Loft insulation and joist depth

[ColinM50]

I keep getting calls from these "energy advisors" telling me I can get a free top up of my loft insultion and when I ask them what to do about all the stuff in my loft which is boarded out, I'm just met with "hmm, you'll have to empty the loft before we can do it". Which isn't much help and really means I won't be able to store stuff in the loft anymore. All they've suggested is that they'll rest the stuff on top of the new insulation. which is no solutiojn at all.

I'm sure others have had the same problem, so what did you do?

Existing joists are 75mm deep and we've got 75mm of insulation and boards down resting on the joists. I'd really like to increase the insulation to 270mm as recommended now and I've thought of making up some dummy joists as the pic below. So sort of 50x50 timber top and bottom with the metal webs between and screw/nail the bottom beam across the existing joists at 15 inch centres and then after the insulation's been put in, screw the flooring on to the top 50x50. Does that seem sensible?

Now comes the real question. What's the metal web called and where can I buy it? Tried a couple of builders merchants locally and they all suck their teeth and tell me I'll have to have the joists made up properly but can't help other than that. Anyone know please or get any other ideas?

[VxDuncan]

You can buy little standoffs in B&Q to give you extra height if needed. Alternatively, you can do what I did and "cross brace" the joists - that is add a second layer of joists at 90 deg to the existing to give you more depth, put the boards on those.

Watch the existing joist are all level though - add packing (/cut out the new joists) to ensure you don't stress the old ones and crack the ceiling. I did this to get around 150mm of insulation which made a fair difference. couldn't easily go any higher with the layout of my loft/hatch. Plus, the benefit of extra insulation isn't linear - you don't get twice the energy saving for twice the insulation thickness - I assume it's an inverse-square law.

[944fan]

You should aim to get as much insulation down as possible. Especially as it is currently free. That funding goes at the end of the year and wont be back in that guise.

150mm of insulation gives a u value of about 0.29 on a slate roof, 300mm gives a u value of 0.13, so double the insulation will give you double the benefit as the rate of thermal transmitance with 300mm is more than half what it is at 150mm. Current building regs have a target u value of 0.16, which equates to about 270mm, but i think most schemes are topping up to 300. They are unkikely to just top up to 150mm as it wont give them enough carbon credits, which is why they are installing it for free.

[ColinM50]

Thanks VX and 944, but what's the name/cost/supplier for the metal bits so I can make these dummy joists and get the bods in to put the insulation in?

[matts4]

Isn't there a danger that the cost of time, materials and parts will offset any potential savings? (Over say, a 8-10 yr period)

[944fan]

Interstial condensation can be a problem when the roof void is improperly vented but even the thickest of installers should be aware of this and should check.

[Tom_C76]

ColinM50 said:

Thanks VX and 944, but what's the name/cost/supplier for the metal bits so I can make these dummy joists and get the bods in to put the insulation in?

Those are called Ecojoists and they're structural floor joists, not spacers. They're assembled by machine too, pressing the nail plates into the timber, not suitable for DIY.

As others have said, just cross new joists at right angles to the existing, top up insulation should run across existing joists anyway.

[TA14]

Tom_C76 said:

They're assembled by machine too, pressing the nail plates into the timber, not suitable for DIY.

This is the main point. I soppose that you could use protective pieces of timber, clamps and screws but it would take ages.

[Craikeybaby]

I'm going to get some of these for when I board my loft, as the insulation is already over the level of the joists.