Hi all,
I was planning on flooring my loft but there seem to be a lot of cables crossing the beams. Obviously whoever rewired the house didn't really use the loft so just threw the cables wherever to get to their destination. IS it a difficult job to reroute these along joists etc. Was maybe going to get a spark into do it but no idea the complexity of th job....
Any advice appreciated.
Joe.
The right way to do this is to drill holes in the centers of the joists, and route all the cables through them. However, barring a rewire, this probably isn't possible.
The trivial way is to take some wood of the same width as the joist, and
1/2" thick or so, and apply over the existing joists, not covering where you want the cables. This can be bits of the flooring chipboard or whatever.The better way is to do this with 2" thick (or more - depending on the thickness of your existing beams) of the same width as your joists, the full width of the attic (removing a couple of tiles may make it easy to get these in).
Take these beams, notch the bottom of them where the cables are needed to pass through, then glue and screw to the existing beams. This gives extra space for insulation, and makes the beams much firmer.
If you just board over the existing joists you won't be able to accommodate the recommended depth of insulation. What I'm planning to do with mine is fit 8"x2" (8" side vertical) at right angles to the existing joists, 370mm apart. This conveniently allows the lighting electrics to be left as-is. There's enough slack to allow it to be moved aside where necessary. I'll then fit insulation, then put chipboard over it. That'll give me 11" of insulation (270mm is recommended, this is a little more) and all the storage I need.
If you want to re-wire, allow 1-3 weekends (depending on house size), a reel of 1mm sq T&E and maybe some junction boxes. It's not hard assuming you understand a) how a lighting circuit is set up, b) how your particular house has been done & c) are competent!
Mark
8 X 2??????????
Why are you cutting your loft space down, adding a load of extra weight, creating lots more work and generally making a complete hash of it? - why not just use 2X2? - all the weight is going onto the bedroom ceiling joists anyway, adding all this extra weight will cause cracking of the ceilings....your ideas WRT insulation aren't good neither, putting insulation over the cables is bad...if you do go ahead with the 8X2's, just leave the gap underneath as it is....if you want more insulation up there, just thread it under each cable prior to putting cross joists in.
To the OP: just use 2X2 or 3X2's at right angles to the existing joists, lay chipboard on it, enjoy.
Mark,
Is there a reason you're choosing 8" deep, aside from making your total depth 11"?? I'm kinda doing the same thing, but wasn't planning on doing it at right angles to the existing joists (which are 4" deep and only 1.5" wide :S). Are there advantages to strengthening rafters at right angles?? I was only going to add an extra 2" in height to cope with about 5 pipes from the CH system that run through mine. I guess I'm kind of worried about losing head room!! (I'm tall.. there's not a lot to start with!!) Now I'm thinking upping the insulation from 4" to 6" isn't really good enough!!
Regards, _____________________________ The Christmas Grimch
I suspect lack of payback calculations. Taking as am example my loft, it's got 6" of rockwool, with 6*2" beams at
24" centers, what's the average power leaving through the ceiling?Let's call the roof area 100m^2 of rockwool, and 8m^2 of wood. Assuming 10C difference, the rockwool loses around 2.6W/m^2, and the wood about 8W/m^2, or 260W + 64W.
(Average U value of 3.2)
Call it 320W.
My initial suggestion was to add 2" along the beams. This will drop the heating requirements by a third, to 200W, and will use around (assumign timber is 300 quid per cubic meter) 120 quid of timber, and 100 pounds of rockwool.
Putting the 2" beams crosswise is better than this, but not a whole lot, as the rockwool dominates the heat loss by a factor of 4:1, so if you put the second layer crosswise, you gain relatively little. I did not do the exact numbers, but they will be around 180W.
Adding another whopping 8" of rockwool and wood will cost around 800 quid, and save another 90W.
90W, assuming it's on-peak electricity used to heat it will cost about 25p/day to heat, and will payback in 10 years, neglecting interest.With interest, or with off-peak electricity, or gas heating, it will almost certainly not pay back ever.
No. It is almost always worse to do this, unless the only load up there ever will be sparse point loads.
If the loading is 'stuff spread at random', then adding a ton of wood up there which does not add strength is not going to help with ceiling cracking from the load of stuff up there. It will help with ceiling cracking from people moving around.
It adds no strength in the short direction, only spreads the load.
