I’m looking to lay some boards in my loft this weekend and just wondering if I should lay timbers across the joists to raise the board height and allow for a decent level of insulation or just reduce insulation and trust that boards will trap some of the heat?

The joists are not particularly deep and so I wouldn’t get much rockwool in without possibly pressing on the back of the lathe and plaster ceilings which I defintely do not want to do! If I need to raise height and lay rockwool both in joists and between new timbers do I need to worry about air flow under the new boards at all?

I’m a novice diyer if that makes a difference!

If you are thinking of walking on the boards, you will need to lay thicker joists and support them at the wall
The joists in he ceiling were not designed to take any weight, just to hold the lath and plaster up.
If you do start to walk on them expect your ceiling plaster to develop some cracks at best

I did for the section of the loft I wanted to board for storage

then just roll it out for the for the rest over the top

As easygirl said…

They’re not joists. Just rafters.

Thanks but it’s a victorian house – surely everything was over engineered! 😉 Or have I made a big and wrong assumption there…. a very decent joiner fitted the bigger loft opening and said nowt. To be fair I don’t want to make a room up there only storage but even then I’d be walking up there a little. Think the rafters are c 4×2 so pretty decent and supported by a much bigger timber across the centre plus a few brick walls on the 2 room and landing below. I have wandered around standing on the rafters and never seemed anything but totally solid so I assumed spreading the weight would be fine. They also supported the old water tank for the heating which is now redundant so I’m guessing that was pretty heavy.
Do I need to cancel my order with the local building supply merchants as I’m not putting joists in. Not this weekend. Not any weekend!

http://www.tameside.gov.uk/tmbc8/note07.pdf

4×2 at foot centres is what I’ve got, sounds similar
Larths have been removed and plasterboard put up instead

I don’t go hiking up there or store weights or anything, just a few bits and pieces suitcases etc

don’t go putting a car up there bit by bit or a running machine 🙄 and you’ll be fine

Be aware of any electrical cabling too. Do not lay insulation on top of cable, especially SHOWER CABLES. They are very likely to overheat and be a fire risk if covered.
Also, do not trap cables between boards and joists.

Jerry. Eco Electrics, Sheffield

Infront of me right now I have U values. Assuming your un-squashed insulation is a buliding regs friendly 250-270mm, you’re looking at a U value of approx 0.18 to 0.16.

Squashing your insulation to the depth of a joist (assumed 100mm) will reduce your U value to circa 0.34.

So you will reduce the effectiveness of your roof insulation by just over half.

My victorian loft is 4×2 at 1ft centres. Plasterboard ceilings. I laid more timber (2×2 ish as thats what I had to hand) on top and then boarded, with the gap filled with insulation. (don’t press it down at all, the more it plumps up the better it works)

I have a silly silly scary amount of stuff up there, tools and all sorts stacked high in boxes and I’m up there shifting stuff around every couple of days. No sign of the ceilings cracking, but I did check regularly when I started to fill the loft.

I used some 50mm plastic conduit to give the cables (lighting only) some breathing space where it is under the boarded areas.

Fix 3 x 2 beareares over the existing joists but at 90 degrees to them plenty of room for air cicualtion, and running pipes or cables. then insulate between the rafters, and hold the insulation in place with chicken wire..

Oh and a good tip, paper overalls and a descent fan to give you some cooling air and a decent light, never use a halogen on as if it falls over youll have a majorfire.

I put 6″x2″ timbers at the side of the original 4″x2″, i used two lenghts 5 meter and 4 meter, i sat them on the load bearing walls. i cut them to fit into the eves and bolted them to the original timbers. i also fitted bracing timbers to the roof struts.

Right thanks all. So much for my easy couple of hours work tomorrow and job done idea. ‘Feel’ like a few 22mm boards and a well spread out average amount of normal household stuff wouldn’t be a problem but now have those doubts….
Oh and a halogen bulb = major fire in my loft is super scary! I might take a bucket of water up there with my AAA powered headtorch and 1meg candles rechargeable torch just in case.