What is the price? Cellotex has low U value and thin but look at the price.
I would be inclined to look into laying this stuff over the joints (eliminate thermal bridging) and put chipboard over.
Never thought I'd say this, but you talk much sense, Drivel. I don't think anyone credible has come up with a valid reason for not doing what I propose. The only question is whether 270mm is really necessary, which is academic having spent the money, but I wouldn't mind seeing Ian's revised calculations for curiosity if nothing else.
Mark
by all means use the insulation which you have already purchased, but a valid reason is that there's way too much weight in 8 X 2 timbers, hence my suggestion to compress the fibreglass
Use 1.5mm. Oversize because of being covered in insulation. It is called derating.
There is no such thing as too much insulation. Fuel is not going to get cheaper.
1mm is already oversized in domestic lighting situations.
Tell me about it... We helped BP pump up an extra 70,000 barrels from the North Sea in the last year... did they put the price per barrel down when they sold it?? Did they fu.... _________________________ Grim
I realise your advice was meant for Mark's particular case, however... How could I go about laying insulation over the joists if I'm not putting my second layer of joist at right angles to the first?? As I mentioned before, I'm only planning on adding 2 inches to the height - to work round water pipes (with their foamy insulation thingamybobs). Would the addition of the chipboard over the top of the lot not help with the insulation of the entire area as a whole? ______________________________ The Grimch Reaper
I doubt it'll be too heavy - it'll be spread out and form a strong structure. If I detect any flex or ceiling problems I'll strip it all out. I'm more concerned about the weight of the 18mm chipboard flooring than the
8x2...Compressing the fibre will reduce its insulative properties. It's not the weight of fibre that does the insulating, it's the volume of trapped air. So it doesn't make sense. If it did we'd all be insulating our homes with 10mm solid fibreglass! Anyone know what boats are like in the winter?
Mark
You could fill the space between the joists with rockwool / fibreglass. Then lay a foil covered PIR foam ridgid board (i.e. the stuff celotex / kingspan is made from - but find another brand for half the price!) over the top and screw your chipboard down through the PIR into the joists.
The ridgid foam is plenty dense enough that it will not crush under the distributed load of the chipboard and anything on it.
If you're covering the pipes you won't need the foam insulation on them, but there may be no advantage to removing it. I saved money by not buying pipe insulation for my plumbing in the loft at floor level, as I knew it would be covered by my new floor & insulation.
Yes, although chipboard is not good insulator itself, the fact that you're sealing over the fibre will mean reduced airflow and hence convection, so should help.
I think I get the gist, but what you say isn't 100% clear. I think we need a diagram.
You can get 22mm chipboard, buy 18mm is pretty heavy and surely strong enough. The thicker stuff is for joists that are further apart.
If you really can't add more than 2" why not just fit 2x2 at 90 degrees? 6" is the minimum level recommended by NEF, so you'd have a reasonable level of insulation. See
Mark
Counter means at 90 degrees.
22mm chipboard on 1x1 or 2x1 batons will make the floor more rigid.If he can't go above 2 inches then he is better off buying high performance insulation. He has no option and to do all this work an not improve the insulation value is pretty dumb.
I would check with the makers first.
You will find this is a solution they suggest. It is commonly used for construction of warm deck roofs as well. Some of the insulation manufacturers also make a board which consists of insulation bonded to ply for just this purpose (Tuf R Deck, Temp Check Deck etc). Having said that, the ply they use is usually too thin for construction of a flat roof that will last.
The problem is the insulation foam resting on joist 2" thick. I have seen cheap ply run across joist and foam insulation board over then chipboard. A ply-insulation-chipboard sandwich. The ply spreads the load.
This method is the best as thermal bridging is 99% reduced. Only the screws act as a bridge.
It is not a problem in practice. PIR foam is more than rigid enough to support this load. If you place PIR foam on the ground and stand on it, it does not crush - and that is just with a foot sized area.
I built the flat roof on my loft with 80mm PIR foam boards laid directly onto the firrings, and then 18mm WBP ply over that. Even driving 5 1/2" screws through the deck into the firrings/joists, to the point where they were pulled right into the ply there was no noticeable deformation in the foam.
The celotex Tempchek deck product is made like that - although it only has 12mm ply on the top surface. With the thicker board on top there is no need for a board under unless you need it for fixing into (not much of an issue on a floor)
It is also much easier to do than extending the joist heights.
Humble pie time - discussed it properly with my structural engineer neighbour. Conclusion is that I do need to span the structural walls and ensure the weight is supported properly by the walls and not the ceiling joists. The good news is that using 8x2 means they'll be strong enough for the span.
Mark
As I said. Support the 8" joists on the gable ends. Joist hangers are available for this
If you are going to the hassle of increasing joist size to that extent, you may as well do the structural calcs to show they meet the requirements for a floor in a habitable room, and do the work on a building notice. Then you have the foundation work for a complete loft conversion in place should you need it later.
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