170mm joists

[Toby]

Hello all,

Next job is to insulate the loft - I purchased some 100mm and 170mm rockwool when it was on special from nPower.

Now the 100mm is easy now I have removed all the old filthy, patchy squashed insulation, but I want to board out the entire loft too, so I assume I just need to buy 170x50mm joists, and install these across the current joists (I have made sure all my cables are clipped to the current joists, and they are only for lighting (on a 6A RCBO), no electric showers, or socket circuits, apart from one 20A "Loft power" radial, installed against the party wall, with the cable going straight down, so only 270mm will be covered with insulation, but the other side of the cable will be against the brick wall)

Or would it be better to attach the 170mm timber along the current joists, and have the Rockwool all in one direction? I assume it will help distribute any loads and keep it all rigid if it is laid across, rather than along?

The loft will just be for storage, plus a 40 ish U 19" rack for all my CAT5 and RF to terminate in (It's a little on the large size, but I got it free), plus various other equipment (This is installed up against the party wall, and will be a lot lighter then the old water tank that was up there, so I don't think it will be too heavy (I will be providing some thermostat controlled, forced ventilation to the cabinet from the outside of the house, for the summer) (That is another question, where can I get a thermostat with an adjustable hysteresis of, say, 10C?

Thanks!

[Tabby]

?title=3DInsulationNT

[Toby]

1. The best is to add more timber along the top of the existing joists to deepen them, gluing and screwing it in place. This provides space and greatly strengthens & stiffens the floor stucture. 2. The simpler option is to insulate the existing depth, fit the floor, then lay out more roll insulation on top of the floor. This can be rolled back or just left in situ when boxes are placed there.

AND

From title=Strengthen_a_Wood_Floor

Increase Joist Height

A joist can have its depth increased by gluing and screwing more wood along the top. The new wood runs along the old joists, not at 90 degrees to it.

This is often a good solution, especially for loft floors. It does however raise the floor level to a small extent.

The new wood is glued in place & screwed every 6".

Its not essential to use full length timber. Using cut timber finger jointed gives almost the same rigidity. Square cut wood butted firmly end to end also gives a lot of additional rigidity, if not as much as continuous or finger jointed timber. Use a mallet to prevent gaps between ends.

Using reduced length timber avoids the need for a crane and part removal of roof covering.

[The Natural Philosopher]

insulation wise its slightly better to cross the joists to reduce cold bridging and draught channels. Board on top.

[Tabby]

We calculated this a while ago, the difference is very small, so really it comes down to what's msot convenient for the OP.

NT

[Toby]

I think it will be easier to cross them, this presumably also means there is less weight of the insulation ion the plasterboard, as the second layer will be partially supported by the existing beams.

I just cant see why the Wiki says to do it the other way, if there is a benefit (the cold bridging) no matter how small, to do it the other way?

[TheScullster]

"Tabby" wrote

snip..........

Not sure on the cost comparison, but you might consider:

Laying the dense foam type insulation across the joists - then laying board over this and screwing through. OR ISTR B&Q sell chipboard with foam insulation pre-bonded, to be installed in a similar fashion.

Advantages are - no additional joists - thinner insulation needed for desired thermal improvement (therefore less height loss).

Phil

[Tabby]

A lot of loft floor structures are somewhat weak, adding the wood along the joists is a big strength benefit, adding wood acrross them adds much less strength/rigidity.

NT

[The Natural Philosopher]

less rigidity, maybe. Not less strength if the cross beams are end supported.

[Chris J Dixon]

The centre area of my loft was already boarded with a mixture of materials that had accumulated over the years.

I only wanted to provide for storage, a shallow pitched roof gives little room to work, and I didn't particularly want to have to take everything out of the loft.

As some of the boarding was securely nailed down, had fibreglass insulation beneath and could not be lifted intact, I first ensured everything was secured, roughly level and all joins were supported by joists.

I then covered this with two layers of Knauf space board insulation,(1) with the joints staggered. As I needed to hold everything down, and the insulation will dent from point loads such as bony knees and storage box feet, I covered it with hardboard (2) and screwed it all down to the boarding (3).

I was very pleased that the installation went exactly as planned.

As I think I have already posted, I have used B&Q's cheapest ( £3 per big roll) insulation to top up the unboarded areas as far as is practical, and given the water tanks an additional wrap, having pulled out what insulation I can reach from under the them.

(1) Bought 32 sheets 1200 mm x 500 mm x 50 mm from B&Q when the price was low.

(2) Three full sheets cut across into 500 mm strips at B&Q - half the price of ready cut ;-)

(3) M6 x 120 mm woodscrews were on offer at CPC.

Chris

[Tabby]

Adding new end supports is more work, but it could be done. For decent sized joists thats fine, for 1.5x3 it would be far stronger to add them in parallel, glued and screwed, since 6" carries more than twice the weight before excessive deflection than 3".

Re strength, its rigidity that counts, if the structure flexes too far the plaster breaks up. Usable strength.

NT

[andrew]

I discussed this with our local BCO, I wanted to increase the insulation depth and allow for a possible future loft conversion. She was fairly adamant that the strength be provided by sistering an 8 by 2 to the current ceiling joist and through bolting.

With the guys at work producing many squarish offcuts of 30mm plasterboad faced celotex I'm currently using no more nails for a crazy paving effect on the bedroom ceiling :-), saves the company 80quid a tonne in skipaway.

AJH