thickness of heat sink for ufh

[scottishjohn]

I have been reading and following lots of topics and the one that keeps coming back again and again  is excesive solar gain and how it causes over heating in near pasiiv houses

  so my suggestion for discussion  is

"do we really need such a thick slab to act as a heat store "

heating thick floor takes a long time ,the house does not loose heat quickly if is well insulated + then out comes the sun and you need to lower the house temp .

why not just a thinner slab so it can change temp quickly

how will it be any more expensive just to heat the floor when you need it and  so get round the over heating ,

excessive solar gain onto a floor that is already up to temp must be a waste of energy in first place  -use the solar gain ,  to supplement the floor heating

and not cause other problems ,which you may have to use energy to correct 

dropping floor temp of a large mass will not happen quickly and will again be a waste of energy

 

[lizzie]

Interesting thread

[scottishjohn]

It may depend on where you live ,,as in scotland there will or maybe will be less thermal gain than in S/E england

[ProDave]

I am in the minority here using a 30mm thick dry sand / cement "biscuit mix" for the heat dispersal medium in my suspended timber UFH.  I am very pleased with the way it works, FAR better imho than spreader plates.

 

We don't get overheating issues but as mentioned that is probably due to our location in the north, rather than any particular design genius.

[joe90]

I am in the middle, unable to use ground slab due to many things so went fir 70mm liquid screed and that works well.

[ragg987]

5 hours ago, scottishjohn said:

why not just a thinner slab so it can change temp quickly

Not sure I follow the logic. Solar gain is usually more of an issue in the shoulder months when the sun can be bright but low enough. During this period, a low energy house needs very little heat input, so ufh / slab will only be about 1 or 2C above room temperature.

 

As the room warms from solar gain, the slab emits less and less energy. Once room temperature is above that of the slab then the heat is transferred into the slab. Delta T dictates this.

 

In theory, a thicker slab will provide a better "cushion" so might be more beneficial.

[Jeremy Harris]

5 minutes ago, ragg987 said:

Not sure I follow the logic. Solar gain is usually more of an issue in the shoulder months when the sun can be bright but low enough. During this period, a low energy house needs very little heat input, so ufh / slab will only be about 1 or 2C above room temperature.

 

As the room warms from solar gain, the slab emits less and less energy. Once room temperature is above that of the slab then the heat is transferred into the slab. Delta T dictates this.

 

In theory, a thicker slab will provide a better "cushion" so might be more beneficial.

 

This is exactly how it seems to work in practice for us.

 

I'd not want to have a floor that didn't include a fair bit of heat capacity in our house.

[Ed Davies]

Obviously the sun angle onto the windows is a big part of the reason for the difference in solar gain in the shoulder months vs in mid summer but I wonder if where the sun is shining into the room also matters. In mid summer it'll be shining directly on to the floor so heat will be directly absorbed in the slab without increasing the air temperature in the room that much. However, in the shoulder months it'll shine deeper into the room and hit walls and furniture more so cause more directly perceived heating as they'll have less immediately accessible heat capacity so rise in temperature quicker.

 

Once heat is in the air and other lightweight contents of the room it'll only transfer to the slab slowly because the thermal resistance of the interface between the air and the floor is something like half as much again for heat going downwards as for heat coming upwards.

[scottishjohn]

2 hours ago, Ed Davies said:

Obviously the sun angle onto the windows is a big part of the reason for the difference in solar gain in the shoulder months vs in mid summer but I wonder if where the sun is shining into the room also matters. In mid summer it'll be shining directly on to the floor so heat will be directly absorbed in the slab without increasing the air temperature in the room that much. However, in the shoulder months it'll shine deeper into the room and hit walls and furniture more so cause more directly perceived heating as they'll have less immediately accessible heat capacity so rise in temperature quicker.

 

Once heat is in the air and other lightweight contents of the room it'll only transfer to the slab slowly because the thermal resistance of the interface between the air and the floor is something like half as much again for heat going downwards as for heat coming upwards.

can,t directly agree with that 

if floor is absorbing heat ,then it will also give it out at same rate as normal 

and if slab rises to above the temp set -heating will turn off  until it goes below that again .

the problem I am seeing from what you all write is that floor is up to temp already  ,then you get solar gain , so house temp rises and because floor does not change quickly even if it turns off you get the unwanted extra heat  absorbed into the floor.

