celotex insulation of floors and ceilings

[Stephen]

Hello,

I lifted a floorboard in the lounge and found that the joists are about 4.5 inches thick. I was wondering whether I should put celotex between the joists; I have read about people doing that on this group before.

Is it a case of getting the thickest you can, so in this case 100mm celotex?

I tried to use the calculator on the celotex web site. It said that

75mm celotex had a U-value of 0.28 W/m^2K and 100mm celotex had a U-value of 0.25 W/m^2K

Assuming the room is 4.2 m by 3.5 m, am I( right to think the heat loss if the temperature difference is 25C would be:

for 75mm celotex: 0.28 * 4.2 * 3.5 * 25 = 102.9W

for 100mm celotex: 0.25 * 4.2 * 3.5 * 25 = 91.9W

If I've done this right, there is only 10W difference, which I don't think would be noticeable. I still think I should get the thickest sheets that I can for maximum effect, but it would seem that *if* there is a big difference in price between the thicknesses, using the

75mm celotex would not make much difference to heat loss.

I realise that this would not be the total heat loss and that heat would be lost through the joists etc.

I used the temperature difference of 25C based on it being -5C outside and 20C inside. Is that a reasonable figure or should I use an external temperature of -20C for extreme winters, like we had a few years ago?

What I would really like to know is what would the heat loss be without the insulation. Then I could see what the saving would be and how worthwhile it would be. What is the U-value for air?

At the other end of the house, I have a loft conversion and looking through a hole in the plasterboard ceiling, I could not see any insulation. I think the conversion was done about 2006. Was there no requirement for insulation then?

I was thinking of taking the plasterboard ceiling down and insulating between the ceiling joists. This would leave all the structural woodwork above uninsulated to allow air to circulate around that but would keep the room below warm. Actually, it's the opposite effect I am after: the room gets hot in the day from the sun hitting the roof, so I thought that insulating would keep the room cooler.

Thanks, Stephen.

[Tim Watts]

Even 25mm will give an *big* gain over nothing, not just because of the insulation, but because you are sealing off the draughts.

Me - I'd go with 50mm. 100mm is excessive and 75mm as you note does not really gain much extra and it's harder to work with - bulkier, trickier to cut and fit etc.

Unless you are planning underfloor heating, then it does (as the floor is at 40-odd C so heat losses are magnified).

Be sure to check that there will be ventilation from outside *under* the celotex otherwise you risk rot in the timbers.

[Stephen]

Thanks for your fast reply.

I haven't noticed any draughts yet but perhaps I have not been here long enough to notice any. Perhaps the tongues seal any gaps between the floorboards and stop any? Of course, if I lift the floor to fit the insulation, I will have to cut the tongues.

When I lifted the floorboard, there was 18" or so of space beneath the floor* and I could feel a good draught down there, so hopefully there's nothing to worry about there.

• I remember thinking if the gap had been a bit bigger I could have crawled under there to wire. I thought it might be a bit too claustrophobic at the height it was.

[Tim Watts]

Sounds fine - just watch it when you get to the end of the joist run that there isn't an airbrick at joist height that you are about to block. If you up, just taper the insulation up and over it so the air flows below.

Good luck :)

Another easy way of fitting the celotex is to put it on top of the joists with one edge just lipped inside the next joist and run a saw down the other joist. This will give you a very slightly undersized fit which (assuming a few nails of screws to support it) can then have a thin bead of PU foam run down both sides, which ensures and excellent seal.

[John Rumm]

Probably not worth it if doing just between the joists, since you will be left with a cold bridge on the joist itself. Its one of those cases where "some" is a vast improvement on none, but then you get diminishing returns thereafter. Also keep in mind that losses through the floor may be dwarfed by losses elsewhere depending on the general level of insulation.

Yup, celotex has a K value of 0.025, so divide that by the thickness in meters to get the U value. Hence 0.025 / 0.1 = 0.25 W/m^2 K

The sum is basically correct, but its ignoring some important details:

1) Like the insulation would not be continuous - you would actually have a smaller area at the U value of the insulation chose, and then a second area with a U value of the joists alone.

If you can tolerate the rise in floor level, you could insulate between the joists, and also "over" them (i.e. between the top of the joist and the underside of the floor boards). Chances are that 50mm between joists, and 25mm over the whole area will out perform 100mm just between them.

2) Air changes result in heat loss as well, so eliminating those though the floor will make a big difference as well. 3) The temperature at the floor level will likely be quite a bit lower than the room average - so that reduces losses a bit anyway.

