Retrofit design

Underfloor insulation guidance

DESCRIPTION
Suspended timber floor is to be insulated whilst floors are up for other work in the house.
LOCATION: Ground floor
EXISTING FORM / CONDITION: Uninsulated suspended timber floor with solid floor in kitchen
ESTIMATED CURRENT U-VALUE: unknown
MIN BUILDING STANDARDS U-VALUE: 0.7 W/m²K
TARGET U-VALUE: 0.25 W/m²K possibly less for solid
RELEVANT STANDARDS
Building Regulations Part L1B, Part C, Approved Document 7 Materials and Workmanship and BS 5250
INTERFACE WITH OTHER ELEMENTS
Link to solid floor in kitchen may be a thermal bridge, this junction requires a detail to show how the two insulation types can be joined continuously if possible. If not the thermal bridge will be in the middle of the house so should not be a major concern but you should monitor for any signs of moisture in the future.
INSTALLATION SEQUENCE IN RELATION TO OTHER SITE OPERATIONS
Installation as and when floors are lifted.
PRODUCT SELECTION
Product: Mineral wool PIR insulation for solid floor
Form: Rigid batts, large boards
Vapour permeability: Mineral wall only
Accessories: Insulation netting, fixings, skirtings and other finishes as required
WORK SPECIFICATION
PRE-INSTALLATION INVESTIGATION
The contractor is to confirm the dimension between u/s of joists and ground level and the joist sizes. A minimum of 150mm ventilation is required. The position of the underfloor vents should be noted as the insulation should not obstruct the vents.
2. Contractor to confirm the ventilation to the underfloor area is in compliance with Building Regulations Part C and BS5250 and the existing vents provide no less than 1500mm2/lm of external wall or 500mm2/m2 floor area whichever is greater. If insufficient free ventilation area is available then additional vents should be installed to achieve adequate ventilation.
PREPARATION
3. Remove existing floor boarding and set aside for reuse. Remove skirting and trims, and set aside for reuse where in good condition.
4. If the ground is restricting the 150mm ventilation space in certain limited areas, these should be levelled off and the earth removed to achieve a 150mm free air space throughout. The disposal of the earth should be agreed with the client who may want to dispose of it on their land.
5. If additional wall vents are required these should be installed. Ensure existing ventilation bricks are clear inside and outside.
6. Ensure the joists are sound, dry and free of rot, and the tops are clean and free of nails and screws etc. Check joists ends are inside cavity wall insulation so thermal bridges are avoided.
INSTALLATION
7. Install insulation supporting netting. Securely fix to joist sides and tops making sure the net does not sag between joists.
8. Install insulation the thickness of the joists to fit tightly between joists. A perimeter insulation strip should be fixed between the wall and the joists running parallel to it. There should be no gaps in the insulation.
10. Solid floors to be insulated with PIR boards 10-25mm thick as much as the kitchen deisgn allows. This can be laid on a level screed joints between joints should be checked and there should be no gaps in the insulation.
11. Finishes to wall and surrounding areas to be made good as required.
SUPPORTING NOTES
Insulation materials for suspended timber ground floor structure: timber fibre insulation is not appropriate as it may rot should it become damp over time. Vapour impermeable plastic insulation materials are not appropriate as they would prevent the moisture and humidity transfer in and out of the building increasing the risk of condensation.
Insulation materials for solid floor: a suitable board for tiling over which is vapour impermeable.
Airtightness: All gaps around the perimeter, adjacent to joists and between insulation boards should be minimised to prevent humid air penetrating the construction, condensing and increasing the risk of rot.
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