Doubling up rafters

Hi

Why don't you go for PIR insulation material and save yourself all the hassle - 50mm between rafters leaves you a 40mm air gap which will be fine and add 50mm over the face of the rafters and if you want to go the extra distance use Kingspan K70 or similar board with plasterboard finish instead of the bare 50mm board and trim out with 2 x 2 around your roof lights!

Regards

I am using Kingspan TP10 or Celotux equivalent which is a PIR isn't it?

And I have been told that the insulation between rafters must have a lower u value than the stuff over the rafters so that you don't get condensation between the sheets.

If you use 50mm and 50mm the u ratings would be the same.

So 75mm and 25mm works, or 100mm and 25mm.

That's just what I have been told and read anyway, so I could be wrong.

Hi

One good quality PIR (Kingspan/Celotex etc) are as much the same as one another, although I would suggest reading the technical literature before taking the plunge!

Someone's been stretching your imagination a tad too far about the condensation issue, may be an issue if you were using different types of insulation material!

If you use a PIR insulation - say T10 between rafters and K18 over the face of the rafters you are not going to get any problems with interstitial condensation as PIR has a relatively high vapour resistance, to the extent that no vapour barrier is required - had you been using fibreglass it will have been a totally different situation.

A 50mm air gap for ventilation is the maximum that you need to go to, rather than what some people seem to be quoting as a minimum, as there is little if any benefit in going above 50mm, some people don't advise a 25mm ventilation gap but 40mm (3.8") is going to make neglible difference to 50mm. So 50mm PIR between rafters that leaves you 50mm over rafters and you will also find that a 50mm thickness is easier to handle than the 75mm version!

Thinking about our situation in a bit more depth, if you use 25mm between the rafters you could be inviting trouble as the insulation in general will make the external facing side of the construction a lot colder and you may well find that the 25mm will not accommodate the freeze and thaw cycles that will occur in the worst of the winter months and you may well get a significant amount of condensation between the insulation panels something you are unlikely to have to entertain with a 50mm thickness! Also, moisture, timber and a lack of ventilation do not bode well!

The above scenario is a change to the normal problems associated with condensation which are related to warm moisture laden air from the occupied area coming into contact with cold air/surfaces and reaching saturation point and resulting in surface and interstitial condensation.

Whatever you do, you will need to maintain an air gap between the insulation and the underside of the roof covering which will entail the use of eaves and ridge vents to provide proper air cirulation.

The choice at the end of the day is yours - so the best of luck with whichever venture you decide to take!

Regards

I'm building a small extension with new walls on two sides... 2.50 x 3.00
http://imageshack.us/photo/my-images/69/sideelevation.jpg/
The image above is obviously not to scale.. but gives you the basic idea.

I've been told by building regs I need to double the rafters around my two 800 x 1100 roof windows.

The window weighs less that the tiles it is replacing.. so if its not a weight issue ? At the moment we have installed 170mm x 50mm rafters with 250-300mm spacing across the roof... and with the span being 2.50m.
Is the narrow space between the rafters.. and the relatively narrow span across the roof sufficient to compensate for the lack of double rafters around the window ?

Rooflite's site seems to indicate you can install with single rafters.. but has no clear info on installing to a new roof. Can anyone could point me to a place on the net for chapter and verse on this question ?

http://imageshack.us/photo/my-images/849/rafterconfig.jpg/
This is basic rafter/window configuration (again not to scale)

Recrem said:

Can anyone could point me to a place on the net for chapter and verse on this question ?

Click to expand...

I'm not aware of such guidance, the by the book response from Building Control is that you pay a structural engineer to confirm the rafters do not need doubling up or you just double them up. Generally its just cheaper/easier to add 2 extra rafters. Curious as to how you ended up with rafters @ 250-300 ctrs though when 600 is the norm?

Being "self build" we figured we'd generally add extra rafters and extra thick to give extra support for the windows... that way we couldn't go wrong.

So we've added extra rafters and now need to add even more rafters..

I'm starting to wish I'd just gone out and bought a shed..