To insulate or not to insulate, under tiles that is the question!

Fash · 2006-09-12T13:22:51+00:00

Situation: Basement floor, 175mm reinforced concrete over hardcore onclay but ~2m underground. I want to put travertine tiles (12mm) overelectric underfloor heating for comfort in the winter and primaryheating in the summer when the rest of the house is warm anyway.The U-value calculator say's given the area and perimeter that U-valueexcluding floor coverings is 0.67. Manufacturers of insulated tilebacking board (marmox etc.) say that even with 6mm of insulation underthe heating elements I will feel the benefit of quicker warm-up andlower running costs. The U-value calculator says that with 6mm ofinsulation the overall U-value is ~0.63 i.e. not very different.Since the heat-loss from the room is largely unchanged I can't believethe argument about reduced running costs is true? Certainly a gain of0=2E04 in U-value doesn't justify the =A3400 cost of the insulation. Isthere something I'm missing?On warm-up time surely 12mm thick tiles will still heat up faster thanthe 175mm concrete so even in the warm-up phase the effect of the 6mminsulation will be pretty much nothing. Again am I missing something,apart from the desire to get the cost down?I accept that if I had 50mm of insulation it would make a difference,but this is a basement with finished headroom of 6'6" so I don't havespace to play with.Should I spend less money (since it's cheaper) on a bit more wallinsulation to keep the overall heat loss down?Advice welcomed.Fash

Sep 12, 2006 23 Replies

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Christian McArdle 20 years ago

Indeed.

What is the intended finished floor surface? If carpet or wood based, I wouldn't use UFH at all.

Certainly. Personally, if I really wanted UFH heating in there and there really was only a choice between 6mm and 0mm insulation, I would still go for the 6mm. Note that 10mm boards are the same price and the additional 4mm is worthwhile.

What you haven't considered is that if the insulation is adequete, then the wall temperature will be much higher. Provided the ventilation is up to spec, which seems to be very much the case with heat recovery ventilation mentioned, then the relative humidity at the vapour barrier will be much less than 100% and no condensation at all will occur. As the temperature drops through the insulation, what little air (with closed cell insulation) that gets through will be very low in absolute humidity due to the vapour barrier, so no interstitial condensation will occur there either.

Christian.

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Fash 20 years ago

Thanks again, I'm signing it off now. I would put the 6mm in if it weren't so damn expensive but 10mm is really pushing the headroom issue (partly because the guys who poured the floor were worried about not having enough concrete so ordered the extra 0.5m3 which meant the slab ended up about 10mm thicker than planned so I had less to play with than I thought. BCO is very understanding about stair headroom and stuff like that but I don't want to push him too hard. The problem with the condensation is that the ventilation is inside the room (for the occupants) and doesn't ventilate as well the space behind the insulation and in front of the membrane. Essentially there is likely to be a humidity gradient through the wall.

Anyway the arrangement of the membrane should ensure that condensation on that interface isn't an issue so I will be insulating between the studs for the dry-lining. Before anyone comments on bridging by the studs and using over stud insulation, I know, I know but I don't want to keep bringing the walls in. Anyway U-value for the wall construction even accounting for the bridging is pretty good (I'm going for ~0.3 although heat-loss calculation was based on 0.6). I'm really not going to notice this room on the heating bill.

Thankyou to everyone.

Fash

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Christian McArdle 20 years ago

Can this be sorted with a few airbricks?

The vapour barrier and insulation will prevent condensation coming from the inside.

U-values are not the only reason to avoid cold bridging. One of the main problems is that you will get localised cold lines, which will lead to condensation where the studs are. The condensation might not be excessive, but can often lead to obvious staining from moulds. Can you do the final "plasterboarding" with something like Kingspan K17/K18? This is most typically done using a 37.5m board (25mm insulation+ 12.5mm plasterboard). However, thinner boards are available and will make a significant difference to the cold bridging situation.

Christian.

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