Hi all,

I have a family member wanting to insulate their loft with loose fil cellulose, without disturbing the recently skimmed and decorated ceiling below.. This is a ventilated, cold loft space situation - insulating the room below.

Would a breather membrane over the top of the insulation be a good way of dealing with airtightness in this situation? (With Orcon F and Vana tape to deal with penetrations)

Slightly concerned about trying to tape and seal the membrane, from what I've heard cellulose insulation is very dusty.

Any thoughts, most appreciated.

Thanks
Chris

Maybe Im misunderstanding things but cant you seal all penetrations/gaps/cracks before the insulation is laid? Fidly to do at eaves level but less so than trying to seal a membrane over the top of the insulation. Youd likely need to lift the perimeter of the the roof from the outside to get a 100% seal from a membrane to wallplate

If you are concerned about wind disturbing the insulation in the eaves fit sections of thin ply 25mm below the felt and between the rafters to divert any wind over the top of the insulation. Fill up to the underside of these boards with insulation.

Hi Chris,

I used to use a lot of Warmcel before I got fed up of dropping a trail of it all over clients' houses, and of discovering that laying it in apex voids in high summer (approx 40 degrees in the apex) causes the Warmcel to stick to the sweat and turn to papier mache (cracking arms!)!

I never had any significant issues with slumping but subsequent access is an issue - having to trawl through it and back again makes reinstatement to the original depth pretty hard.

I think if it were me I'd consider laying Intello or VCL over the ceiling (and up and down the joists) rather tan just trying to seal the penetrations, but undoubtedly the latter will help quite a bit.

Chris,

no idea if this applies in your particular case, but often, a good deal of vapour removal from houses is via the ceiling into the roof space, which being well ventilate, is removed to the outside. Be aware of the possibility you could increase the humidity internally, if you start to apply "DPM" (I think you mentioned), or other vapour controlling membranes. You could improve the heat retention problem, but worsen the humidity problem. I undertsand the desire for air tightness, but that can be achieved with VPM's rather than VCL's.

Posted By: djhUsual solution is mechanical ventilation (preferably MVHR) but there are alternatives.

I'm planning to put in MVHR in my current house but in the meantime I've found humidity monitors in each room and opening windows when required to be quite effective.

I'm still looking forward to MVHR as that will be one less job to do, more effective and more efficient but in the meantime this works quite well.

Posted By: djhIt does cause the heating bills to increase of course.

How much do you think though? Because what I was surprised to learn (shouldn't have been a surprise) is that when you have higher humidity you have more thermal mass (the water in the air) to heat up... so reducing the thermal mass in the air should make it easier to heat.. so lower heating bills

There must be a happy medium in there somewhere.

But yeah, looking forward to MVHR

Posted By: derekeBecause what I was surprised to learn (shouldn't have been a surprise) is that when you have higher humidity you have more thermal mass (the water in the air) to heat up... so reducing the thermal mass in the air should make it easier to heat.. so lower heating bills

I don't think that will be very important in the grand scheme of things. It's true that higher humidity air will take a bit more energy to heat but the higher humidity air is already heated when it goes out the window. The incoming air is lower (absolute) humidity so doesn't take quite as much to heat up but then all the humidity that you add to the air within the house has to be warm as well (it often will be already if it's in your breath or from the shower or evaporated from a cup of tea but you have to pay for that heat somewhere). MVHR will automatically exploit the difference in humidity levels (unless it's a humidity-recovery enthalpy type, which gets even more complicated) to recover the energy. But all these numbers are small compared to the main event. You heat the house because heat is lost through (a) the walls and roof etc and (b) the air exchange with outside. (a) is bigger than (b). Humidity is a fraction of (b).