how to stop wind in standard roof

riccol1966

Hi, my attic is currently insulated with appx 350mm of fibreglass between ceiling joists, pushing up above the joists. But in a standard cold roof like mine, wind comes in from the eaves and then blows through the attic, and over the fibreglass. Any wind moving like this across non-rigid insulation can reduce the insulation performance by 60-80%.

So considerations to fix are:

1) lay more insulation across the joists at right angles to the existing. That sounds great until I realised that the joists would then be covered and you can no longer walk around the attic!
2) lay a floor on the attic to trap the air, which would require raising the joist level so as not to compress the fibreglass. Then there’s the usual issue with handling pipes, cables etc.
3) Insulating the rafter area with rigid boards. This would mean that the attic space itself becomes insulated, with the wind channelled between the roofing felt and insulation board, via the necessary 50mm air gap.

If I go with the final option, am I wasting energy as I am now insulating the entire attic. That's the big question. Not sure how the laws of heat movement apply and would this actually be a benefit or a drawback. Any heating engineers on the threads who could advise or has anyone done something similar?

Oh, one final question : does anyone know how to calculate the insulation area for a pitched roof so I know how much I need ?

sydthebeat

your problem is excessive ventilation, so you wont solve this with upgrading insulation. I assume your dweling is located in a very exposed site.

Where did you hear that wind blowing over quilted insulation reduces its performance by 60-80%?

You need a certain amount of ventilation in your attic to prevent internal condensation becoming a risk.

Have you a 20mm continuous strip running all around your soffit? or have you intemediate ventilation strips every, say, 1.2 m or so?
Have you ventilation at the ridge level?? ridge vent tiles?


By the way, if you have 350mm fibreglass up there, and assuming you do not have 350mm joists, you should have laid the fibreglass over the joists at right angles to the layer under for better performance and prevent cold bridges. To do this properly and retain use of the attic for storage, it means joisting out again over the structural joists. This is what you should do, and floor out over these new joists....