Rasing Joist for Loft Insulation


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Established Member
8 Feb 2005
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North Yorkshire
Hi All,

I've just had a chap around to advise about topping up our loft insulation. I was amazed that regulations now specify 10" of insulation. This is well above the height of our joists, which are about 4".

In order to keep some boarding down for storage, I guess the three choices are:
1) A thinner (more expensive) insulation material
2) Only use 4" insulation under the boarded area.
3) Raising the joist height under the boards

We are getting a council rebate for the work which cuts the cost down considerable (well worth looking into). This probably means option 1 is out, and I'd rather avoid option 2. So it's probably going to be option 3, raising the top of the joists.

Has anyone done anything like this? If you've got any advice, or even better, photos, it would be a great help.

Many thanks
cambournepete":t3c79a6l said:
I just put new joists across the existing joists and put insulation between them and then the boards on top.

Thanks for the reply. When you say across, do you mean inline or perpendicular to the original joists
I have done this for two or three customers, and find no problem. You will lose head room so just check there is no problem there.
maltrout512":18288mbv said:
I have done this for two or three customers, and find no problem. You will lose head room so just check there is no problem there.

Presumably you have to set them into the wall, so that not all of the weight is on the old joists?

Much easier if the new "joists" are at 90 degrees to the old ones. Better for heat loss as well, because the insulation will then also run at right angles, and there will be no chance of a continuous gap alongside a joist from top of ceiling to top of insulation.

BUT.......do pay attention to the eaves. Most rooves with traditional felt require eaves airflow, and it is easy to block this with the additional insulation. You might even have to resort to some boards between the joists to maintain a 2" airgap under the felt. If you don't maintain airflow, there will be condensation within the roof-space, and I have see rooves only 15 years old fail as a result.

Hi Andy,
I did this a couple of years ago and raised the joists by fixing 6x2s on top of the existing joists in the areas where I wanted to keep the floor boarding.

For the other bits I just cross layed the insulation on top of the existing insulation (i.e. at 90 degrees to the joists). There's 2 thicknesses sold (100mm and 170mm IIRC) so you can build up two layers to the required thickness.

There's a bit of an issue with electrics if you have your upstairs lighting cabling attached to the joists. The extra insulation layed over the top could cause the wiring to overheat. Some people advise removing the wiring from the joists and laying it on top of the insulation - other people will tell you this breaks the regulation for fixing cabling. :?

You can calculate the capacity of your cabling, taking into account factors such as insulation. If you're inside the cable spec then you're OK.

Hope this doesn't make it worse,

How did you actually construct the new joists. I was thinking something a bit like one of these:

Neil, Yes, that's an option I've seen mentioned in various forums. I guess as long as the new joist structure weight isn't too great, the old joists should be OK to support the weight. At the moment, they're OK taking the weight of me and a load of boxes!

Mike, that's something I'll check once (if) the work is done. The bit I want to board is in the centre, well away from the eaves opening. So I just need to make sure the insulation is kept back at the edges.

Simon, good point. The only electrics in the loft is the upstairs light ring. I can always bump this up a gauge. More of a concern are the ceiling mounted spot lights. These are going to need to be kept clear.
AndyG":2la78ioh said:
cambournepete":2la78ioh said:
I just put new joists across the existing joists and put insulation between them and then the boards on top.

Thanks for the reply. When you say across, do you mean inline or perpendicular to the original joists
There not much weight on them so they're jsut connected to the existing joists.
If you run your joists perpendicular to the existing joists then the top up insulation will sit on top of the existing joists. When I enquired about doing this I was told it was a definite no-no because moist air rising from the room below would get trapped under the insulation and condense there, where it could cause problems with the existing (structural) joists.

Quite how this would be worse that the effect of having boards covered in boxes I don't know, but that is what I was told. I therefore placed 'top up' joists on top of the existing ones and boarded on top of them.

When we moved in to our house we had to have the roof re-done. We had the felt replaced with Tyvek breathable membrane. We then had the rafters insulated in-between. It was then covered perpendicular with foil bubble wrap stuff which was taped to form a vapour barrier. Seems to work fine and not hassle with the joists. This isn't viable if you have the old style felt!

whoever told you that was wrong, I'm afraid. It is standard practice, and indeed, best practice.

The logic is that if moisture can get through the first layer of insulation (which it can) it will also get through the second layer. You are unlikely to get interstitial condensation within the depths of mineral wool we are talking about, and even if you did, the hygroscopic nature of the material means that the vapour will be emitted later to the cold face (aided, of course, by the airflow through the loft).

Of course, none of the above applies if using boarded insulation such as Kingspan.

Mike Garnham":1f5vg3vz said:

whoever told you that was wrong, I'm afraid. It is standard practice, and indeed, best practice.

I did a bit of googling after I posted and what I found confirms what you are saying. Wish I'd been told that back then, because in that particular loft it would have made things much easier. :roll:
Building control insist new joists are at 90 % to the existing joists spreads the load better and is less likely to cause any ceiling cracking
Hi John,

don't ever let Building Control insist!!! Their job is to check compliance with the Building regulations......not to stipulate how you comply. If you can get calculations to show that doing it in chocolate ice-cream is strong enough, then that is good enough for them. Furthermore, a suggestion from Building Control in one circumstance can often be interpreted by Builders as a requirement in all circumstances......as a result, there are lots of myths out there!

I can certainly think of situations where it would be structurally advantageous to build up along the joists rather than across them.......second best in insulation terms, but in some circumstances it may be stronger. Every case should really be assessed on its merits, but in general terms, across the joists is best.

depending on how you use the roof space, you may find it more advantageous to insulate the rafters rather than the ceiling joists .

this will keep the roof space warmer, but the total insulation ( R ) value the same


the option is then to add polystyrene slabs with plasterboard over the top - no woodwork necessary, just long screws !

it makes the loft area much more inviting and usable - but if you only go up there once a year for the crimbo decorations, then it dont much matter !

thats a good point tusses - my current plan for my mothers gaff (where i currently live) is to up the insulation between joists from 100mm to 300mm , floor over that with chipboard , and also insulate 100mm between the rafters - using that stuff made out of sheeps wool rather than polystirene. before hard boarding over that and painting it white (to help with loft lighting.)

i also intend to box round the water tanks with a double skin of plywood with at least 100mm of insulation between them and a removable lid featuring the same width out of solid insulation board.

the concept is to get the old dears house insulated to the point where it can be heated soley by the log burner ( i get logs free from work) meaning she wont need to run the stupidly expensive gas CH

we just have a log burner (free pallets)

old cottage - 4 bed, circa 1800


central heating ... pahhh !
I had to get a structural engineer out to examine my Roof and loft space after a previous occupier had decided to remove some of the supports :shock: As it transpired the roof was OK and did not need any remedial work but i did ask about needing to build up the joist level for insulation

He told me to put extra ceiling bearers at 90° to the existing joists and attach them to the gable walls with joist hangers, then build up the existing joists in between the new ceiling bearers to the same height.

I was told to use Coach screws to pull the existing ceiling joists and the new top up joists together, all in all it made an excellent job of it, it wasn't cheap though, the attic floor is solid as a rock now though.
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