Kingspan

[Shift]

Just had a flat roof converted to a pitched tiled roof and planned to insulate with kingspan. I was going to fill the joists with 150mm as they are over 150mm deep but builder recommended 100mm between ceiling joists and 50mm on top of that. The reason for this he said is the wooden joists act as a thermal break. Buying 100mm plus 50mm works out quite a bit dearer than getting 150mm would there really be that much difference.
Where is the cheapest place to buy kingspan, tried local builders merchants,ebay etc,cheapest I could find is £47 for the 150mm there any cheap online stores or does anyone have any they wish to sell.

 

[Deluks]

You will probably get away with 30mm in place of the 50mm, the difference in thermal loss will be negligable.

I've never understood the panic about joists being a thermal break, being that wood is an extremely bad conductor of heat. Ever burnt yourself using a wooden spoon?

 

[cmother1]

The building inspector for our extension said we had to put 100mm between the joists and 50mm over them. I seem to recall, though, that this was to achieve a 100mm air gap above the insulation.

However... a friend of mine has a very similar extension a couple of years older with all the insulation between the joists. You can now see dark lines down the ceiling where the joists run. Whether this is due to thermal bridging, the tanalising or whatever, I don't know. But it's definitely visible.

 

[alittlerespect]

Timber is a relatively good insulator, but in comparison to modern insulating materials its about a 1/4 as efficient.

For example, say 100mm timber joist - U value across the grain 1.5W/m2K compared to a fibreglass insulation material of equivalent thickness giving a U value of 0.4W/m2K.

Meaning there is 4 times the heat loss through the timber than the adjacent insulation material, although my initial thoughts are that I would not have expected there to have been such a problem in such a short period of time, which does question whether or not the friends house had insulation put over the top of the joists?

Problem with insulating the ceiling is that the roof void becomes a lot colder in winter as there is less/neglible heat escaping from the living area to the loft and any exposed timber ceiling joists will consequently be a lot colder than most people would anticipate at it is the temperature differential that is the most likely cause of the thermal bridging.

Regards

 

[ColJack]

couldn't you buy 150mm and cut to a wide "T" shape? so that you have a 50mm "lip" on either side to sit on the joist? then tape the joints?

 

[freddiemercurystwin]

Phone up Kingspan technical http://www.insulation.kingspan.com/uk/services04.htm and talk to them, they'll tell you exactly what thickness you need above and below the rafters to comply, it will depend on the spacings of the rafters which determines how much bridging is present. Why people don't use the techy services of these firms more is a mystery to me. Try Encon for prices after you know what thickness you can get away with. http://www.enconinsulation.com/images/UserFiles/File/BG13-50.pdf You'll also need to ensure you leave your 50mm airspace below the felt if the roof has a felt underlay.

 

[Shift]

couldn't you buy 150mm and cut to a wide "T" shape? so that you have a 50mm "lip" on either side to sit on the joist? then tape the joints?

I did think of that but thought there would be too much waste, another way I thought of doing it was to have the 150mm sat on some supports in between the joists so it protrudes 50mm above joist, then cut strips of 50mm thick kingspan to fill in the gaps above the joists.

 

[Shift]

Phone up Kingspan technical http://www.insulation.kingspan.com/uk/services04.htm and talk to them, they'll tell you exactly what thickness you need above and below the rafters to comply, it will depend on the spacings of the rafters which determines how much bridging is present. Why people don't use the techy services of these firms more is a mystery to me. Try Encon for prices after you know what thickness you can get away with. http://www.enconinsulation.com/images/UserFiles/File/BG13-50.pdf You'll also need to ensure you leave your 50mm airspace below the felt if the roof has a felt underlay.

Thanks for the info, will give them a call.