Authors - Mike George, Andrew Geens and Max Graham: (12-03-06)
Stimulating simulations
The article in the last issue of Building for a Future ‘Beware the blanket approach!’ [1] attracted some controversy, particularly regarding the insulation of solid ground floors. This generated a very stimulating debate on the BFF feedback forum [2].
As a result of this debate we invited Mike George, Andrew Geens and Max Graham to respond further ...
The lively responses on the BFF forum arising from our previous article (See: http://www.buildingforafuture.co.uk/forum/winter05/index.php?DATEIN=tpc_xjyppelzh_1134158985) have not only given invaluable feedback but have adequately highlighted the need for some further research to be carried out. In addition it has to be noted that some errors appeared in one of the tables in the original article. These are acknowledged as being limited to the descriptive tabular model data and refer to insulation thickness and associated u-values. These errors in no way affect the model data input, analysis or conclusions made in the previous article ...
Andy Wilson: (15-03-06)
RE: Stimulating simulations
This second article answered some of the questions I had raised in relation to the first one. Certainly, it suggests that the generous amount of floor insulation recommended as best practise by (for example) the Energy Saving Trust may in fact be far too generous.
There are however, a lot of other variables which would be need to be factored in before concluding that the most cost effective solution for floor insulation is for a fairly minimal depth of insulation to be deployed.
The first variable is the relative costs of heating versus insulation. While one might expect these two costs to rise roughly in tandem, this is by no means a certainty. The cost of heating may rise more more rapidly than the cost of insulation materials.
The second variable is the relatioship between the costs of installing floor, wall and roof insulation. The labour costs associated with floor insulation are often much lower than those associated with wall or loft insulation of the same standard.
The third variable is the thickness of the floor screed. In Ireland, for instance, it is recommended that floor screeds poured over insulation are a minimum of 65mm deep. Standard practise is for 75mm or deeper. These deeper floor screeds are much less prone to cracking. These are, I believe, essential in the case of screeds poured over substantial depths of insulation ( we will come to that in a minute).
Deeper floor screeds will increase the thermal mass of the floor. This, I think, will tend to increase the benefits of under floor insulation.
The fouth variable is what I would call the cold feet factor. I have already commented on this in my reply to the previous article. Irrespective of total heat losses, cold floors are synonymous with cold feet which in turn will lead some degree of discomfort among the occupants of the building. The most effective way of warming up cold feet is by reducing body heat losses elsewhere. Putting on more clothes may be one solution. The other solution is to set the thermostats a degree or two higher. The net result of the second solution is that more heat energy is required to achieve the same degree of comfort.
This may be a very difficult variable to simulate but one would imagine that it is best is that the floor temperature is at least as high as the air temperature inside the building. This would suggest that there may be advantages in increasing both the thermal mass of the screed and also of the depth of insulation below it.
This brings me onto the fifth consideration: under floor heating. In Ireland, for example, there is a growing tendency towards under-floor heating, in spite of the much higher installation costs when compared to conventional radiators. In buildings with under floor heating, the temperature of the floor screed may be as high as 35 Celsius. The difference between the floor temperature and the ground temperature is likely to to much higher than would be the case in floors with no underfloor heating.
Heat losses through the floor are potentially much higher in buildings with under floor heating than in buildings without. The benefits of under floor insulation will be proportionately greater.
One final point. As far as I could see, there was no mention in either of the two articles as to the relationship ( in terms of elemental area) between the floor, walls and roof of the 'building' used in the simulation. Given that exact fuel costs were quoted, presumably the floor area, wall area and roof area were precisely quantified. Some further information about this would be extremely useful.
Mike, Andrew and Max: (16-03-06)
RE: Stimulating simulations
Hi Andy, thanks for your feedback. We will respond to your points in turn. All of our comments refer specifically to the research at Glamorgan to date
Variable1:
We agree that the cost of energy is likely to outstrip material costs. However an initial payback scenario for floors is as follows:
A floor having 65mm insulation will take around 40yrs to recover the material cost of the insulation alone [based on current fuel costs]. Even if energy costs double- 20yr payback.
Variable 2:
While the labour costs of elemental insulation may indeed be as you say, the overall costs per unit area of say roof insulation and floor insulation are closer together. This has no bearing on the energy consumption, only the payback costs which are a very complex and subjective analysis not considered in the articles.
Variable 3:
I [mike] completely agree with you about the screed thickness having layed many cubic meters myself. I have modelled various scenarios [regarding similar changes to thickness of screed]. I have also varied the location of the insulation [above and below concrete].
In my opinion, the results do not demonstrate any significant difference in heat loss and do not therefore increase the benefits of insulation.
Variable 4:
We do not accept the cold feet argument as being important when considering the amount of energy required to provide so called comfort. What is important is to save energy due to heat losses. We believe the most effective measures to achieve fabric savings are already in place and the way forward is to make further savings by lifestyle changes. Simply put, wear slippers; buy local materials/produce; buy a bicycle to go to work on etc etc.
Variable 5:
We accept that underfloor heating is a completely different situation which needs to be evaluated differently. This has not been studied and we are not convinced that it can be effectively modelled using software currently available.
Final point:
Regarding elemental areas, these are precisely quantified within the model. This data however remains un-published at the current time.
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