raising joists for loft insulation

coolpran

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I want to insulate my loft and still be able to use it for storage.
The current joists are 100mm high X 50mm and I want to increase their height so as to fit in 270mm of insulation and board on top of it.
I am thinking of fitting these 175mm X 50mm wooden planks (http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=9276000) at 90 degrees to existing joists.
Fixing these with angle brackets at regular intervals (http://www.screwfix.com/prods/61576/Building/Builders-Metalwork/Angle-Bracket-Heavy-Duty-50-x-50mm-Pack-of-10).
Then I am going to put 1 layer of 100mm insulation between existing joists, and another 170mm on the raised joists, leaving about 5mm gap for ventilation/wiring.
On top of all this, I will be fitting standard chipboard (http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=9276437) with screws.

The span of the roof is 4.8m (W) X 9.5m (L) and the joist run lengthwise, i.e. 8 joists at ~60cm gaps covering the 4.8m width.
I will try and attach pictures of the roof truss and joists for clarity.

My questions are
- is the recommendation to raise the joist at 90 degrees or along existing joists?
- does raising the height by 175mm seem excessive considering I am going to board on top? can I get away with raising by 100mm only?

I know this question has been asked before, but none of the answers seem definitive.

EggMella

Check out this product...
http://www.knaufinsulation.co.uk/press_releases/deck_it_out_with_knauf_insulat.aspx

I'm not sure how it will compare in terms of cost but weighed against time & effort it should be a reasonable alternative.

coolpran

thanks eggmella. i had seen that, but could not find any retailers which stock this product

27col

What you are proposing doing will be expensive and time consuming. It will also add a lot of weight into the roof, before you start to consider the weight of what you are storing. The Knauf board seems to be a good idea. Why do you not ring them to find out where their suppliers are situated. It might well be just as economical to to it that way, and certainly a lot simpler to install.

Kiran

B&Q stock the Knauf boads, however they ar quite expsensive. If you are thinking of adding a sub frame then it would be wise to fix them to the gable end wall as opposed to have the whole weight bearing on the existing joists. Joist hangers from a timber merchant woud do the job!

frankie

EggMella wrote: »

Check out this product...
http://www.knaufinsulation.co.uk/press_releases/deck_it_out_with_knauf_insulat.aspx

I purchased some of this from Focus about a month ago. They had a BOGOF deal on at the time.

coolpran

Kiran wrote: »

B&Q stock the Knauf boads, however they ar quite expsensive. If you are thinking of adding a sub frame then it would be wise to fix them to the gable end wall as opposed to have the whole weight bearing on the existing joists. Joist hangers from a timber merchant woud do the job!

sorry to sound like an idiot, but which one is the gable end wall?
so i fix one of these http://www.screwfix.com/prods/33789/Building/Builders-Metalwork/Joist-Hanger-50-x-200mm-Pack-of-4 to one wall and the other end rests on the existing joist, is that right?

also, i forgot to mention, the roof truss is W-type shown here http://www.tpub.com/content/construction/14044/css/14044_91.htm. Does that make any difference?

27col

The gable end is the wall that runs parallel to the trusses. You might well have two gable ends if your house is semi detached.

coolpran

thanks for your advice guys. i am thinking of buying the joist hangers now. need to see if I can fit the 4.8m piece of wood through the loft hatch.

anthonykeates

Does anyone know whether it's worth the extra cost of installing 270mm of insulation over say 170mm in terms of what can be saved in energy costs?

latecomer

If you are doing it yourself it wont cost much at all particularly with the current offers that are on. No idea what the energy saving would be though.