C was recorded last year, so while full time, central heating systems may not be needed, cooling almost certainly will be wanted.

As an ASHP can do both heating and cooling, apart from the system size, a system will almost certainly be needed (not everyone likes the high temperatures that I do).

So I am not sure there is much saving to be made really

 

We've had highs of about 30 deg and the house didn't get above 26deg. Like you say, smaller windows, especially to the west. It would have been much cooler if my family didn't insist on leaving the windows and doors open while wandering in and out of the garden all day. 

 

A single A2A unit is a fraction of the price of a full ASHP and UFH setup. A passive house is like a canal boat. Once it's moving a tiny amount of power can keep it steady state. 

 

 

 

 

[joe90]

2 minutes ago, Iceverge said:

A single A2A unit is a fraction of the price of a full ASHP and UFH setup. A passive house is like a canal boat. Once it's moving a tiny amount of power can keep it steady state. 

I like that analogy 👍, why my build had a 4Kw ASHP.

[SteamyTea]

56 minutes ago, joe90 said:

4Kw

No it didn't.

kW

[SteamyTea]

1 hour ago, Iceverge said:

We've had highs of about 30 deg and the house didn't get above 26deg.

Last year the heat wave missed us, even though we did have a severe heat weather warning.  2022 was worse as we had similar OAT but clearer skies.  This did cause my house to get to a mean of about 27°C inside.  For an experiment I covered the inside of the windows in tin foil, this reduced the temperature a lot.

Not very practical as no light at all comes in, and you can't open the windows.

And it looks silly.

But may be a useful idea if made into easily fitted panels (see my other thread about that).

Edited January 23 by SteamyTea

[joe90]

5 minutes ago, SteamyTea said:

No it didn't.

kW

I blame auto correct 🤷‍♂️(plus I wanted to see if you were awake yet).

[SteamyTea]

Just now, joe90 said:

I blame auto correct

I have just done a blog post about insulation and commented on my phone not being able to insert a superscript minus sign, and if it is an R-Value or a R-Value.

[Thorfun]

On 21/01/2024 at 08:49, Norbert said:

The spreadsheet does not give the option to put in air tightness values, but based on comments above I understand that this becomes more important as other factors are improved.

this isn't true. Jeremy's spreadsheet most definitely allows you put a target ACH in. i used it to figure out my preferred target ACH.

[JohnMo]

3 minutes ago, Thorfun said:

this isn't true. Jeremy's spreadsheet most definitely allows you put a target ACH in. i used it to figure out my preferred target ACH.

Agree it lets you add ACH figure, but not a direct airtightness figure. If you were ventilating at 0.5ACH with MVHR, you add that figure and the efficiency of the MVHR.  The spreadsheet assumes you are pretty much airtight and well insulated.  If you know a target airtightness you can say add 10% of your airtight figure to the ach and drop the MVHR efficiency slightly. But if your airtightness is 3 or below, and you are using MVHR it's good enough as is - nothings exactly correct, but better than a ballpark away.

[Bramco]

1 hour ago, SteamyTea said:

For an experiment I covered the inside of the windows in tin foil, this reduced the temperature a lot.

Not very practical as no light at all comes in, and you can't open the windows.

 

We have the MBC 0.14U walls as above and blown cellulose vaulted roofs - think we got up to between 25 and 30 degs during the hot weather but it wasn't enough for us to wish we'd put a cooling system in.

 

One thing we do have are lots of sliding patio doors. The ones on the south elevation all have overhangs and external blinds. So in the summer the sun never really reaches the doors and in the spring and autumn when the sun is lower in the sky, we can tilt the blinds to reduce the solar gain. In the winter it's great, the sun streams through the doors giving great solar gain.  Bit more expensive than silver foil but a little more versatile....

 

Simon

[Alan Ambrose]

@Bramco - can I ask what kind of blinds you chose? I'm imagining shop awning style as they're tiltable rather than the external vertical roller ones I was considering myself? Or are they tiltable in the venetian sense?

[Bramco]

Tiltable venetians.

 

They were supplied through hallmark blinds - but essentially they are made somewhere like Poland to order.  Here's the link -> https://www.hallmarkblinds.co.uk/external-blinds. Scroll down to Trojan exterior blinds.   We looked at quite a lot of other suppliers, who either weren't interested or were stupidly expensive.

 

We also hate curtains but these work effectively for our master bedroom at night.  I think there is an interlocking version but we just went for the standard version and they are fine.

 

There are various Somfy controls, either hard wired or wireless and there's an automatic system to raise the blinds if it's too windy outside - we've disabled that - it's not a great look looking for the switch to bring the blinds back down again when yr stark naked - fortunately we're in the countryside though.

 

Simon

[Thorfun]

3 hours ago, Bramco said:

Tiltable venetians.

 

They were supplied through hallmark blinds - but essentially they are made somewhere like Poland to order.  Here's the link -> https://www.hallmarkblinds.co.uk/external-blinds. Scroll down to Trojan exterior blinds.   We looked at quite a lot of other suppliers, who either weren't interested or were stupidly expensive.

 

We also hate curtains but these work effectively for our master bedroom at night.  I think there is an interlocking version but we just went for the standard version and they are fine.

 

There are various Somfy controls, either hard wired or wireless and there's an automatic system to raise the blinds if it's too windy outside - we've disabled that - it's not a great look looking for the switch to bring the blinds back down again when yr stark naked - fortunately we're in the countryside though.

 

Simon

we have the Hallmark Trojans as well. supplied by a local dealer https://www.powellblinds.com. we planned for them from the off and so they are hidden behind the cladding above the windows. when they're up you can't tell that we have them.