Insulation thickness between joists under ground floor

[shadyron]

Hi there!

My house is 1950s-ish and I've got a large void under the ground floor. At present, I've got 50mm of sheeps wool insulation between the joists here, but having just been reading up on this, I see that this is far from adequate. However, there seems to be no consensus on exactly how much I should have. I know that even with 50mm it's probably achieving a u-value of around 0.34 (ish). Would increasing this to 100mm or even 150mm bring a huge advantage?

Putting in the above has already has already given a big increase as regards the overall feeling of warmth in the house (and, indeed, the heating is staying off for longer, too!).

Any thoughts anyone? For info, it's this stuff I've been using: http://www.blackmountaininsulation.com/index.php/products/sheeps-wool-insulation-products - the tech info says that the U value is 0.78.
Note that I'm not trying to do something mad like bring my 1950s house up to modern construction standards, just trying to stop is leaking out heat like a sieve.

Cheers

 

[tony1851]

A good level of underfloor insulation nowadays is 100mm Kingspan or equivalent between the joists.

 

[mointainwalker]

I personally believe the real necessity of underfloor insulation in a suspended-floor is draught-proofing and once that is achieved actual insulation is less important due to the relatively low floor temp.

For multiple reasons i.e. cost, eae of fitting, tightness to joists ( cut oversize ) I would always favour fibre-wool underfloor. IMO increaring what you have will certainly not bring you a[/b] huge[/b] advantage, but if it's cheap enough then another 100 mm would be useful . I would have said 50 mm too, but as this is much less popular than 100mm, will not cost much less.

 

[shadyron]

I would have said 50 mm too, but as this is much less popular than 100mm, will not cost much less.

The pricing is totally linear, so the sqm cost is entirely the same (although doubles if you go for 100mm over 50mm but that's to be expected, if you see what I mean) so I'll probably just bung another 50mm all round - it'll probably be about 100 quid all in to (if that) to do the whole underside of the house.

Thanks so much.

 

[pinenot]

I'm with mountainwalker on this one, the added insulation will bring little in the way of cutting your heat loss.
As hot air rises and escapes, it is replaced by air (cold) as a means of balance or equilibrium if you will. This air comes through gaps in greater percentage than materials. 1950'ish built houses were built with unintentional gaps, this worked well because the fires needed to have a good feed of air in order to work.
So we come to plugging the gaps (air tightness) by the way this is the main constitute of passive house (passivhaus) design, where the building is pressure tested to achieve very good gap minimizing and very small quantity of air leakage. The achievement of this also achieves heat retention, which is where you have come in with your post.
Windows, doors and joints between walls and floors, are possibly the main culprits, http://www.superhomes.org.uk/ offers a wealth of information on this subject, but in the meantime if you can hire a thermal camera (not the cheapest route) you will get a pictorial record of all cold spots and leaks. The next best thing is a simple lit candle, which you pass around all windows and external doors. Take the candle to a door, working quite gently so as not to bend the flame, pass the upright candle about an inch from the door/frame clearance gap and watch for a flicker in the flame this is a draft. Ditto at windows, side lights, letterbox and any other orifice to the external. Gaps at floor/wall joints should be filled with an expanding/contracting filler and I've had good results with dry glazing tapes on the bottom of skirting boards also...pinenot

 

[shadyron]

I'm with mountainwalker on this one,

Thanks very much for that - very useful advice. I've already replaced the front door with a GRP door (replacing a single glazed metal job, which, frankly, urinated warm air like I had money to burn). Appreciate what you're saying about the unintentional gaps, though. To be fair, the house itself doesn't cost a great deal to run in terms of gas - we're paying around £1.70/day to keep it (it being a typical three bed semi) heated/heat the water which isn't too shabby, although I'd hope that'll fall pretty soon as I've just rectified a couple of issues with the heating too.
That said, though, I'm doing what I can to insulate where I can as, when it comes to sell, it makes the picture much nicer.

Cheers

 

[tony1851]

the added insulation will bring little in the way of cutting your heat loss.

If that's the case, how come Building Regulations look for a U-value of 0.22 for floors?
In most cases it is difficult to achieve that without insulation. Granted it is not as cost-effective as roof insulation inch-for-inch, but it does make a noticeable difference to both comfort and overall heat loss.

 

[freddiemercurystwin]

pinenot is a PM don't you know, say no more!