Boarding and insulating my loft

[Kevin652]

I have a project to board my loft and perhaps improve the insulation. The objective is light storage, not living space. I wanted to check it out with you guys who have more experience than me, just in case I am making an expensive mistake.

The house was built in the '80s, and is about 7m wide by about 10m front-to-back. The roof is held on 12 frames of a complex "W" shape, made of 70mm x 35mm timber, and pitched about 63cm apart. The ceiling joists are filled to their full 70mm depth with horrible old glass-fibre insulation. There is no insulation on the slope under the tiles. There are proper ventilation grills in the eves at the back and the front of the house. The previous owner put some old chipboard on top of the ceiling joists, but I think the chipboard has taken on some moisture and is getting weak.

My proposal is this: to add 50mm x 50mm cross-joists pitched at about 50cm apart, and lay light T&G boards (19mm thick, 9cm wide) on those, therefore running in the same direction as the original frames. I reckon this works out at about 9Kg per square meter, or 10Kg if you include the cross-joists.

How do I add extra insulation into this mix? I propose to put solid sheet insulation between the new cross-joists, resting across the original joists, to the depth of the new cross-joists, i.e. 50mm. This means the boards would rest on the insulation as well as on the cross-joists. But would this be inviting condensation? Do I need an air gap between the floor and the insulation?

The sheet insulation I was thinking of is some green foam sheets they sell here in Luxembourg (maybe also in the UK?), which weighs about 1 or 2 Kg per square meter. (It is the insulation that weighs that, not Luxembourg or the UK ). Or perhaps expanded "cottage cheese" polystyrene sheets. Which is better? Is it OK to put that stuff over fibre-glass?

What about adding insulation under the tiles? 70mm does not give me much to play with, especially if I have to leave an air gap between the tiles and the insulation. Any advice? Should I perhaps use 30mm sheet, then cross-batten to a depth of 50mm and fill the gap between the cross battens?

Thanks in advance for your advice.

Kevin

 

[PrenticeBoyofDerry]

You have 70mm ceiling joist, how is the span being supported?
What structure are the walls?
If you have good support for the joist, it is recommending in the UK to have 270mm of insulation in the loft space.
Run new timber/joist perpendicular to your existing joist, the ideally want to be spaced at 400mm centres this will allow for loft panels of 2400mm or 1220mm in length to be installed with minimum of cuts. (not sure of sizes in Lux, may have to do some research and plan)
The insulation (use rock/mineral wool if available) will be laid in between joist, creating a full fill, with no gaps.
You must however leave space at the eaves for ventilation.
If the loft is for light storage it will be pointless insulating the slooping roof.

 

[Kevin652]

Thank you for your reply PrenticeBoy. I looked at my notes again this morning, and I see the ceiling joists and slopes may actually be 92mm deep. I think some of the triangulation struts are 35mm x 70mm, hence the confusion. I shall measure again this evening.

The joists run front-to-back on the house, supported on the concrete / block front and back walls. So they do a massive 10m span, although they are all partially rest on internal walls of the upstairs, which are made of a sort of solid plaster block (10cm thick?). One of these walls, near the middle of the span, is the continuation of the central load-bearing wall of the house.

For the decking, I was going to use 9cm wide (19mm thick) softwood tongue-and-groove which is quite cheap here, and comes in 2.10 m lengths. If I span 5 cross-joists per board, that puts the spacing at 42cm. But insulation seems to come in 60cm widths here, so mathematically the best spacing would be 65cm. 3 widths of insulation @ 60cm + 3 joists @5cm would come to 195cm, so I would have to cut the boards to that length. But I take it from your reply that you think that 65cm is too great a gap between joists, right?

Also, you recommend mineral wool. Is minreal wool that loose stuff that you just spread out? Can I put it on top of the existing fibre-glass? I have an aversion to fibre-glass cos it makes me cough every time I go up in the loft ... what do you think of solid sheet insulation? Is it a no-no?

Thanks again.

Kevin

P.S. Re-working the maths, it seems to be begging me to space the joists at 35cm intervals and cut my insulation into (or buy) 30cm strips. 35cm x 6 = 210cm so each board rests exactly on 6 cross-joists. Except I might have to miss out a cross-joist in the middle of the house where the strengthening struts come down. I'll end up with more joist than insulation at this rate

 

[PrenticeBoyofDerry]

If you space your centres at 420mm that should be okay, 600mm plus is way too much.
The minerial/rock wool insulation normally comes in rolls, some are an irritant to skin/eyes/mouth but some rolls are now made from recycled plastic.
You could use loose fill granuals, slabs or PIR boards. It's realy up to ypu just check out the thermal values so you are getting a good insulator. I recommend the loft rolls as they do the job, easy to install and normally cheaper.
If your board out with the standard T&G flooring remember it's best to keep the joints staggered, very much like laying laminate.
Not sure what the building regulation are in Luxemberg, but in the UK electrical cables within thermal insulation has to be calculated and some times needs to be upgraded, if it can not be routed safely.

