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How low a U value is good enough?


cbk
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Hi guys.  I'm at the early design stage for a new build and trying to figure out just how low a U value is good enough.  I'm also undecided between a TF build and an ICF build but based on the budget quotes I'm getting,  the TF approach is looking a lot more expensive.  So I'm wondering is there a simple way to model what my actual heat demand will be using the various build systems to see how much of a difference a 0.15 value makes compared to a 0.12 value.  Some time ago I found a thread (and now lost it!)  with some spreadsheets that seem to work this out but I'm a bit stuck to know how to adapt to my build.  Any help would be appreciated.  Also thoughts on the ICF v TF comparison welcome.  For context I will be project managing the build but looking to use a local builder and trades for most of the build work.

 

 

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6 minutes ago, SteamyTea said:

Is that exposed wall area.

and it is kWh.

 

Makes sense - more coffee needed this morning. Still seems a pretty low amount of heat (a 10 x 10 two story house brings a similar figure)

And it is And ?

Edited by Trw144
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I’ve said it before and I will say it again. 

Do people get hung up on the u value of an element, when it’s all about good details and getting the whole construction right. 

 

My icf block walls  come out at 0.22, yet  I think as a whole structure the house is excellent. 

Good walls, good roof, good floor, good windows, good air tightness, and a brilliant builder turns into an excellent house. 

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I feel that you have to consider all the aspects of the build together, foundation, walls, roof, windows and doors. No point in having an unbalanced approach, with low U value walls, but not enough floor insulation, remember heat will always go to the coldest surface first. The level of air tightness, and how you are going to achieve it, is also very important. High levels of air tightness mean you need to think about MVHR. Low U-value doors and windows with just a standard install will not give good air tightness. Different build approaches create different issues with penetrations, cold bridges, etc. The number and size of the windows will greatly effect the U value of the whole wall. 
 

What is your ultimate goal? You could engineer a house that needs very little heating, but costs or aesthetics might be an issue. Looking at any of the straw bale house case studies is a real eye opener for your preconceptions. A highly insulated house doesn’t have to be expensive. 
 

With ICF, the concrete is the air tightness layer, so the junction between the top of the walls and the roof is the critical area where you could create cold bridges and lose heat. A big potential drawback to ICF is that any future remodelling can be very difficult, because the concrete walls are full of steel rebar in addition to being 100 - 150mm thick. 
 

On reading a lot of the threads here during the early stages of my journey, the thing that surprised me the most was that cooling, needs to be higher up on your list of things to worry about. Also try to factor in the plant room in the floor plan design.

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41 minutes ago, Nick Laslett said:

I feel that you have to consider all the aspects of the build together,

I agree, I don’t know exactly what my U value is but I built with large cavities, double glazing (that’s engineered to nearly triple U value) lots of under slab insulation and roof insulation. Great attention to airtightness and details such as doors and orientation. All I know is my heating is only on for a couple of months a year. I do have a woodburner which some say is against “passive” but we love it. 

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49 minutes ago, Russell griffiths said:

Do people get hung up on the u value of an element, when it’s all about good details and getting the whole construction right. 

When good design, detailing and construction are a given, then the U values of all elements and the airtightness are the most important factors to produce a comfortable, low energy house for a particular area. If the U values are too high it won't be a low energy house.

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1 hour ago, Russell griffiths said:

Do people get hung up on the u value of an element, when it’s all about good details and getting the whole construction right. 

Could not agree more, it means nothing to have three foot of top quality insulation and quadruple glazed windows if the detailing is shite and cold air bypasses these marvels, yes have a plan and some carefully worked out ideas but the execution of the detail is where you should be absolutely sure of achieving your goals. If your not doing the work yourself then you really need to understand how this detail is going to be achievable. 

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Thanks Guys, lots of very helpful content.  To try and fill in some of the gaps this is where I've got to so far:

  1. House is about 220m2 on a south facing plot in Shropshire.  Lot of glazing on the south side (including a 5.5m x 2.4m slider) but design features a balcony to act as a brise soleil for ground floor and roof overhang to protect first floor from overheating.  Architect has confirmed there is less than 2% chance of any overheating.  There is a risk this design has overdone the shading so I potentially may lose some of the winter heat upside!
  2. Current plan is for EPS raft foundation with a U value of about 0.11, trussed warm roof should be about 0.12
  3. Preferred supplier for Windows has Uw value of 0.79, Slider will be 1.1, entrance doors about 1.0., and there are 6 rooflight that will be 0.1.
  4. Design includes MVHR and target is to achieve high levels of air tightness.  I'm  learning that detail here is the key and probably my weakest link as I plan to use a local builder for much of the work so will be a challenge to ensure attention to detail.

So, with all these  elements pretty much decided I'm trying to settle the question of just how low do I need to go for the walls.  For the ICF option the difference between 0.15 and 0.12 is only about £5k for block supply and I'm assuming labour etc. will be the same for either.  The other difference is in wall thickness and I will lose about 75mm which is OK if there is a real benefit in the lower U value, otherwise it would be nice to keep the extra space if the higher value is sufficent.  Not yet managed to get to the same level of analysis for the TF options but I guess it will come down to the same trade-off.

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6 minutes ago, cbk said:

The other difference is in wall thickness and I will lose about 75mm which is OK if there is a real benefit in the lower U value, otherwise it would be nice to keep the extra space if the higher value is sufficent.  Not yet managed to get to the same level of analysis for the TF options but I guess it will come down to the same trade-off.

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.

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Just now, joe90 said:

Extra insulation works on diminishing returns, the first inch makes a big difference but the more you have you receive less benefit per inch.

yep, and this is making me reconsider our (I really should say 'my' as SWMBO doesn't really care and just wants the ****ing house built!) target U-values. I'm wondering if I should reduce the 80mm PIR to 50mm. this will change it from approx 0.12W/m2K to 0.15W/m2K and save quite a bit of money. so I'm trying to weigh up the costs and make a decision. but part of me is thinking I don't want to add more insulation at a later date and if/when building regs start to mandate <0.15W/m2K i'd rather our building still conform so maybe just go low now and not worry about it in the future.

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8 minutes ago, Thorfun said:

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


join the club!! . @SteamyTea will be along in a mo with a bar chart/graph to show what your savings will be! And your pay back time. I worked on the assumption that even if I have to use electric heaters for the few weeks a year to keep warm when it’s really cold (like now) it’s worth it. I ain’t going to live long enough for some pay backs ?.  
 

saying that, I am sat in my solar heater (south facing conservatory), temp up to 27’, bifolds into the house open so heating the house for free ?

Edited by joe90
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We decided on the timber frame route and selected the company we wanted to go with which gave us the U vals for the slab, walls and roofs. We're now selecting the window supplier and have a target U value based on making sure that we can achieve an SAP 'A' rating. Turns out to be 1.2.  Now the issue is what type of window, Alu-cald, alu, etc. combined with which glazing.  I guess this is what the OP is done but the other way round, select the windows and then work out what U value you need for the walls.

 

Simon

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1 hour 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.

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.   

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