If the floor could change quickly then the problem would be lessened .

trying to work out how much heat you will get ,so you could turn off the UFH before it gets to target temps,saving energy and letting sun top it up  is a very hard thing .

air in its self is a very good insulator so your solar gain is coming from the sun heating something solid --

so temp rise is a result of that hard surface letting the heat out again 

 the energy to heat the house will be the same no matter if it comes from a large amount stored in floor  or a smaller heat store of thinner floor ,but which then turns on heat source sooner to top it up 

maybe for  you guys with excess PV the sunamp  is very good idea so you store the excess energy ready to be used when needed be it for the UFh or DHW 

using air con to combat this which is costing energy seems wrong  in our very changeable Uk climate .

fine if canada where they have proper seasons -- 

 

 

 

Edited April 14, 2019 by scottishjohn

[scottishjohn]

8 hours ago, ProDave said:

I am in the minority here using a 30mm thick dry sand / cement "biscuit mix" for the heat dispersal medium in my suspended timber UFH.  I am very pleased with the way it works, FAR better imho than spreader plates.

 

We don't get overheating issues but as mentioned that is probably due to our location in the north, rather than any particular design genius.

my present ufh/suspended floor  on an old TF house works very similar which got me thinking  more about this .

Ifeel there is a mid way to get best of both and full use of free suns energy without more technology to harness it

[Temp]

There is a lot to be said for shutters.

[lizzie]

I find - so far - my house is difficult to control its either too hot or too cold.  That could be because of the issues I had with mvhr not being set up properly and ufh still isn't properly sorted (fingers crossed man is coming tomorrow).

 

I am all in favour of exceeding building regs for insulation as I think its low bar but I am wondering if there will be significant benefits to this way of building (which is not cheap) when its all up and running properly.  Time will tell I guess.

[Jeremy Harris]

13 hours ago, lizzie said:

I find - so far - my house is difficult to control its either too hot or too cold.  That could be because of the issues I had with mvhr not being set up properly and ufh still isn't properly sorted (fingers crossed man is coming tomorrow).

 

I am all in favour of exceeding building regs for insulation as I think its low bar but I am wondering if there will be significant benefits to this way of building (which is not cheap) when its all up and running properly.  Time will tell I guess.

 

The key is being wholly in control of the design and build process.

 

I don't agree that it's "not cheap" at all.  Our build is a similar thermal spec to yours, from the same supplier, and came in at ~£1,380/m².  If I'd been "hands off" and used sub-contractors for everything it would probably have come in at around £1,600/m², which is still well inside the sort of normal self-build range.  We could have pushed the cost up to around £2,000/m2 or more by going for a higher spec, but we already have a reasonably good finish, with solid oak joinery everywhere, loads of travertine flooring, a kitchen that cost around 10% of the total build cost, etc.  The core, well-insulated and airtight structure, was only around 30% of the finished build cost anyway, most of the cost was in all the other stuff, that played little or no part in making the house thermally efficient.

[ProDave]

26 minutes ago, lizzie said:

I find - so far - my house is difficult to control its either too hot or too cold.  That could be because of the issues I had with mvhr not being set up properly and ufh still isn't properly sorted (fingers crossed man is coming tomorrow).

 

I am all in favour of exceeding building regs for insulation as I think its low bar but I am wondering if there will be significant benefits to this way of building (which is not cheap) when its all up and running properly.  Time will tell I guess.

I don't know why you are having the issues you are.  I know mine is a different make and design of house, but it shares many of the well insulated, air tight, low energy credentials.

 

I have been very satisfied with the performance. I particularly like how it maintains such a constant temperature. Nothing happens quickly, it won't cool down quickly and it won't heat up quickly. Maintain the correct low level heat input and it maintains a comfortable temperature 24/7 regardless of what it is doing outside. And the controls for that are nothing more than 3 room thermostats controlling the 3 UFH zones downstairs and a conventional central heating time clock (mainly so I can have the heating off at night for a totally silent house).  My house has also proved that in a well insulated house, even in the Highlands, you don't need heating upstairs.

