Extra 'foil' insulaiton in attic

Our attic has the usual fiberglass insulation between the joists, with some DIY chipboard flooring on top and oceans of junk.

I would like to improve the insulation.

I'm thinking a handy solution might be to use a staple gun to fix a layer of that thin foil on plastic type insulation to the underside of the rafters. I could do it all in an afternoon and still keep the floor area intact.

Is this a good idea?

Are there any possible problems with moisture, as I don't think this stuff is breathable?

Thanks for any advice.

I don't think it'll be very effective tbh. (Properly) insulating the rafters creates what is known as a 'warm roof' but unless you do it properly, it's a waste of time (and money).

If you already have chipboard down you could lift it and lay 50mm rigid kingspan/xtratherm etc. at right angles to the joists and then relay your chipboard on it. It's not 'handy' but it'll work and work well.

Whatever happens you need to allow ventilation between the underside of the roof (the felt) and anything you insulate with ans you should allow cross-ventilation (ie, a flow of air from the fascia through the attic to the other fascia). if you don't ventilate you risk dry rot.

If you don't mind spending an extra few quid you could leave your chipboard down and lay the rigid insulation on top of it, then re-sheet with new chipboard.

As already explained it would be a waist of money insulating the roof. What you need to do is to bring your attic up to reg. and lay 300mm if possible, rockwool is better to work with than fibreglass. You will have to lay between the joists and over the joists.

I am in similar situation,no insulation on roof joists and just have fibreglass insulation between joists on the floor no flooring on top,

I was told to get kingspan insulation between the roof joists and leave 50mm space for air flow. I would then put similar on the floor joists I would think. The roof insulation would keep the cold out and then the floor insulating would stop the heat from the house going to the attic. Is this not the best thing to do.

I picked up rolls of Knauf space blanket in Homebase last week. They have 100/150 & 200mm thickness rolls for half price and layed it on top of my existing insulation. Its a lot easier than doing the roof space