Below Deck Insulation, What is below deck insulation

Below deck insulation speaks to what is below deck insulation and why below deck insulation.

It is when batt insulation of mineral fiber or glass fiber is installed either between framing members or on top of a suspended ceiling assembly.

The building code normally requires a ventilated air space between the insulation and the underside of the deck to dissipate excess water vapour. Below deck insulation is relatively economical and trouble free, but it leaves the deck and the membrane exposed to a full range of outdoor temperature fluctuations.

In cold climates when enclosing humid spaces, a vapor retarder is recommended on the warm conditioned side of the roof insulation to control the diffusion of water vapour into the insulated portions of the roof where condensation could occur.

Spray foam insulation can also be installed below a roof deck. When air impermeable foam insulation is used, the risk of moisture accumulation within the assembly is reduced, and ventilation between the deck and the insulation may not be required.

With this type of roof the insulation is placed between the flat roof joists at ceiling level.

To prevent condensation forming when the insulation is placed at ceiling level, a minimum of 50mm gap is provided for ventilation above insulation. This may involve the use of 50mm x 50mm counter battens or firrings reducing to 50mm. A vapour barrier should also be placed on the warm side of the insulation, i.e. between the insulation and the ceiling finish.

The roof should also have ventilation openings of at least equal to a 25mm continuous gap running the full length of the eaves on two opposite sides.

The external walls need to be extended up to the underside of the roof insulation to prevent cold bridging.

All roofs should be insulated. The thickness of insulation will depend on the type of insulant used and whether you are constructing a cold or warm roof.

Insulation levels have increased dramatically. Typical details being used at the moment include:

When constructing flat roofs please consider the following, materials used must be suitable for use in flat roofs, roof needs to achieve an external fire rating, vapour barriers should be used, flat roofs should be anchored to walls to prevent uplift, flat roofs should be laid to falls and flat roofs require regular maintenance.

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