Rockwool fibre insulation v Cellotex boards
- Thread starter Jupiter01
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It's twice as effective in straight lambda (insulative) value terms, but when it comes to the practical details of installation, celotex is a pain in the arse because of the difficulty of sealing it up to form a homogenous layer
When you take it on balance with the rest of the wool, the effect of the higher insulative value diminishes. Ultimately, it's something like "Compared to 300mm of wool, 200mm of wool plus 100mm of celotex might save you an extra tenner a year on your heating bills" - you have to then work out how long that saved tenner will take to pay you back. Throw in the time to fit, the umpteen cans of expanding foam, and crawling round on your knees in the loft risking falling through the ceiling, and it rapidly becomes a waste of time and money
For any insulation product well joined to its adjacent layer, it makes no difference whether you buy 2x100 or 1x200. If the jointing was crap, you'd be better off with the thicker one. The reason why youre advised to wear 3 thin jumpers when going for a hike rather than one thick one, is that you then have an element of control over how many layers you're wearing half way up the mountain, your body temperature being variable, to feel adequately warm. a thick jumper is either on or off and youre either boiling or freezing. This doesn't apply to insulating your loft because you don't nip up and roll out an extra blanket when the weatherman says it's going to be cold, or roll them back up in summer.
Thanks. That was a very insightful and helpful response.
A couple of follow-up's if I may please:
Should the longevity of the two options play a part in my decision?
Also, in most parts of the loft I can pile in 300mm of wool of I wanted as I wont be boarding it out but there is a central section which I do intend to board and it is the entire length of the loft and around 1.5metres wide. The ceiling joists are 70mm high and I was originally planning to add 30mm battens which would then provide space for 100mm Celotex. I am concerned that if I go with rockwool here too, it may be on the low side?
Thanks again. That was a very helpful response and I just want to make sure that I make the correct choice now that I will have all my ceiling boards down before I re-board and skim
A couple of follow-up's if I may please:
Should the longevity of the two options play a part in my decision?
Also, in most parts of the loft I can pile in 300mm of wool of I wanted as I wont be boarding it out but there is a central section which I do intend to board and it is the entire length of the loft and around 1.5metres wide. The ceiling joists are 70mm high and I was originally planning to add 30mm battens which would then provide space for 100mm Celotex. I am concerned that if I go with rockwool here too, it may be on the low side?
Thanks again. That was a very helpful response and I just want to make sure that I make the correct choice now that I will have all my ceiling boards down before I re-board and skim
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Longevity isn't an issue. Both will last a very long time. In all my years I can barely think of a loft that was insulated with pir foam. It just isn't done. Mineral wool is ideal for the job, does it well and is cheap. I really think the pir option is a non starter for all sorts of reasons. As for boarding out. My advice is top up with mineral. If you want a platform make it high enough to get the mineral underneath. I've just specified one 450mm high - i.e. 450mm over the top of the ceiling joists. This allowed 400mm of mineral to run under with a space above. If you plonk board directly on top of mineral - even battened off - you will get condensation on the underside. Bad idea.
Thanks @jeds.
Everything you have said makes perfect sense and applies in my situation except, I have reservations about the central section where I will be boarding out. This section is already at a crawling capacity. there simply isn't the room to have 300mm insulation here (I could have 400mm in other areas if I wanted to - not an issue). This would make it difficult to crawl too. This is why I was wondering whether I should go with 100mm PIR here. Hope that makes sense.
Everything you have said makes perfect sense and applies in my situation except, I have reservations about the central section where I will be boarding out. This section is already at a crawling capacity. there simply isn't the room to have 300mm insulation here (I could have 400mm in other areas if I wanted to - not an issue). This would make it difficult to crawl too. This is why I was wondering whether I should go with 100mm PIR here. Hope that makes sense.
ok, that might be the bit I missed. I wouldn't try to fit pir boards between joists. An option might be to start with mineral between joists and then lay continuous pir boards over the mineral. Will these board need to take traffic - i.e. crawling over? How much space do you actually have?
Hi John,
Yes the boards in the centre will need to take the weight of someone crawling across. I would estimate the current height from the top of the joist to the highest point of the truss at 1.5m. The area I am looking to board is around 1.5 metres wide too (central section of picture inserted below). Note that there is a triangle which means it's restricted further on this sides.
