Increasing height of joists to add insulation and board uo

11fernsroad

Hi

On a mission to sort out my loft,where there is adequate insulation material but all dumped in a corner or some areas left clean.

Plus there are about 30 downlights,and three bathroom extractor ductings,loads of electric cables and 2 TV aerial cables.

Plan is to increase the height of joists (at present 8 cm) to the  to add another 10 cm height or more and distribute the loft wool.

We would then like to board over it(at least in the centre bit for storage as well safe access in case of maintanance work)

What is the cheapest way to increase the height of the joist.I checked loft legs and they are expensive for my project.

joists are 60 cm apart.

Loft area is 6 m x 12 m

Thanks

GDB2222

You can get special legs to fit on top of the joists. 

Edit - I could not remember what they are called. Apparently, loft joist legs!

Mutton_Geoff

You don't need loads of insulation under a boarded section as the boarding over acts as an insulated box. Loft insulation only works by trapping air to stop any exchange of heat. A boxed in section does this anyway. Minimise your boarded section to a small area near the hatch.

Fit loft caps over all your downlighters, run 100mm insulation along the joists then top up with 200mm at 90 degrees to that so you have a total of 300mm insulation.

If you only have 3" joists in the loft, they are not going to take much weight (even standing on them could crack the plasterboard ceiling below). I would think about decluttering rather than increasing the load on the ceilings by boarding it all out.

grumbler

Mutton_Geoff said:

You don't need loads of insulation under a boarded section as the boarding over acts as an insulated box. Loft insulation only works by trapping air to stop any exchange of heat. A boxed in section does this anyway.

Do I understand correctly that you are saying that rockwool adds nothing in terms of thermal insulation? If so, this is incorrect. It significantly reduces convection - on of three ways of heat transfer. That's why they now fill cavity in walls

ComicGeek

Mutton_Geoff said:

You don't need loads of insulation under a boarded section as the boarding over acts as an insulated box. Loft insulation only works by trapping air to stop any exchange of heat. A boxed in section does this anyway. Minimise your boarded section to a small area near the hatch..

Nonsense. Standard loft quilt insulation only works by virtue of small pockets of air within the insulation - and air pockets above and below the quilt have limited benefit. Hence squashing it down massively reduces the insulation properties. 

A boxed in section of loft would only have the insulation properties of thin bits of wood, which is very very limited.

Best thing is use rigid insulation boards between joists in the boarded areas, but keep these areas to a minimum and use cheaper quilt insulation loose in the other areas. 100mm rigid board is about equivalent to 200mm unsquashed quilt. 

11fernsroad

Looks like Loft legs is the best option then.

Of course the company will ask to fix them at minimum distance,but if I were to use the chipboard to basically have safe access in the loft without any heavy storage use,what would be maximum distance apart I can fix the loft legs?

Mutton_Geoff

ComicGeek said:

Mutton_Geoff said:

You don't need loads of insulation under a boarded section as the boarding over acts as an insulated box. Loft insulation only works by trapping air to stop any exchange of heat. A boxed in section does this anyway. Minimise your boarded section to a small area near the hatch..

Nonsense. Standard loft quilt insulation only works by virtue of small pockets of air within the insulation - and air pockets above and below the quilt have limited benefit. Hence squashing it down massively reduces the insulation properties. 

A boxed in section of loft would only have the insulation properties of thin bits of wood, which is very very limited.

Best thing is use rigid insulation boards between joists in the boarded areas, but keep these areas to a minimum and use cheaper quilt insulation loose in the other areas. 100mm rigid board is about equivalent to 200mm unsquashed quilt. 

You have misunderstood me. I meant it is not worth building up the height to accomodate 200/300mm of insulation to then board over it. I know how insulation works, it's main function is to prevent the movement of air and therefore heat exchange between warm/cold areas.

Boxing in straight over the existing joists in a small area (as suggested) and fill that with insulation (but don't compress it!) is what I meant by "you don't need loads of insulation", ie 200mm+. I'm talking about a square meter or so. Putting loft legs in to allow 200+ insulation then boarding the whole lot is wasteful imho.

Norman_Castle

11fernsroad said:

Looks like Loft legs is the best option then.

Of course the company will ask to fix them at minimum distance,but if I were to use the chipboard to basically have safe access in the loft without any heavy storage use,what would be maximum distance apart I can fix the loft legs?

If a company is doing the work they should be advising. The legs or boards should specify recommended distances.

https://www.wickes.co.uk/Loftleg-Raised-Floor-Loft-Legs---175mm-Pack-of-12/p/100412

https://wickes.scene7.com/is/content/travisperkins/Insulation-Fixings-Loftleg-Raised-Floor-Loft-Legs-175mm-Pack-of-12~L3100_100412_TECH_0

Investigate the cost of adding ceiling joists or similar at 90 degrees to the current ceiling joists spaced to accommodate whatever flooring you are using. This might be cheaper and should spread the weight.

GDB2222

11fernsroad said:

Looks like Loft legs is the best option then.

Of course the company will ask to fix them at minimum distance,but if I were to use the chipboard to basically have safe access in the loft without any heavy storage use,what would be maximum distance apart I can fix the loft legs?

Not a good idea! You have very small joists. Fewer legs means concentrating the load on every other joist, say. Or, even worse, every third one. Plus, the less support you provide, the thicker (and heavier) the boards will have to be.  So, you'll pay more for the boards, and have the heavier load concentrated on fewer joists. 

phil24_7

Are both ends of the joists sitting on external walls or is the centre supported on a centre load supporting wall?

What is the span of the joists?

If the joists are supported on both ends by the exterior walls, I would be looking at glueing (structural glue) and screwing 120mm (or larger epending on span) joists to the top of the existing joists to give you a sound floor capable of holding a reasonable load.

I'd then fit roofing batten to the edges of the joists, with the top edges 100mm from the ceiling below which would allow at least 100mm of Celotex etc to be intalled level with the joists.  You could then run 25mm of celotex running across the joists to eliminate thermal bridging or just board over the joists.

fenwick458

I used a mixture of loft legs and 3x2 CLS when I did mine (in my house now ). found out it was a lot cheaper to just add in a few bits of timber for £15 and it saved about £50 on loft legs, they are quite expensive if you put in as many as they reccomend!
about 10 years ago in my old house I re did the loft boarding, previously it was about 20 old doors just sat on top of all the insulation. there was plenty of insulation, but it was all squashed flat, and there were a few gaps near the eaves. I used loads of 3x2, raised it up so there was a 300mm gap for the insulation, which swelled up to fill the gap, and made sure it was all even right into the corners. i'm positive it was warmer in the house ever since i did that. meant i had a load of extra storage space too