 

[mointainwalker]

In the past year or so ( around here anyway) the old yellow fibre-glass insulation has received two similar alternative products.

One is brown and causes no irritation at all 9generally speaking) and is a couple of percent more expensive than the yellow fibre. the other is white and is being promoted as ecological, wonderfully easy to handle etc etc.
this is considerably more expensive - don't know by how much.

Fibre wool has the same insulation value as polystyrene and is about 60% cheaper. It is generally sold as a package 120 cm long, which may be two separate rolls of 60 or one roll of 120. They now generally print markings at 10 cm intervals on the backing paper so cutting it is easy although a bit of a nuisance to roll it out to cut etc.

The backing paper - which is actually a vapour barrier - should be laid TOWARDS the ceiling.

 

[Kevin652]

Thanks mointainwalker.

I had a look around the DIY shops here and the insulation all seems to come in rather wide rolls. I did however find some brown fibre insulation at 50 cm wide so I wonder if I can get away with my cross joists at 52.5 cm intervals (my boards are 2.10 m long).

The roll looks good value at around €5.50 per sq.m, and has a metal foil vapour barrier, which makes me wonder .... is it OK to lay that (silver side down) on top of and across the existing yellow fibre? The existing joists (and fibre layer) are 10 cm thick.

There are recessed lights in a couple of the upstairs rooms and there are some holes in the existing yellow fibre to allow them to stay cool. I hesitate to cover the lights , but if they are covered at a height of 10 cm with this second layer, will the foil vapour barrier also help dissipate the heat from the lights? Perhaps I should put some sort of metal guard around them before I put the second layer of insulation ... maybe a modified aluminium container from the Chinese takeaway. WDYT?

The only other thing about this insulation is that it is 140 mm thick, and I was counting on having my cross joists only 60 mm high (and 40 mm wide). That means I either have to squash the cross-insulation down to 60 mm, or double up my cross-joists and squash the insulation down only slightly to 120 mm. Which would you do?

Sorry if I sound like a newbie to DIY ... but I am, and I want to get it right.

Thanks
Kevin

 

[mointainwalker]

The lights in your ceiling are a problem. The heat from the bulbs will draw the moist warm air from the living area up like a chimney into the insulation/loft where it will condense ( very undesirable). This is why there should be a vapour barrier.

However the lights also need to able to dissipate their heat or they risk overheating with early failure or possible fire risk( depending on bulb type/wattage )

If you want to keep them and insulate sensibly, the only thing I can suggest is putting a fire-resistant box ( many people make this out of plasterboard) over it and putting your insulation over that.

If you squash fibre insulation, you reduce its insulating effect to approximately the compressed thickness but you can buy insulation in nearly any thickness so go to a reasonable DIY centre or builder's merchant to get the closest available to 60 mm/120

I would suggest 120 mm if you can do it, as 60 mm is really too thin. EU recommendations/regulations now require 260 mm : IMO over the top but I have 200 mm where I can.

This fibre you found at 50 cm sounds rather odd. Since the "standard" spacings for joists are 60 or 40 cm, all fibre insulation I have ever seen has been in 120 cm rolls as this is divisible by both 40 and 60 to leave no waste off-cuts.

P.S. Your insulation wasn't particularly cheap. I paid E 5,30/m2 for 200 mm roll from a French DIY shed a few weeks ago.

 

[ajrobb]

You don't want to hear this, but as your ceiling probably has a U-value over 0.35 W/m²/K, you are supposed leave it alone or improve it to 0.16 or less (Approved Document L1B Table 3)

Untitled

• ajrobb
• 9 Mar 2011
• 4

That would take 250 mm of mineral wool. However, as laying loft insulation without altering other thermal elements is exempt from Building Control, who's to know?

 

[ajrobb]

must check country

• must check country

 

[Kevin652]

Not to worry, but thanks for the information anyway. I'm not sure how to find out about the building regs here, and anyway most of the local standards are a sort of hotch-potch of German, French, and Belgian. We had a German roofing contractor here to give us a quote and he said he couldn't touch it because it was "a French roof".

I very much doubt it meets anyone's standards as it is at the moment. What is the best way to learn about U-values and how to calculate them?

Considering LEDs or low energy bulbs for the downlighters.