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Both warm roofs and cold roofs have their own advantages and disadvantages. Which is the better choice for your flat roof will depend on your own situation. The issue of insulation is a controversial one and warm roof vs flat roof debate is centre to this.
A warm roof is where the insulation is laid above the decking and the vapour control layer. The vapour control layer is to stop moisture moving up into the insulation. The process of installing a warm roof offers a greater level of thermal insulation, for the whole property. And it is easy to install if the existing roof deck is in good condition. The main advantages of a warm flat roof are as follows: it is economical to install as all the work is carried out above the roof joists, the insulation is in a continuous quilt , and there are less things which can go wrong. The disadvantages of a warm flat roof are: it has a higher roof depth which means it may not be suitable in certain situations where there a constraints on height. And the final finish is soft. To avoid a soft finish, decking 1 can be laid above the insulation or a harder waterproofing membrane chosen.
A cold flat roof is when insulation is laid between the roof joists and above the vapour control layer and the plasterboard. The space between the insulation and the decking is empty. The flat roof design for a cold roof should allow for at least a 50mm gap between the top of the insulation, and the roof deck above. This gap is known as a cold roof space. It should be vented to allow for the circulation of air. This is normally done with a grill attached to the wall the roof joins to. This grill is normally finished with a lead flashing. Between the wall and the roof a gap is left for air to flow into the cold roof space below. On cold days, the air in the cold roof space can be very cold and cause a significantly lower temperature in the room below.
The main advantage of a cold roof is it is not so deep in height. This is particularly beneficial with retrofit 2 and new builds where things like door thresholds constrain the height the roof can be. Additionally, with a lower roof depth the ceiling of the room's below can be higher.
The disadvantages of a cold flat roof are:
1) It is a more complicated construction.
2) It is a more expensive flat roof construction.
3) Increased probability of thermal bridging (through the roof joists and the joist hangers).
When it comes to the depth of the roof, a cold flat roof beats a warm flat roof. As the insulation of a cold flat roof is between the joists. This does not add any extra height to the roof, which prevents the ceiling height from having to be reduced if there are constraints with things like door thresholds above the roof.
Cost And Time For Retro Fit Installations
When it comes to retro fit installations it is possible to install a warm roof over an existing flat roof. As long as the current flat roofing system is in good condition, glue can be applied on top of it and the installation fixed on top of it. Obviously, this means the installation of a the new roofing system will be a lot faster and cost less. When it comes to a cold flat roof the decking needs to be lifted to install the installation in between the joists. Alternatively, the ceilings below can be removed and the insulation installed. Both of these options take time and will be more expensive when compared to a warm flat roof. Additionally, with a cold flat roof, ventilation will need to be installed which will take more labour time and result in a higher cost.
When it comes to thermal bridging, it is more effectively stopped when using a warm flat roof construction. The reason for this is because a warm roof is one continuous quilt on insulation. Whereas with a cold roof the insulation is placed in between the joists 3. There are two reasons why thermal bridging is more likely in a cold roof. First, the joists are not insulated and second, gaps in the insulation. With a cold roof thermal bridging will always be able to occur through the joists. And the degree of thermal bridging between gaps in the insulation will depends upon the quality of the installation. A correctly installed warm roof construction will stop most thermal bridging. Whereas, a correctly installed cold roof construction will have some degree of thermal bridging. The main reason for a warm flat roof having superior performance when it comes to thermal bridging is the continuous quilt of insulation takes away the need for cutting in between joists and removes the problem of mistakes happening when cutting the insulation.
If you would like RJ Evans to provide any information related to advantages and disadvantages of both warm and cold roofs, please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.