Minimum Insulation Thickness For Flat Roof

Flat roof insulation thickness.

Minimum insulation thickness for flat roof.

When building a flat roof it is vital that a degree of falls is built into the construction to ensure rainwater run off. With a flat roof this issue does not arise but the u value constraint and the amount of insulation needed is just the same. A lower u value indicates better insulation properties hence u value requirements usually specify a maximum value. The definition of a flat roof according to bs 6229 is having a pitch less than 10 o to the horizontal however some in the industry would consider up to 15 o as their definition of a flat roof.

The chosen insulation laid on top of this should be able to take the weight of anything put above it on the roof without compression or protected from compression by further sheeting. U value of a flat roof to comply with part l of the building regulations 2010 all roofs to rooms except those in unheated buildings should be thermally insulated to a maximum u value of of 0 18 w m k. Achieving a u value of 0 20w m meaning your roof will be better insulated than your walls will need 120mm thickness of rigid foam or 200mm of mineral wool or natural insulation. Others would treat even steeper pitches as flat roofs.

See the department of energy s doe ranges for recommended levels of insulation below. There are two types of flat roof. State applicable energy ashrae zone minimum r value state local ms mississippi ashrae 90 1 2010 2 r 20 3 r 20 state local mo missouri 2012 iecc 4 r 25 5 r 25 state only mt montana 2012 iecc 6 r 30. Cold roofs and warm roofs.

In this case ventilation is required and the roof can still become wet with condensation which may cause materials to decay. It should be of the maximum available thickness. However if we do not include removal of old roofing you are looking at about 7 50 to 9 00 per foot of roofing plus about 2 50 to 3 00 per foot of 5 thick insulation say about 10 12 per square foot. Typical recommendations for exterior walls are r 13 to r 23 while r 30 r 38 and r 49 are common for ceilings and attic spaces.

Cold roofs have the waterproof layer placed directly onto the roof deck and the insulation inside the roof just above the ceiling. If you are upgrading the roof to meet current thermal requirements then the thickness of insulation will vary depending on its thermal conductivity. The typical u value of an existing uninsulated flat roof is 1 5w m2k.