I currently have 4" of 'loose' fibre glass insulation in a 1930's semi. I may add some extra joists at 90 degrees to add another 4" of insulation. As I understand it, its the air that gets trapped in the insulation that provides the insulating effect. Therefore, there's no point in buying the thickest pack of insulation around and squash it into 4", rather get one which is closest to the thickess I require. Should be cheaper (per m^2) and provide similar insulation to a thicker one squashed to 4". Or am I wrong?
I have just added another 4" of insulation, (roll type), and just laid it on top of the existing insulation, running on top of the joists, with the lengthwise joins actually on the joists. This way, I know where the joists are if I need to walk in the loft. Total cost about £30.00, for a two bedroomed semi.
Why are you going to lay another layer of joists ? Why not just roll your new insulation at 90 degrees across the top of the existing stuff that's already there ? The insulation doesn't need to be between joist to be effective, in fact the looser and thicker it is, the better it works. The fibre insulation is very cheap to buy on its own, so please don't go to expense of laying joists on top of joists just to be able to squeeze insulation between them. It's just not worth it.
I guess I assumed everyone would remember my other posts! I'm planning on boarding out the loft, and therefore need the extra joists to add height for additional insulation whilst hopefully providing a little extra strength.
Unless they're firmly supported by supporting walls at both ends, they'll simply add weight to the existing potentially overloaded joists - but not strength!
The only way to gain extra strength is to make the existing joist thicker in both surfaces. Adding joists on top of joist is only adding more weight to the joists underneath the new ones, so that is a definite no, no. The whole purpose of joists it take the load they are designed for, and because you've put thicker ones on top, doesn't mean you've made anything stronger. Try putting the ends of a matchstick across two points and pressing down on it until it snaps. Then try putting another matchstick at ninety degrees across the lower one and press down on them both where the meet. I think you'll find that lower matchstick still breaks at about the same pressure.
After reading your other post, I think you find that when you take away the joists that are already running at 90 degrees across the top of the joists and then lay your new boards at 90 degrees to the existing joists, you're actually doing the same job as the timber that is already there. Once you've boarded over the joist you should actually see an increase in the insulation properties of the ceiling because you've created a sealed unit with the existing fibre wool that's there. So you shouldn't really need to lay another layer of insulation on top.
If the area of the loft is boarded over right up to the edges with the new chipboard flooring systems that are currently on the market, then they to add some insulation properties to the ceiling because of their construction.
Try to make the job as simple and as cheap as you can. You won't gain anything hugely different from what you already have, unless you intend to increase the size of the existing joists and make them deep enough to able to take two layers of insulation between them. A loft area must be allowed to breath and move with different weather conditions, so the space must be ventilated properly to prevent moisture from gathering and then dripping through. So to seal the loft space to tight will mean you are actually going to create more problems for yourself in the future.
Try doing a web search for roofing construction and get some tips on what a roof and loft space actually do for a house. I think you'll be amazed at what you find out.
I was aware of that (hence I think I've generally said 'possibly at 90 degrees'...) - I did expect though that it would spread the load more. Whereas I could put a box down which may sit on a single joist, if the cross joists were screwed together, then that joist is not going to move down without bending the joist on top, which would also require the joist next to it to bend... which would spread the weight over two joists. The question is though is whether the boarding itself would do that anyway (though chipboard is possibly not great at resisting being bent compared to 4x2's) and whether the additional weight added by the joists would outweigh the benefit added by spreading the load....
So if I was boarding out 4" insulation, it may not be worth trying to add extra insulation (which would require adding extra timber to give the height, either cross or on top of existing joists) as it would seal the joists off.
I doubt I'll board it out right to the edges (its pitched on 3 sides, which limits access, plus these sides tend to be in the middle of joist spans, which would be weakest). At the edges then I can just lay extra insulation on top.
Air is the insulator used in loft insulation. The thicker it is the more air is trapped and the better insulation.
By boarding the loft, you automatically trap air under the boards, and provided that air doesn't circulate, I would think that there is no advantage in providing a thicker layer below the boards. In fact if you sealed the gap between ceiling and board completely, you wouldn't need any fibreglass, the air would act as the insulator as in double glazing.
The fibreglass you have there will impede the air circulating and so I would just go ahead and board over, paying attention to general loft ventilation, especially over the bathroom, as discussed.
Colin
PS Don't forget to box in any halogen downlighters if fitted
But access to the edges doesn't have to be any thicker than the thickness of the boards, because you'd slide the boards in to the edges of the space. Then just fix them down where it is most convenient to do so. If you go with a flooring grade chipboard system, then the tongue and groove around the edges of the boards will be enough to hold them together in the short distances you can't reach right under the eaves. So you'll still literally be able to board the loft right out to the edges and make the space between the ceiling and the new loft floor into a sealed unit with the existing insulation still between it.
The reasoning for not boarding right to the edges is two fold. The boarding is mainly for storage, and storage at the edges would be difficult to access (plus loading may be an issue when its in the middle of the spans - not boarding saves me thinking I can put things on it!). The edges form a significant proportion of the loft area, which means I could do it quite cheaply if I just boarded the area I would store in. Of course, if the area I'm boarding is a small proportion, then the insulation benefits from it wouldn't be so great.
