Should loft ventilation be blocked off if you insulating between the rafters?
I use my loft for storage and it is insulated between the joists and boarded over. To prevent condensation the loft is ventilated via grilles in the sofits. I have read that the 6 inches of insulation this gives me is not enough. I don’t want to put more insulation between the joists as this would mean re doing the boarding and adding timber to support it, I had a leaflet through the door for insulating under the tiles / felt between the rafters. But I can’t see the use of this if there is a continuous air flow through the ventilators that will keep the loft space at outside air temperature? I thought that stopping this ventilation would cause high levels of condensation so is this type of insulation a waste of money?
First of all, you have to understand what the sofit vents are for. They are NOT there to prevent condensation as you think. They are there to provide airflow to the underside of the roof sheathing in the event the sheathing gets wet as well as to ensure that the underside of the roof stays at or near the outside temperature to prevent ice damming. Now, as far as insulating between the rafters goes, before you install insulation between them, you need to install what are called rafter vents. They prevent the insulation between the rafters from contacting the underside of the sheathing. The rafter vents provide about 1 inch of space for airflow on the underside of the sheathing (the person who posted prior to me is wrong…you do not need 3 to 4 inches of space for airflow) The rafter vents are cheap. They’re made out of foam and can be bought at Home Depot or Lowes for about $1 each (each one is about 3 or 4 feet long). You would install them between the rafters from the sofit vents all the way up to the ridge beam of the roof. This will allow airflow to the entire underside of the roof. You then staple insulation WITH A VAPOR BARRIER between the rafters. The insulation will come in contact with the underside of the foam rafter vents. This prevents the insulation from touching the sheathing. It is the underside of the roof that must remain at or near the outside temperature, not the entire loft. Understand? Good. 
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