Say your joists are 125mm deep and 50mm wide. Lets say your joists run the 3.5m span of the room. On 400mm centres you will have 4.2 / 0.4 say

11 joists. That's 11 x 3.5 x 0.05 = 1.9 m^2 of timber. Allow an inch for floorboard thickness, that's a depth of say 150mm. Timber K value is 0.14, so that is 0.14 / 0.150 = 0.93

So your 100 mm celotex calc becomes:

for 100mm celotex:

0.25 * (4.2 * 3.5 - 1.9) * 25 = 80W

for timber:

0.93 * 1.9 * 25 = 44W

Total 124W (note I ignored the insulating effect of the floor boards on the insulated bits)

If we re-do with say 50mm between and 25 mm over we get an effective

75mm between joists (i.e. 50 + the 25), and 25 + the timber elsewhere.

So the U value for the floor area becomes 0.28, and for the timber + insulation 1/( 1/0.93 + 1/1 ) = 0.48

for 75mm celotex:

0.28 * (4.2 * 3.5 - 1.9) * 25 = 89.6W

for timber + insulation:

0.48 * 1.9 * 25 = 22W

or 112W total

So cheaper and easier to fit, also more effective.

Unless you are sizing a heating capacity to cope with worst case situations, its more relevant to use the average temperature while heating. So even picking 10 degrees for a heating season average would not be unrealistic. You will find conduction losses through the floor are likely dwarfed by air leaks.

You can compute the sums for just timber, but that is only part of the story. Much depends on the amount of air flow under the floor.

I had a problem with wood rot under the floors in some parts of the house when we moved in - that was down to condensation and lack of ventilation mostly. Adding more air bricks to get a cross flow of air made a massive difference to that, but also made it much colder at floor level in the winter. So I added dampers to the air bricks to allow the flow to be reduced in the winter.

In 2006 there would certainly have been fairly stringent requirements - they are slightly relaxed for the sloping parts of the roof in a loft conversion. In 2004, when I did a loft conversion I seem to recall that the minimum level of insulation on the rafters was 50mm between and 25mm under.

I did 50 between and 30 under in the end:

Yup, that could work. If the roof sarking is air permeable then you can almost fill the depth of the rafters. If its traditional underfelt, then you need to leave a 50mm gap between the back of the tiles/felt and the insulation (plus have ridge and soffit ventilation) to protect the timbers.

Alternatively you could slap insulation backed plaster board straight onto what you already have - you will lose a couple of inches of room space, but its a much easier job!

[GMM]

I'd agree with Tim: I had a floor replaced by a pro builder and he used

75mm celotex, as it conformed to regulations. Certainly, it made a huge difference to the room, but I suspect a great deal of that was due to draught exclusion by the celotex and the T&G boards he used to replace the old square-edged ones. 50mm would be easier and cheaper all round, and probably equally efective, but still rigid enough to stay in place, so I would give that a try if had another one to do (and stuff the regulations).

My celotex was cut to make a tight fit and laid onto roofing battens nailed along the inner faces of the joists to make a 1" ledge. Adjacent boards were taped with aluminium tape where they butted together. Seemed the simplest way to do it, but the lad cutting the foam boards had a great eye for it, and I would probably find myself needing a bit of expanding foam to fill any gaps.

[harry]

Be awate you can get "seconds" for your insulation. About half price, has very minor defects. Can be repaired with fixing foam if necesary.

Google "rigid ++insulation board +seconds + your area".

It's virtually impossible to fit this insulation accurately. Cut it about an inch undersize & fill the gaps with fixing foam. Cheapest fixing foam is from Screwfix I think.

There must be no gaps anywhere or the whole project is defeated.

[harry]

Best fitted from underneath if possible.

If you have to lift floorboards, chuck them away and fit flooring grade chipboard. Without tongues, floorboards are very weak. Also you will have big problems with skirting boards.

[robgraham]

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John - I'm just picking up on your comment about a damper on the airbricks, as I reckon that my underfloor is excessively vented. I would sorely like to do underfloor insulation but not at the expense of destroying the old f loor boards, so reducing the airflow in winter would be an option.

I've got grills rather than bricks - how did you do the damper and how are you sure that you haven't over-damped?

Rob

[Dave Liquorice]

Do you have bare boards or are they covered with carpet? The tongue and groove slows down a draught and if the boards are tight stop it but you still have that cold air moving against the underside of the boards taking heat away.

Only one, once you have got a board out you can remove all the fixings for the next board and slide it out. Removing the fixings will be the hard bit...

I'd do the sums for 50 mm. But agree 100 mm is on the wrong side of the diminishing returns unless it's the same price or cheaper than 75 mm.