Depending on the cost of the boarding, I may well board it all (or the vast majority).
The small packs I have already (2 packs of 3) say they're designed for 45cm spacings. As I've got 30cm gaps (which should work out to be 35cm centre spacing), these would probably have to have a proportion trimmed off of each, which would be wasted. I'm hoping to go for large boards and cut them to an appropriate size for my spacing. This should be cheaper than using pre-cut boards - shouldn't it? Any idea of the normal size of these boards, and their relevant costs (18mm thickness - unless I can get away with less as I'm using 35mm spacings rather than 45?)? I'll look at our new "The Timber Store" which has recently opened in Guildford - hopefully that'll be cheaper than B&Q etc.
I've got a jigsaw which I expect I could use to cut down these sheets (probably using a bar to aid as a straight edge). The other option I have is to invest in a circular saw - though I'd prefer to keep costs as low as possible. Do you think a jigsaw would be up to it?
Thanks for everyone's advice - its been very useful!
I've done a couple, one where the joist spacing matched the board length ( i.e. a fairly new property ) and another on an older ( approx. 35 yrs old ) property where the joist spacing was incompatible.
In both cases many boards needed cutting to fit round fittings, e.g. chimneys, water tanks etc.
I used a jigsaw for all the cuts and found it perfectly adequate to cope, but get a couple of *good* blades.
Another point worth mentioning is that cutting the boards with a jigsaw makes a large amount of dust. A solution is to mark them up a few at a time and then take them outside or into a workshop to cut, either that or get a good mask for protection and be prepared to suffer a berating from the misses for all the dust that blows out the hatch into the landing or bedroom underneath.
But if you use flooring grade blockboard, then the spacing in the joists doesn't matter because the tongue and groove style means they fit together anyway. As long as the tongue is in the groove of the other board next to it, then the boards will take the wait of someone walking on them. So all you're really cutting off is the end of a full row of boards. That's the beauty of this type flooring system.
I take it that flooring grade blockboard is not the same as flooring grade chipboard? From what I've read about the chipboard, the boards must join over a joist? This would require cutting 27cm off a 8' board. Is this actually the case with 18mm chipboard T&G flooring? If so, that would be great. The flooring does have T&G on the short edge, not just the long edge.
I've been following this thread with interest as I'm in much the same position as David. My house has 4x2 joists (not measured the spacing yet) and I'd like to board at least the centre section but probably a bit into the eaves as well. My original plan was the same as David's - Put cross joist across the existong one to give extra height to allow for more insulation and ventilation but I take your point about extra loading.
I'm a bit confused about your previous advice here though - If I lay 4 inches of insulation between the joists and then board over am I likely to run into condensation problems due to lack of ventilation to the insulation? Is it generally OK if I were to not board over the bathroom but to add extra insulation here instead?
I take your point about the boarding preventing air circulation and acting as insulation but how does this compare with more fibre? The current suggested minimum is 200mm (and I note that some people have gone upto 450mm - Surely there's a law of diminishing returns here!) The suggested solution would only have 100mm insulation, and boards?
And if you cut to the length of the spacing of the joists, how much of each board do you loose ? Quite a bit I'd imagine. So if you keep all the tongue and grooves together, no matter where the joint ends up, you be able to board out the whole loft, right up to edges, and it should all be strong enough for you to use as storage space.
When one board ends up with an overlap on one joist, then the other board is banged in against the edge of it, it will still have enough strength to take the weight of an average bloke, even if the joint ends up in the middle of the gap in the joists, because of the construction of the boards.
Try placing a board between a couple of bricks on the ground and then stand on it in the middle. You should find that the board will quite happily take your weight, not jumping up and down on it of course, but it should be able to bend to a hell of a lot before you can break it.
Sorry. Flooring grade blockboard and flooring grade chipboard are one in the same item.
There certainly is a law of diminishing returns. If you look at the U value of a roof with 100mm of insulation and work out the heat loss for the area of a typical house it is generally a great deal less than that which is lost through the walls and windows.
Doubling it to 200mm reduces that heat loss of course, but generally to the tune of tens of watts. One can argue that the insulating material is cheap enough so that the ROI on doing this is short but it is rather like changing a 100W bulb for a 40W bulb while all the time there is a 10kW heater running.
If the job can be done easily, then fine, but otherwise, if there is a bunch of cost to install it such as timber etc. and that is not otherwise useful, then going mad with roof insulation is not the most cost effective way to deal with energy cost reduction.
As Andy Hall says, there is not a huge difference in the effect of laying extra insulation when the same can be done by trapping the air between the existing material with boards on top. The insulation works by stopping air movement through the space between the boards and the ceiling, hence not allowing colder air to travel through and cool the space down. The trapped air is then heated by the very slow convection through from the room below and has to stay where it is.
So by increasing the thickness of the insulation your actually just slowing the air lose a very minuscule bit more than one layer does. But then your giving yourself more work to try and increase the height of the space you need before you can lay boards as a storage platform. Doing the math shows that it is really not worth that kind of expense to gain so very little more.
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