Multifoil insulation: helping out with the loft conversion


I want to make my loft warmer, can Multifoil insulation help?

The TLX Helpline receives a lot of enquiries from people saying that they want to make use of their loft by making it a bit warmer (but they don’t want to do a full loft conversion) and can they use either TLX Silver over the joists or under the rafters? Or, can they use TLX Gold when re roofing?

When we give the answer: “No, using our products here is not appropriate or desirable, and there is no halfway house between having an insulated cold ventilated loft and a full loft conversion as far as insulation and ventilation goes”, it’s hard to concisely explain why this is the case.Multifoil loft conversion u value 0.18

“I want to board over the joists so I can use it for storage”

“I want to use it as an occasional guest room”

“I want somewhere for the grandchildren to play”

“It’s not going through building control”

No planning permission necessary?

Although you do not need planning permission to convert a loft, Building Control should be notified whenever there is major building work. Their brief is not only to ensure adequate insulation, but that it is safe from an access, electrical and fire viewpoint – which if people are using the area on a regular basis should be an important consideration.

Be aware that if the work is not signed off by Building Control then when you come to sell it, that ‘deluxe loft room’ or ‘hobby room’ will not count as an extra room without the appropriate certification, but could even be a liability as it would be assessed as being inadequately insulated and possibly unsafe in other respects, and so would need bringing up to standard.

There is no halfway house...

BS5250 Control of Condensation does not recognise a halfway position between either putting all the insulation at a joist level, or all at rafter level - you have to either have a cold loft (nearly always ventilated) or else a habitable room in the roof with heating and ventilation. If the insulation is at joist level the current recommendation is to have 100mm of 0.044 W/m²K loft roll between the joists, and 170mm laid crosswise over the top, achieving a U-value of 0.16 W/m²K. If the insulation is at rafter level a U-value of 0.18 W/m²K is required (provided this is technically feasible).

12-15 litres of water vapour enter a loft every day!

It is estimated that 12–15 litres of water vapour enters the loft from below each day (you would only expect new houses to have vapour barriers under the ceilings to prevent this), and this is swept away by the ventilation. So whilst it would be fine to use TLX Silver multifoil on the ceiling of the room below to prevent water vapour entering the loft, if it were used over the joists or under the rafters where the temperature is much colder, it would only trap it and cause condensation.

Scenario one:

“I don’t want to put all that loft roll down – I need to store things there” e.g. a boarded-out loft with 100mm of loft roll underneath the boards. If the boards are close-fitting it is possible that condensation is already occurring underneath them, since they are on the cold side of the insulation, at outside air temperature (since loft is ventilated). The advice to a house owner would be to top up the insulation to 270mm, and ideally ensure there are gaps in the boarding to allow the escape of water vapour. The ventilation may need to be increased, since because less heat is escaping from downstairs the loft will be colder and condensation more likely. If a storage area is required then either consider a limited area without the additional loft roll, or use shelves hanging on the rafters, or else raise the loft floor using loft legs to accommodate the 270mm of loft roll.

Scenario two:

“I want to board the loft and put insulation at rafter level so I can make use of it” - to keep their Christmas decorations warm, for example. Doing a proper loft conversion would mean insulating to a U-value of 0.18 W/m²K, which is very expensive. Owners often think all they have to do is put a bit of insulation board up (as indeed advised by some DIY websites!) or possibly to use TLX Gold (insulating breather membrane) when reroofing. However, if the loft continues to be ventilated, then cold outside air is being introduced and any insulation above this is ineffective - a bit like standing under a gazebo on a windy day – you may get some benefit but it’s hard to say how much if any. If instead the eaves ventilation is blocked up to keep the heat in, then water vapour continues to arrive in the loft from below, but cannot escape. So unless a good level of heating and ventilation is maintained – which is clearly not the case if it is only being used for storage – then condensation will result.

The situation is even worse if there is already mineral wool under the boarding, as less heat is entering from below, but just as much moisture.

Silver multifoil loft conversionWhat about condensation risk?

In the UK climate you might expect there to be some condensation on a few days in the year, but this should not exceed some droplets on the roofing underlay, and it should evaporate when the temperature rises. It is when the humidity levels are persistently high, or condensation is dripping, that mould will start appearing and the timbers may rot.

Scenario two would definitely be inadvisable at all times, but can the additional heating costs incurred by not insulating to 270mm of loft roll as in Scenario 1 be justified in terms of storage benefits?

We can help you with a condensation risk analysis - call our technical department on 01204 674730

Is there a cost saving?

An uninsulated loft has a U value of 2.50 W/m²K, and with only 100mm of loft roll it is 0.42 W/m²K. The recommended 270mm achieves 0.16 W/m²K. For an uninsulated semi with a roof area of 100m² on a cool autumn day with the outside temperature at 10°C, to keep the temperature of the rooms below at 20°C takes (2.5W x 100m2 x 10K) 2500W – 2.5kW – the amount of heat entering the loft from below. Having 100mm loft roll reduces this to 420W, and with the recommended amount to 160W. So that extra storage cost incurred by not topping the insulation up to 270mm would be around the cost of 260W - £6 - £7 per week – less in summer, more in winter. This is possibly a reasonable price to pay if otherwise you would have to hire a storage unit, but a lot in terms of a domestic energy bill.

You can find out more about managing condensation risk HERE

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