Is that 18" from the underside of the floor boards or the bottom of the joists? If the latter I don't think it would be too bad, a lot would depend on the ground, rough and stoney, smooth packed earth, concrete?

I can't decide if fitting from below might be less hassle than lifting most/all of the floor. If the boards are fixed with square cut floor brads they ought to come out fairly easily, annular ring shanks or similar are another matter. At least with lifting the floor and snugly fitted insulation you can tape all the joints to keep the warm moist air of the room out of the now cold void.

Cutting. I'd try a couple of jigs to hold the insulation with one edge slightly overlapping the joist edge to account for a saws kerf. With a vertical bit to butt against the joist side to get quick and repeatable positioning. Have an arm across to the next joist to support the insulation and use the side of that joist to guide a hand saw. Probably mark a cut line along the top face as well, perhaps dead above the joist edge to make the cut block slightly wider at the top and thus a real snug fit. Turn the insulation over for the next cut.

This assumes that the joists are fairly straight but they don't have to be parallel.

[John Rumm]

I got some ali hit'n'miss sliding vents and fixed them over the air bricks. You can then slide the vent to throttle the total opening size.

The house originally had air bricks only on two (non opposing) sides, so there was little cross flow. I added 9 more in total.

Only by trial and error really - make a guess at setting them, then have a feel around under a floorboard later[1]. Prior to installing the extra air bricks you could find droplets of water just hanging off the underside of joists and floorboards in some places. I run the vents full open in the summer, and then mostly close those on the most windward sides of the building in the winter. It does not cut off the flow, but stops a gale blowing under the floor!

[1] You can also be a bit more scientific if you have a moisture meter, and can take readings from a number of test locations.

[Stephen]

Thanks for such a detailed reply.

[snip]

I think insulating on top of the floorboards would be impractical as it would involve having to trim the bottom of the door and would create a small step, which could cause trips.

I wondered whether you could insulate across the joists from underneath but I suppose that would prevent the air from circulating around them. I've never understood why joists under the ground floor and in the attic have to be ventilated but those between the ground floor and first floor do not.

[snip]

Thanks for the useful calculations. I will be calculating radiator sizes soon as I want to upgrade my old boiler for a new condensing one. I think there were two calculators recommended here: Myson and someone else. I'll have to download them.

[snip]

I'm sure I could see the rafters so they can't be insulated under. I was only viewing through a hole though, so when the time comes, I'll have to take down the ceiling and get on a ladder to have a closer look.

How would I know if the sarking was air permeable? There is no ridge and no soffit ventilation.

That's the kind of thing I was thinking, except I was going to insulate above the ceiling rather than just below it. I will start with insulating under the floor first and insulate the loft conversion in the near future.

I did also make a post about insulating walls but I cannot see it nor any replies. Has anyone else seen it or shall I repost?

Thanks, Stephen.

[Stephen]

I'm not sure I like the idea of chipboard; it would be too difficult to lift in the future. At least with floorboards, you can just lift one at the edge of the room without causing massive disruption.

These floorboards are 22mm thick. My old house I think there were only

16mm, so I hope these will be nice and strong?

Thanks, Stephen.

[Stephen]

At the moment bare boards but it may get carpet in the future.

I've only lifted one board at the edge of the room and it was just an estimate. Perhaps I'll lift a few more and have a go at crawling underneath! It was earth but not rocky or stoney as far as I could see.

I think cutting to get a n airtight fit will be the most important and most difficult part.

Thanks for all your help. Stephen.

[The Natural Philosopher]

because the internal ones are not exposed to cold that can cause condensation.

[The Natural Philosopher]

The trick with chip is to jigsaw a bit out with an angled bladed, then it wont fall through when you replace it and glue it back in. a gap filling glue tales care of the saw width.

[The Natural Philosopher]

what is important is getting a continuous vapour barrier and no places draughts can go. I used decorators caulk and expanding foam to make sure insulation is absolutely continuous and there are no gaps, and metal foil tape to ensure the vapour barrier is OK

[Dave Liquorice]

You won't get an airtight fit but certainly pretty snug with no or extremely low amount of free flow. I'd still tape across all the joins between joist and insulation and any joins in the insulation, to keep the warm wet air of the house out of the cold void where it would condense and be a damp problem.

[Stephen]

Thanks. The caulk sounds interesting; I might use that. I'm not fan of expanding foam because I seem to get it everywhere! I've been buying the applicator guns from tool station but I find they only last a year and then the insides get seized (foamed?) up and they have to be replaced. Perhaps it is infrequent use? At least a year is longer than I would get from a handheld can.

Thanks, Stephen.

[Stephen]

Will aluminium tape stick to the joist?

Thanks, Stephen.