 

A "control system" that is over shooting and under shooting (too hot or too cold) probably has too much "gain" which in this case means too much heat input. Or at best it is not tuned correctly.  A lot of my professional life was control systems and I have seen plenty that are unstable.  The trouble is the average "heating engineer" probably know sod all about real control system theory.

 

Lots of things will interact so you need to sort them one at a time.  Get the MVHR balanced first.  If it is still under and over shooting, the problem is the UFH.  You want that to be producing gentle heat at just the right rate, not bursts of too much heat (which will overshoot) then by the time it eventually cools down, it will take time to heat the slab again from cold, and while it waits for that it will undershoot.

[lizzie]

@JSHarris I think you will find prices have gone significantly from when you bought yours.  I don't know what size your house is but my frame and slab (without the groundwork prep) alone came in at circa £550sqm plus they could not do the roof so I had to have someone else do the roof trusses which added considerably to the cost of actually getting a shell building (not a watertight one of course). 

 

Then the 3g windows, mvhr, ufh etc all big ticket items for someone who has employ people to do everything, I am not a DIY person as has been shown all too often on this forum LOL.  My core costs are well in excess of yours percentage wise.

 

I am single storey too which is also more expensive to build.

 

It has cost me quite a lot sorting out problems with the whole thing too, it was not a good situation in any way but I am not counting that when I say it is not cheap to build this way.  It may have been a few years ago but not any more. I think it is comparable to brick and block and in some ways may cost more as contractors charge more as there are fewer that do this sort of build whereas brickies are more plentiful if you see what I mean.

[joe90]

I have not “been on top of my finances” but just done a “back of fag packet “ calculation and my 240sq m build in brick and block has cost me about £1100 per sq m. I was quite hands on.

[Jeremy Harris]

48 minutes ago, lizzie said:

@JSHarris I think you will find prices have gone significantly from when you bought yours.  I don't know what size your house is but my frame and slab (without the groundwork prep) alone came in at circa £550sqm plus they could not do the roof so I had to have someone else do the roof trusses which added considerably to the cost of actually getting a shell building (not a watertight one of course).

 

I accept that, but you've already mentioned here that your build costs were over £2k/m², IIRC, so it looks like the well-insulated structure was still around 25% of your build cost, maybe a fair bit less, unless my arithmetic is haywire, so as a proportion of total spend, the insulated structure is far from being the dominant cost element.

[lizzie]

@JSHarris if only.... you have no idea on what it cost lol and I’m not saying on a public forum.

 

If you take the cost of your frame and divide it by your sq m it will be a  lot less per sq m than mine.  I know the frame supplier costs increased quite a lot in the years between you and me.  Not inflation just market.

 

That way gives a better indication of cost of type of build rather than £sqm finished house as people have different ideas on what is basic, good and high spec for finishes which really affects the numbers.

 

The point I was trying to make is that timber frame construction (shell not finished ) is no longer a ‘cheap’ option as it was somehow perceived to be in the past.

 

The self build calculator (costs at June 2018) show in their examples timber frame with timber cladding  to be more expensive than block and brick. 

[Jeremy Harris]

@lizzie, I think what you're saying is that the insulated frame was even less than 25% of your build cost, so that makes it relatively cheaper, as it didn't dominate your build cost.  You could have used any build method available and it wouldn't have changed your final bill by much (given that the insulated structure is only a modest part of the total cost).

[LA3222]

@JSHarris in £/m2 how much was your frame/founds?  @lizzie other issues aside, I think that when she refers to "Not cheap" it is regarding the frame (how I read it)  Time and again you use your build as evidence to suggest it is cheap - but as Lizzie rightly points out, there's been a lot of water under the bridge since you built your home and times are very different.  What was cheap may no longer be so.

[lizzie]

50 minutes ago, LA3222 said:

@JSHarris in £/m2 how much was your frame/founds?  @lizzie other issues aside, I think that when she refers to "Not cheap" it is regarding the frame (how I read it)  Time and again you use your build as evidence to suggest it is cheap - but as Lizzie rightly points out, there's been a lot of water under the bridge since you built your home and times are very different.  What was cheap may no longer be so.

yes spot on thats exactly what I was trying to say. 

