DIY Doctor UK DIY Forums

Hi,

I'm looking to improve the insulation in my loft by placing a Knauf 'space blanket' over the existing insulation. Once i've done that I intend on boarding the loft with Chipboard. Once i've installed the space blanket the top of the insulation will be tight against the top of the Joists meaning no void whatsoever between the insulation and chipboard.

The literature I have from Knauf is obviously telling me to ensure there are no cables burried UNDER the space blanket due to the risk of overheating. My concern is that if I place the cables over the space blanket, then proceed to bury them between the insulation and chipboard - am I creating a new risk of overheating?

Likewise if I do need to create a void and I need to fix new timbers on top of the existing joists to then lay chipboard, am I comprimising the structural stability of the joists?

Many thanks in advance.

Hi,

if you add timber to the joists and screw it to them you will increase the loading capacity of the joists - so that is good. If you compress the insulation with floor boarding you will reduce the thermal properties of the insulation as the secret to insulation is simply the trapping of air particles. No air particles equalls poor insulation. So increase the depth of the joists by cross battening and leave a small gap below the floor boarding. Also leave a gap around electric cables otherwise they can overheat.

Aidan

[quote="the specialist"]Hi,

if you add timber to the joists and screw it to them you will increase the loading capacity of the joists - so that is good. If you compress the insulation with floor boarding you will reduce the thermal properties of the insulation as the secret to insulation is simply the trapping of air particles. No air particles equalls poor insulation. So increase the depth of the joists by cross battening and leave a small gap below the floor boarding. Also leave a gap around electric cables otherwise they can overheat.

Aidan[/quote]

Excellent! Thanks for the advice!

Sorry to jump onto this. Do you therefore not recommend boarding a loft after fixing insulation?

My loft is insulated although I'm sure not correctly and was going to update it. Some of it is also floored and I would like ideally floor the rest to store stuff.


Thanks

Hi stuarta,

One of the reasons I suggest a gap between insulation and boarding is in respect of condensation.
A ceiling with mineral wool insulation is classed as vapour permeable. This means that it will allow water vapour (moisture) through it.

Insulation does not completely stop heat loss - it just reduces it. Also the warm air is holding moisture from within the building - we call this relative humidity and it is expressed as a percentage. Air is never completely dry.
So it stands to reason that although the heat loss is reduced there is still a movement through the insulation of moisture laden air. Interstitial condensation within mineral wool is considered to zero. The moisture is held as vapour until it reaches the cold side of the insulation at which point it will be released as liquid water on reaching the dew point. If there is nothing restricting the air movement and the loft is properly ventilated the moisture will simply be removed to the atmosphere. However if there is boarding pressed against the insulation you run the risk of condensation occurring on the underside of the boarding and the top of the ceiling joists. If this is for prolonged periods other problems can develop. this is why I recommend a gap between insulation and boarding for ventilation.
If the space above the insulation was heated then different advice would apply.

Hope I haven't confused you.

Aidan