4" to 6" is a worthwhile upgrade. 6" to 8" is quite debatable, but probably worth it if it's very cheap.If you are doing the gluing + screwing thing, then for best results, support the joist you are doing in the middle, with a hoist, to push it upwards with a force of 50Kg or so, then work from the adjacent beam to attach the 2*2 - lots of screws and enough glue. This pretensions the beam, so that the new composite beam has a long way to sag before cracks appear.
A nice plane, to plane the top of the joists smooth is probably a good idea, as it will mean you need lots less glue, and get a better stronger joint.
Ian, I have been absolutely fascinated by this thread (we are hopefully moving to a 1920s house with poor loft insulation/ boarding, amongst other slight problems) and it is clear that you know what you are talking about.
Can I ask you how you work out these values of heat loss? Please don't think that I am trying to find fault but rather that I would like to do calculations like yours for our new property.
OK, I can google but if you can provide a trusted link or reference that would give me the necessary information to do the calculations myself that would be even better.
It seems that there is clearly a diminishing law of returns here, as in most things to do with physics - once we are in the new place it would be nice to put some numbers into the equation.
Thanks
Steve.
Ceiling joists in lofts are often only 100 mm deep, so you can't do that and comply with wiring regs. Where an unprotected cable passes thro' a drilled hole in a joist it must be at least 50 mm above the bottom (if there is a ceiling below) and below the top (if there is a floor above).
You're OK if the joists are 125 mm or more, or if the cable is protected in earthed steel conduit, or if you can use those protection plates that TLC sell:
Unfortunately, the hard way.
Unfortunately, I have not found a 'nice' solution that works for me - say javascript/java, pick your construction, specify depths, ...
I did it the hard way. google for BS EN ISO 6946:1997 examples
and that will turn up examples of how to calculate the U value, to the
1997 BS standard - which is slightly out of date, but nothing much has changed, and is fine for indicative purposes.Basically. Thermal conductances - measured in w/m/k give the number of watts that a cube one meter in dimension will pass, per kelvin (or centigrade) difference across the opposite faces.
For example, stone is about 1.2, rockwool and many other insulators (polystyrene, compressed straw, wool) about 0.04, kingspan 0.02, wood
0.15.If you have a tenth of a square meter, it passes a tenth as much heat. If the wall is a tenth of a square meter thick, then it passes 10 times the heat.
U value - the number of watts per square meter that a structural element will pass given a 1C difference.
For monolithic structures, this is just the conductance divided by the thickness (in meters) - for example 50mm of kingspan has a thermal conductance of .02 w/w/m / 0.05m = 0.4W/m^2/k
(Or 4 watts per meter square, at a temperature difference of 10C)
For stone, with a conductance of 1.2, you need 3 whole meters (1.2 / 3 =
0.4) to get a U value similarly.Stone sucks (heat).
Now, it gets more complex.
You can't simply add U values of multiple elements in a wall - as that would mean that adding another identical element onto the side of this one would double the heat loss, which is obviously wrong.
You've got to convert into R values, which are the inverse of U values (R value = 1/U value), then add these, then convert back to U values by the same manner.
This all works find for homogenous walls.
It gets complex when you consider plasterboard + lath, with insulation between.
A simple way is to consider these as two seperate structures - in the last post I made, I used the example of 100m^2 of rockwool, and 8m^2 of wood - as 1/12th of the area of my example ceiling was wood.
So, simply ((U value of rockwool) * rockwool area ) * ((U value of wood) * wood area)
-------------------------------------------------------------------- total area
= composite U value.
R values can be thought of as the area that will pass one watt, with a difference of 1C across the structure.
Hope this helps.
Thanks very much for the information and the web link Ian.
I have saved this and bookmarked the page for future use.
The explanation you give has helped me to understand the principle as well as the actual calculations.
I am now looking forward to spending some time (when I have some :-( ) putting some numbers in...
Cheers
Steve
No.
Correct. I have relied on the advice of the energy savings trust, which recommends 270mm:
I've already bought Knauf Space Blanket, the 200mm version, so payback calculations are somewhat academic as the cost (£300 for 60 rolls, or 108m sq) has been sunk. Its R-value is 4.50m sq. K/W and thermal conductivity is
0.044 W/MK.In your calculations you've made a number of assumptions. A 10 degree difference is probably relevant for Oct, Nov, Mar, Apr. 5 degrees Sept & May. 15 degrees Dec, Jan, Feb. Roughly.
Significantly more energy is required to keep a house warm in the coldest months, which is probably why the EST recommends 270mm and why your 10 degree assumption has led you to something different. However, without your workings it's hard to be sure.