[Ferdinand]

7 hours ago, LA3222 said:

@JSHarris in £/m2 how much was your frame/founds?  @lizzie other issues aside, I think that when she refers to "Not cheap" it is regarding the frame (how I read it)  Time and again you use your build as evidence to suggest it is cheap - but as Lizzie rightly points out, there's been a lot of water under the bridge since you built your home and times are very different.  What was cheap may no longer be so.

 

IIRC Jeremy’s build costs were 2014-2017 ish. 

 

Just doing reviews on  a couple of 3 year no-increase rental agreements (no longer offered due to increased legislative risks imposed since) I have with a few tenants, which suggests that +10% is a reasonable inflation adjustment to apply for the 4-5 year period, which makes the @JSHarris per sqm total build cost about £1525 per sqm, or 200k for the cost .. ignoring plot, borehole fun and games etc.

 

That excludes BREXIT, extra inflation if any etc.

 

F

 

 

Edited April 15, 2019 by Ferdinand

[dpmiller]

Frame erected inc supply of all wall and ceiling insulation and PB, plus footings- £230/m2

[Jeremy Harris]

The thing everyone here is missing is that a passive slab is a fair proportion of the build cost for a well-insulated, airtight, build, and that slab will often include the UFH (ours did) in the price.  Comparing an all-inclusive insulated, and guaranteed airtightness, weather proof, house shell (it's far from being just the frame) and passive slab package, to any other build method, needs to be done on a like-for-like basis.

 

For example, you can't reasonably compare the cost per m² for a single storey build on a passive slab, with a two or more storey build, of the same sort of total floor area, as the ground floor cost (which includes the passive slab, UFH etc) will always be a lot higher than the upper floor cost, so a single storey build will always look around 15% to 20% more costly, per m² total floor area, than a multi-storey build.

 

The cost of having 300mm of insulation under the slab is always going to push the price up; it did for our build.  We could have opted for conventional foundations and frame for maybe 20% to 30% less initial cost, but we'd have then had to add loads of insulation, install UFH, and screed the floor afterwards, so spending as much as we would have for a passive slab, and not getting an airtightness guarantee, either.  We had no additional costs for the ground floor at all, as the UFH was in the slab, the surface was dead flat and I was able to lay flooring laid directly on it (and I'm aware that @lizzie's slab had problems that needed rectification, caused by the poor workmanship of a subcontractor).

[LA3222]

38 minutes ago, JSHarris said:

The thing everyone here is missing is that a passive slab is a fair proportion of the build cost for a well-insulated, airtight, build, and that slab will often include the UFH (ours did) in the price.  Comparing an all-inclusive insulated, and guaranteed airtightness, weather proof, house shell (it's far from being just the frame) and passive slab package, to any other build method, needs to be done on a like-for-like basis.

 

For example, you can't reasonably compare the cost per m² for a single storey build on a passive slab, with a two or more storey build, of the same sort of total floor area, as the ground floor cost (which includes the passive slab, UFH etc) will always be a lot higher than the upper floor cost, so a single storey build will always look around 15% to 20% more costly, per m² total floor area, than a multi-storey build.

 

The cost of having 300mm of insulation under the slab is always going to push the price up; it did for our build.  We could have opted for conventional foundations and frame for maybe 20% to 30% less initial cost, but we'd have then had to add loads of insulation, install UFH, and screed the floor afterwards, so spending as much as we would have for a passive slab, and not getting an airtightness guarantee, either.  We had no additional costs for the ground floor at all, as the UFH was in the slab, the surface was dead flat and I was able to lay flooring laid directly on it (and I'm aware that @lizzie's slab had problems that needed rectification, caused by the poor workmanship of a subcontractor).

Dodging the question I posed @JSHarris?

 

I think the system you used is great and if I could afford it then I would use it too.  However for my two storey build the price per m2 is the same as @lizzie price for single storey.  There doesn t seem to be any difference between single/multiple stories which is interesting.  

 

Like I say - a great system but I do believe you should stop peddling the line that it's cheap.  Burying the price within your overall cost per m2 just hides the fact that prices have potentially risen a lot since you commenced your build and skews new members impressions of what is cheap or not.

 

And yes, I agree that comparing like for like is bloody difficult - probably the most difficult aspect of selecting a supplier and an issue I am currently wrestling with.