Hi JW,

I'm not an expert on this subject so hopefully somebody else will chime in, but I do know enough to say that yes there are cold bridging issues. Adding extra insulation underneath or on top of the joists is the obvious way to minimise it but you need to be very careful about possible condensation and rot so make sure to find some good details.

If you're using rigid insulation like XPS you also need to be careful not to leave gaps up the side of the joists that will let cold air bypass the insulation.

The contacts between the joists and the dwarf walls are basically point bridges, so they're not as bad as a linear bridge would be.

Hello, new here!
I'm just starting a renovation of an old cottage in a village in Fife, with internal floors and walls dating from the 40s.

I have read many of the previous threads on this, and have the same thought as the original poster.

I can take up my floorboards relatively easily at this stage and install some sort of insulation and membrane.

And I was thinking that a thin layer, say 10mm, of rigid board insulation (like that thin blue polystyrene stuff?), on top of the joists and the between-joist insulation, would help a lot to isolate the floorboards from the "cold" joists, and would also make the floors quieter to walk on.

So my question is - would it actually be worth doing, can you still get that sort of insulation (last seen when my dad was building our house in the 80s), and is it likely to cause any problems?

Thank you!

Hi Tony, thanks for your comments.

Just to clarify: I'm proposing 10mm on top of the joists *as well* as the usual 150mm of rockwool or similar between the joists. I thought 10mm would be better than none at all, which is the other option! Any more is going to end up making the floors too high. Just not sure if it's a complete waste of effort.

Loft insulation: there's some flat, tired old glassfibre in the loft, from the 70s, under boards. I'll be lifting the boards and putting in a lot more, between and over the beams - I like that "recycled glass fleece" they sell at B&Q, as it's not itchy like glassfibre. Not sure if it's as good though.

External wall insulation: this is tricky, the external walls are 1800s rubble wall to about 1m high, and then double-thickness brick above that. So on the outside, the wall is much wider at the base and then gets thinner. It looks like a nightmare to stick insulation to but I'd really welcome any suggestions.

Of course the internal lath/plaster walls need redoing anyway so I can put insulation on the inside instead or as well.

I should maybe have started a new thread :)

edit: I see you can get 12mm celotex for example.

OK, fair enough.

For the external wall insulation I have a couple of concerns:

I need to read up on this a bit more, but, these walls have no foundation or DPC and it's a cold roof. I'd be concerned about all the heat from the rooms going into the walls and simply radiating up into the loft and conducting down into the ground. Isn't that a problem?

Also, adding 200mm+ to the outside is going to make the wall extend beyond the roof edge and rain gutter! How is that dealt with normally? Should the windows be moved out into the insulation?

Is it really worth doing compared to internal wall insulation, I suppose I'm asking.

Thanks!

Occupancy; 2 people, both working, out during weekdays. Heating will be wood boiler / radiators.

It's quicker to warm up an internally-insulated space, right? But thermal mass is nice...

edit: I'm going to start a new thread rather than continue dragging this one off topic, sorry about that.

<When you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly?>

I had been thinking about the same and though Knauf Space Board might do for underneath
the joists, perhaps fixed with insulation retaining washers/fixings into bottom of the joists.

Posted By: evanWhen you say 50mm batts under the joists (I assume cross-ways) what type of product do you mean exactly? I was thinking of something fairly rigid, polystyrene board of some sort, which can be nailed up to support the material beneath the boards. Is that right?

Polystyrene is OK on top of or between the joists, but I wouldn't use it under the joists because it's not vapour open & there's a risk of trapping moisture in the joists. It's better to use one of the semi-rigid mineral wool batts mentioned by Tony below the joists. Take a look at Rockwool & Knauf.

David

Posted By: davidfreeboroughPolystyrene is OK on top of or between the joists, but I wouldn't use it under the joists because it's not vapour open

Doesn't that depend on the type of PS? I thought EPS is vapour permeable while XPS isn't?

Took a floor up and the joists are supported on 3 more joists, which have very little clearance to the soil. I'm not inclined to try to put insulation around those but will dig some more space under it.
Also should add some ventilation, there isn't any intentional holes I can see.
Suprisingly little rot except where the roof has let the rain in though.

Hey JW. I'm currently swayed by the suggestions made in this thread (thank you!) and am hopeful that the "batts" of insulation won't be all that difficult to secure to the joists, if the floor is up anyway.

I've yet to obtain one of these batts and see what it's actually like though...

I've been looking at this today.

How essential is the layer of plastic beneath the floorboards?

I ask because I've discovered that the floorboards in the main rooms are *very* well nailed down, and will be a lot of trouble to take up. They are tightly fitted tongue and groove, and would be varnished / sealed on top. Would that be air-tight enough?

There's just about enough room to crawl underneath.


On the same subject, I bought some of the 50mm glassfibre batts to have a look at them. They're a bit floppy but a few wire nails at an angle should hold them up OK. The problem with them is the price. £30 for 7 square metres!

170mm glasswool roll from Homebase is just £4 for the same area. And has a pretty similar W/mk value. So I'm highly tempted to use that and just figure out a way of holding it up.

Any thoughts?

The main reason for ventilating under timber floors is to dry the condensation which occurs on the bottom of the joists which is colder. If you insulate underneath the joists they become warmer and the condensation risk is reduced/eliminated. Then you ask the question, so why do we still need to ventilating under the floor? If we don't need to ventilate then why don't we fill up the void under the floors completly with EPS beads? improving airtightness at the same time. Maybe there is no dpc in the walls so the ventilation is to dry rising damp but a French drain around the house should sort that!

When you insulate under the joists the joist ends sitting in the wall can show a 8-10 degree lower temperature than the middle of the joists. The joists will assume a similar humidity/moisture level to the internal RH, this moisture moves towards the joist end by capillary and hygroscopic action and will condense as it gets colder, causing joist end rot if this situation persists for over 30 days. The drying mechanism which was the airflow under the joists is eliminated by the insulation under the joists, so this could be a problem!

Regarding your dwarf walls, which are usually to take out the bounce from the floor and are less structural. You could build these shorter and put a layer of EPS between them and the joists, just a thought!

So if I understand correctly, the main purpose of the plastic layer is to stop humidity from the room soaking through to the joists, where it will make its way to the end, condense and saturate them?

Again, doesn't a good layer of varnish stop this?

Anyway I may have thought of a way to get the boards up - a 6mm core drill used around each nail to lift the board off the nail without wrecking the tongue. Ordered the bit, not tried it yet...

Thanks Mike, interesting article and discussion.

The article is suggesting that a vapor barrier above the insulation can make matters worse.
So that sounds like another reason for leaving it out?

What about radon?

Thanks Mike.

In my case, under the floor isn't intentionally ventilated. But, it's dry and no signs of mould or rot on the timbers - 90 year old untreated timber.

There are no homes above the radon action level in this area according to UKRadon (from their sample).

I can get under there to add insulation, with some difficulty. I can take all the boards up with considerably more difficulty! I could add the plastic layer, I could even add vents.

Currently I'm inclined to put insulation up from the underside and hope for the best...