You don't say whether you've allowed for the efficiency of the heating system in terms of the cost per kWh. A condensing boiler like mine can get up to about 90%, but it won't be at that level all the time. Shall we assume
75%? And that all 75% warms the house, i.e. that once the energy is transferred to the water it is then transferred to the house without waste? Any inefficiency will effectively increase the effective cost per kWh and therefore reduce the payback period.You can reduce the wood area you used significantly, as the whole point of fitting it at 90 degrees is to minimise the area where heat can flow through wood, which is a poor insulator.
Would you like to recalculate and show your workings?
£800? £300 for insulation plus about £70 per room area. I'll do up to 4 of these areas, so total spend will be about £580. However, I consider this to be £300 on insulation and £280 on providing storage. Yes, storage would be a little cheaper with less depth, but not hugely.Presumably invalid conclusions in my case due to wrong assumptions.
I've been worried about the weight, and considered spanning the structural walls and supporting the timbers on them rather than just the ceiling joists. This would be tricky, and I don't believe it is necessary as each existing joist can support 75kg at its centre without detectable flex - they feel very, very strong under foot. I'm therefore confident that I don't need to do this. I have also discussed it with my neighbour who's a structural engineer. Note that this is a 1950s house, so it's quite possible that the timber used will be stronger than today's size equivalent. Loads of people say that today's fast-grown softwoods aren't as good and they certainly rot faster.
It'll add to the load, but spread it very effectively, particularly with 8" joists! For logistical reasons, I'm doing above one room at a time, so can monitor the effect and any downsides, like cracks in the ceilings. I doubt I'll need the whole 100m sq for storage.
So the EST is wrong?
Makes sense in theory, but overkill unless you're going to turn it into a habitable space. I also doubt that 50kg of upwards tension would introduce more than a miniscule amount of flex, and hence would not make a difference once released after glueing & screwing. Have you tested this theory at all?
Double overkill! A really nice way to do it if you need the strength, but think about the time involved. How do you value your time, what's the opportunity cost (e.g. other jobs delayed) and what's the payback period? ;-)
Mark
The EST are frequently worng, when I started installing insulation about 16 years ago, they reccomended 80mm,then it went to 100mm then after a few years it went to 150mm, then 200mm, then 250mm now it's actually 300mm according to building regs and cross laid, IE 150mm between joists and 150mm crossways, covering the joists.
and another point to note - you don't need 200mm joists to fit in 200mm insulation - it has a very similar U-value if compressed down to 100mm.
Given that the insulant is the air trapped between the fibres, I find this a somewhat doubtful suggestion.
Me too, until I visited Owens Corning and saw 1 inch fibreboard being made from 300mm fibreglass and was told that the u-value was of 250mm fibreglass.....they are used in oven manufacture
There is a loss of 50mm there.
What he is doing is increasing the depth of the joist by laminating another joist on top by gluing screwing. Cutting out a small notch in the bottom of the top joist will not (in most cases) infringe the figures you gave as this hole will be in the centre of the new laminated beam.
The reason insulation is counter laid is to cover the joists, which creates a thermal bridge if not covered. The wood can amount to 10% of the loft floor area - significant.
Laminating the joists to create a thicker stronger joist is only necessary if the joist are not supported beneath. Even Paramount walls have batons floor to ceiling every 100mm which will support the joists of trusses above to some degree.
Counter fixing joists above the existing joists at 90 degrees, that are "well" screwed down, with chipboard over, which is "well" screwed down, creates a pretty firm floor and unlikely that the older joists under would sag - you create a sort of box girder out of the whole structure. The counter joists and the chipboard creates racking and tightens the floor right up. Best secure the ends of the counter joists into the gable if possible.
Yes, but a 25mm thick slab that had the same insulation value as 250mm of fibreglass would be a dream product for loft converters and the like. If it were so - I remain sceptical.
Owens Corning, who I used to work for have changed their name (or been bought out by) Knauff insulation has data sheets:
While their compressed slabs at 25mm have a value of 0.034
"Complies with the requirements of the new Building Regulations to acheive a U-value of 0.13 W/m2K"
I did say that the u-values were similar...I certainly wouldn't use 200mm joists in a case like this.
Those are the k values, the amount of heat W that will pass through a 1m thickness for a 1C/1K temperature difference.
If you look at the resistance values 200mm of loft insulation has a thermal resistance of 5.00, 25mm slabs 0.70/0.75
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.