If I packed the existing 70mm joist with 100mm rockwool and then put a PIR board over the top of the joists, how would the chipboard go on and would it be safe to crawl across, etc.?
Yes the boards in the centre will need to take the weight of someone crawling across. I would estimate the current height from the top of the joist to the highest point of the truss at 1.5m. The area I am looking to board is around 1.5 metres wide too (central section of picture inserted below). Note that there is a triangle which means it's restricted further on this sides.
If I packed the existing 70mm joist with 100mm rockwool and then put a PIR board over the top of the joists, how would the chipboard go on and would it be safe to crawl across, etc.?
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Celotex GA4000 claims a compressive strength of > 140 kPa.
Say your mass is 100 kg. Weight on earth = 1000 N.
Say the boards distribute that weight over a circle of diameter 500 mm. But that circle is supported only where it touches a joist. So that could be an area of 500 mm x 50 mm = 25000 mm^2 = 0.025 m^2.
So the pressure on the bottom of the celotex would be 1000 / 0.025 = 40000 Pa.
That gives you a safety margin of 140 / 40, which seems reasonable to me.
Say your mass is 100 kg. Weight on earth = 1000 N.
Say the boards distribute that weight over a circle of diameter 500 mm. But that circle is supported only where it touches a joist. So that could be an area of 500 mm x 50 mm = 25000 mm^2 = 0.025 m^2.
So the pressure on the bottom of the celotex would be 1000 / 0.025 = 40000 Pa.
That gives you a safety margin of 140 / 40, which seems reasonable to me.
My suggestions: Look at loft legs to raise the floor. Easier than adding wood beams and less weight added to roof trusses. Look at rockwool RWA45. This is a sound insulating product, 45kg/m3 (anything over 10kg/m3 is said to be sound insulating so this is one of the better ones). It's also in batts so should be a tight fit between the trusses (gaps will allow sneak sound paths of course).
You should also work the numbers out for heat loss and there a lot of tosh said about loft insulation. For example, let's say you heat a room 4x4m, to 20C, for 180 days of the year and outside is 10C average during that time. U value with no insulation and a roof open to wind is 4.9 so that's Loss = 16x4.9x10x24x180, 3387 kWh per year. Add 50mm on insulation, U=0.8, loss = 3387x0.8/4.9, 553 kWh. Add another 50mm (now 100mm), U=0.45, loss = 3387x0.45/4.9, 311 kWh. Add another 100mm (now 200mm), U=2.2, loss = 3387x0.22/4.9, 152 kWh.
As you can see the returns are reducing. Add 50mm to a bare loft saved a massive 2834 kWh, but the next 50mm only saved 222 kWh (i.e. material cost is doubled, 50mm to 100mm). And then from 100mm to 200mm the saving is 159 kWh (4 x the 50mm material costs).
If you work out the numbers I doubt the energy cost savings would pay back the cost of celotex in your lifetime.
You should also work the numbers out for heat loss and there a lot of tosh said about loft insulation. For example, let's say you heat a room 4x4m, to 20C, for 180 days of the year and outside is 10C average during that time. U value with no insulation and a roof open to wind is 4.9 so that's Loss = 16x4.9x10x24x180, 3387 kWh per year. Add 50mm on insulation, U=0.8, loss = 3387x0.8/4.9, 553 kWh. Add another 50mm (now 100mm), U=0.45, loss = 3387x0.45/4.9, 311 kWh. Add another 100mm (now 200mm), U=2.2, loss = 3387x0.22/4.9, 152 kWh.
As you can see the returns are reducing. Add 50mm to a bare loft saved a massive 2834 kWh, but the next 50mm only saved 222 kWh (i.e. material cost is doubled, 50mm to 100mm). And then from 100mm to 200mm the saving is 159 kWh (4 x the 50mm material costs).
If you work out the numbers I doubt the energy cost savings would pay back the cost of celotex in your lifetime.
Hi, I've got one large sheet of Celotex and could get one more to finish this off. This piece will go to waste otherwise. I am inclined to use regular rockwool between the joists and then place celotex over the joists if it can take weight when we walk on it (after the floor boards are placed on top of it)
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