We have a suspended floor in our Victorian villa. It is supported by 170 mm x 65 mm joists with a maximum 3m span and average 450 mm centres. The floor has been insulated using 170 mm of Icynene between the joists. We would like to lay our underfloor heating pipes directly into the existing floor and bury them in a sand and cement screed. Since we plan to lay underlay/carpet with a combined rating under 2.5 tog, as the final floor covering and we have been advised by the carpet supplier that the screed will be an acceptable finished floor.
Trying to discover which of the tradesmen we have contacted knows what he is talking about has proved difficult at best and more often, downright impossible. I find it hard to believe that anyone can design a successful system without an accurate heat loss calculation yet a number of tradesmen have managed this, including more than one content to rely on sketch plans and less information than provided above on the floor structure. While researching the alternatives on which floor screed is best I have encountered directly contradictory advice which appears to be based on the system sold by the tradesman or his wholesaler rather than the system best suited to our needs.
My greater concern lies with the structural ability of the floor to carry the underfloor heating screed. Also the composition of the floor. It seems a 50 mm screed would be considered structural but a 25 mm screed is not. This raises the question of whether a 25 mm screed is a suitable surface to which to fit carpets. If not then an additional 22 mm flooring board should cure that problem but raise the question of whether is can be laid directly into the screed or if battens would be required.
I am also puzzled by the element of heat storage in the screed. Since it is the mass which provides the heat store surely a 50 mm screed would store twice as much heat as a 25 mm screed and if so does that mean the 25 mm screed provides insufficient mass for an efficient design?
The other big issue is, as has been discussed elsewhere, the ability of UFH to deliver sufficient heat to achieve a comfortable environment. Sensible heating engineers rely on quoting current standard u values for walls, windows, floors and roofs. Generally, the u values for roofs are easily achieved and since they are known for new windows adjustment can be made easily to deal with lower values. Walls are a completely different case. Most u values used are standard for different wall types whereas the thickness and type of internal lining can be significant factors. I wonder whether the most effective way of calculating the heat required is by comparison with he existing system. Assuming the system is adequate then any improvement in insulation would point to a similarly sized (by reference to output) UFH system.
Trying to discover which of the tradesmen we have contacted knows what he is talking about has proved difficult at best and more often, downright impossible. I find it hard to believe that anyone can design a successful system without an accurate heat loss calculation yet a number of tradesmen have managed this, including more than one content to rely on sketch plans and less information than provided above on the floor structure. While researching the alternatives on which floor screed is best I have encountered directly contradictory advice which appears to be based on the system sold by the tradesman or his wholesaler rather than the system best suited to our needs.
My greater concern lies with the structural ability of the floor to carry the underfloor heating screed. Also the composition of the floor. It seems a 50 mm screed would be considered structural but a 25 mm screed is not. This raises the question of whether a 25 mm screed is a suitable surface to which to fit carpets. If not then an additional 22 mm flooring board should cure that problem but raise the question of whether is can be laid directly into the screed or if battens would be required.
I am also puzzled by the element of heat storage in the screed. Since it is the mass which provides the heat store surely a 50 mm screed would store twice as much heat as a 25 mm screed and if so does that mean the 25 mm screed provides insufficient mass for an efficient design?
The other big issue is, as has been discussed elsewhere, the ability of UFH to deliver sufficient heat to achieve a comfortable environment. Sensible heating engineers rely on quoting current standard u values for walls, windows, floors and roofs. Generally, the u values for roofs are easily achieved and since they are known for new windows adjustment can be made easily to deal with lower values. Walls are a completely different case. Most u values used are standard for different wall types whereas the thickness and type of internal lining can be significant factors. I wonder whether the most effective way of calculating the heat required is by comparison with he existing system. Assuming the system is adequate then any improvement in insulation would point to a similarly sized (by reference to output) UFH system.
