Placing attic flooring on top of insulation

26 Mar 2009
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United Kingdom
I am a complete novice when it comes to this type of DIY. So please be patient with me.

Following all the latest news on energy efficiency and saving money i decided to insulate my attic space. Its not a huge attic and is unsuitable for using as another room but until now its been used for storage.

All the new insulation has now been fitted but now it comes above the top of my joists. I have been told if a wish to put flooring back down then i have to make my joists higher to allow space to circulate between the insulation and the underside of the flooring.

How high will my new joists have to be and how much of a gap is needed between the top of the insulation and the underside of the new floor?

How do i fit new joists?

Do i have to leave some of the attic space free from flooring for condensation?

I hope someone can help me as i have had to keep all my attic junk in my spare room for about 2 months now and its starting to drive my crazy.


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typically you can lay 4"x2" (100mm x 50mm) timbers on top of the existing timbers, but at right angles to them as cross-battens, and secure with screws. This will spread your weight while you are clambering about, though it does not make the roof stronger overall, it is more rigid, especially if you use Ply flooring and screw it down (rather than soft, weak chipboard). It enables you to add about 100mm more insulation. Or you can place rigid foam insulation boards on top of the timbers, and floorboards on top of that.

If you want to add real strength you have to run full-length joists so they are supported on the walls of the house. this is not necessary for light storage, though you have to do it if you are making a habitable room or a loft conversion.

A ventilated space of an inch between the boards and the insulation will prevent condensation unless you have a wet house due to wet washing inside, bathrooms and showers without extractor fans, lack of ventilation, and the other usual causes.

What size are the existing timbers; how long are they between supporting walls; how old is the house; what sort of roof design have you got? Photos will help.
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I have been told .... i have to make my joists higher to allow space to circulate between the insulation and the underside of the flooring.

That is not correct.

The insulation should ideally be allowed to expand to its natural depth, which will be higher than the existing joists. That is why additional timbers are added.

There is no need for an air space. In fact, no air voids are preferable

If the insulation will squash down, then it's efficiency is not reduced that much, and it is more than made up for when you then pile up all the boxes on top of the boards
My house is a Ex council property, build in approx 1978/79.

My loft is not being used or will ever be used as an inhabital space as the roof is just far too low.

The current joist height is 6 inches and the new insulation was laid over the old yellow fluffy stuff, so im guessing that the overall insulation is raised approx 2-3 inches above the current joists.

I am planning on boarding over just the middle section of the attic to use for storage and leaving the rest untouched.

If i lay new 3" joists at a right angle they will not be full length so wont be supported by the external walls. I was worried about wight baring as i do store too much junk, but im sentimental and hate throwing things away.

Are the joists easy so secure in place? I am guessing again that i use some sort of right angle bracket and screws?

I do also have another query ..... At the very ends of the attic (where the roof ends meet the floor) there is daylight showing, is this for air circulation?

Does this mean that i will have to secure new joists to the minimum height of the insulation so that the flooring will lie immediately on top of the insulation (without a gap)?

6" is quite a good depth for roof timbers depending on how long it is between load-bearing walls, and how wide it is.

What are these measurements?

Look up "skew nailing" You could use this method but you should use screws, and drill pilot holes before fixing them. It will be cheaper than brackets. The reason for screws not nails is that hammering will crack the plaster.

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