I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete).
The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about
600-800mm deep.
I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions?
remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood.
------------------- I think that rockwool isn't the best insulation to use in this situation.
I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation.
I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc.
There will be a vapour pressure from the living accommodation which is acting to force warm, moist air down via the permeable flooring. This is resulting in interstitial condensation and it's manifesting itself within the rockwool insulation.
This is exacerbated by a poor air circulation beneath the rockwool although even with a dramatic improvement in ventilation, problem 1 is likely to persist. (I don't understand or recommend your intended reduction in airspace)
I don't think (as NT says) that rockwool is the ideal insulant here, probably Kingspan or somnething similar would be more appropriate.
Even if you increase sub floor ventilation, you are still going to have the potential problem of the interstitial condensation and the only way you're likely to overcome this is by placing a vapour barrier (as your friend suggested) directly under the floor decking, between it and the rockwool. I guess this isn't really an option though (is it?)
You must ensure that any damp rockwool is either dried out or removed as quickly as possible. If your joists achieve and retain a moisture content in excess of 20 percent you could easily develop a fungal problem.
MY tuppenceworth would be to suggest you remove the rockwool and replace it with Kinspan or do away with it all together. Although its marginally 'helping' your fuel efficiency, its also having the detrimental affect of lowering the sub floor temperature and thus increasing the relative humidty. This will present you with an ongoing condensation problem...
That I would not dispute. Its a vpuour barrier initself. Celotex even better.
I menat to stop draights coming up through the cracks in the (rockwool) insulation.
i.e. from teh bootom up, a berathable membarne, to stop darughts, then rockwool, then a DPM and tehn borads over teh joitss.
Buit I like poolystrene better apart from teh fire hazrd. Celotex best of all, between the joists, taped down with the special tape and then boarded over.
============= I understand now what you meant. In fact if you don't close the cracks in the insulation (whatever it may be) then there isn't much point in doing the insulation at all.
====================== The Building Regs person I spoke to stated that the fire risk didn't bother him at all. I assume that he meant it isn't a great risk rather than it was me and not him at risk!
The Rockwool needs to be placed firmly against the bottom of the floor so that no moist air can get there and allow condensation inside the Rockwool. A vapour sheet above the Rockwool will also help.
However in any underfloor where insulation is added, it is often helpful to increase airflow by adding a humidity controlled extractor fan in front of one of the air bricks, and ensure there are adequate airbricks for fresh air to enter.
Another way of doing it would be to remove the rockwool, stuff it in a binliner and take it outside, then wrap/'bag' it in thick polythene tube or sheet stapled and sealed with tape, then replace it under the floorboards.
Would then be much nicer to work with under the floorboards if it is wrapped in this way.
the difference is that the underfloor void is constantly wet. If or when any damp gets past that barrier, as IRL it will, it is then trapped, and rot will occur. The floorboards will be nailed thru the barrier, lots of holes.
expect rot. Dryness and air movement are the 2 things that avoid it.
No real problem with nailing through the DPM. Its not like its a pond liner. Its there to reduce teh progress of damp air to the point where natural ventilation can deal with it.
Actually, its dryness pure and simple.
Acchieved by keeping teh rain out, and not allowing condensatuoin to build up.
==================== I wouldn't describe the underfloor void in my house as "..............constantly wet.......". If anything it appears very dry - dry enough to encourage mummification. The problem is that the underfloor is so well ventilated that it creates a very cold floor especially in cold windy weather. I appreciate that there are houses in low-lying ground which do have damp ground surfaces, but it isn't universally the case. I would be very surprised if the insulation I've installed leads to any form of rot.
I too was concerned about potential fire hazard when insulating under my wooden floor, so I got the manufacturer's technical dept number from wickes (was using standard wickes 50mm polystyrene insulation) and called them (can't remember the company name or number offhand, but they're a manufacturer based near Hull & I think they also make Jablite).
I wanted to know whether I should be using insulation containing fire retarders.
They reckon it isn't an issue for ground floors and that the standard stuff is perfectly safe & meets applicable regs.
Before doing something so drastic, I would make sure the underfloor ventilation is clear. If clear then add a few more air bricks and monitor. This is a far simpler, and less upheaval, than ripping out fitted insulation.
No need to compress it, just wrap it fairly loosely. It needs to be compressed a little when pushed between the joists for a snug fit, I'd expect the effect of that on in it's insulation ability is pretty small.
doesnt really solve the problem. Mould wont grow when the woods sometimes wet but soon dries off again, and has airflow, ie as original design. When you put rockwool up it stops the airflow, so instead you get a steady averaged RH, high enough for rot to grow, and no ventilation, again encouraging mould.
There is a cold area (sub-floor space) with a warm area (living space) above. The vapour pressure differential will be (as is almost always the case) such that the vapaour pressure in the living space is greater than that in the sub floor space and thus moistened air will move from the living space to the sub floor space. Sandwiched between these two areas is the Rockwool and naturally there will be a temperature gradient across it - from virtually room temperature at the upper level to outside temperature at the lower level.
Thus, as the warm air moves through the Rockwool it will cool and there will be a point within the rockwool where the dew point temperature is reached. At this point interstitial condensation will occur. (Don't forget that the sub floor space is now cooler than it was prior to the installation of the Rockwool and thus Dew Point is reached more often than before)
To cure this problem the moisture contained in the warm air from the living space must be prevented from being carried through into the sub floor space. The only feasible way to achieve this is to insert a vapour barrier. To me this seems like a very difficult thing to achieve although I suppose that painting the floor with an impermeable surface could conceivably work and, if you really want to leave the Rockwool in place, then this is what you need to try first.
Increasing the ventilation will probably not help the situation. It will make the sub floor space cooler if anything and thus will increase the temperature gradient across the rockwool. Its main effect will probably be to raise the dew point physically higher in the rockwool band (nearer the living area). The ventilation will have little if any effect on helping the moisture to evaporate harmlesslly.
This problem is much more than theoretical as far as the OP is concerned because if it persists (and as winter approaches it will probably get worse) then his floor joists could be in imminent danger of fungal attack.
It's one of those 'good ideas' that can have ramifications which are far worse than the problem you're trying to cure. It's lucky he noticed it, most people would have nailed the trap door down and forgotten all about it (until the floor fell in that is).
Either remove the Rockwool or place an effective vapour barrier between the two areas. Anything else is tinkering :))
Xav
p.s. I'm working on the assumption that his UF ventilation was adequate prior to the Rockwool. He doesn't seem to have had this problem before then...
that's a nice summary and explanation of this type of problem.
the most frightening thing though, as you note, is the number of people who may well have done something similar and now have no idea what is going on down below. While this issue is well recognised when insulating between rafters in lofts, it doesn't get the same exposure when applied to the floor , although it is essentially the same problem.
I've seen some plans for a couple of extensions that were drawn up with a similar set up and no mention of a vapour barrier as I recall. While I had intially apporached the architect who drew these up, to do my extension plans, he was ditched after a couple of meetings, and having given this matter a little more attention I may well have a quick word with some friends who used him for some work a while ago.
As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane.
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