100mm vs 150mm insulation UFH under slab.

[saveasteading]

8 minutes ago, Iceverge said:

Just plug the U values into a model of your house

That is my point really. Doubling the thickness is deemed to double the performance and I doubt that it does.

 

Not saying it doesn't all help, but will be diminishing returns.

 

The suppliers aren't about to test this theory, so the literature and models assume that the second 100mm is as effective as the first 100mm

 

 

[Iceverge]

It only depends on two variables. Unit price of energy and length of tenure. 

 

Here is a crude approximation with equal insulation roof, walls and floor. This is for our house using cheap insulation (0.037W/mK) in a mildish area in the South of Ireland. 

 

 

For most people building a futureproof house isolated from the worst energy price hikes to 20 years+ .   

 

 

Southern coastal areas 200mm is enough. 

 

Most of the British Isles 300mm is a solid bed. 

 

North of Scotland 400mm insulation is best. 

 

 

 

 

[Marvin]

Thanks for the info @Iceverge. It would be nice to have it broken down to cost per m2 so I can relate that to my home, and probably others would too. How many m2 was this over?

 

 

[Iceverge]

Approx 200m2 wall, 100m2 floor and 100m2 ceiling. 400m2 envelope excluding windows. 185m2 floor area. 

 

At the moment floor insulation is about £35/m2 for a U value of 0.1W/m2K, much the same for cavity wall batts and loft roll is only £12/m2 for a flat ceiling. 

 

There's almost no extra labour cost increasing flat ceiling insulation to 400mm ( 2x200 mm loft roll) so I think that is really a no brainer. Going from Bregs backstop to 0.11W/m2k will cost £5/m2. 

 

Similarly floor insulation is really only a material cost so I would use 300mm EPS (2 x 150mm) To go from Bregs to 0.12W/m2K at a cost £15/m2. 

 

Both are peanuts over the scale of a project for something that will last the lifetime of the building. Unlike fancy heating devices or renewables, insulation will pay itself off at some stage. Guaranteed. 

 

Wall and pitched ceilings are more expensive to think about as they come with extra costs in footings, wall ties, window and door openings, timber costs etc but aim for a U value of 0.2 at the maximum. 

 

As usual this is in addition to the usuals about airtightness, MVHR and 3G. 

 

 

[WWilts]

For those on very tight budgets and traditional block/brick construction
Even a few mm increase in cavity width produces a noticeable improvement in U value. Boosts EPC score by up to a point.

With blown in EPS beads increasing the cavity a little bit costs almost nothing more for the insulation.
Easy to tweak the masonry to gain a few mm. Another cost-effective approach.

[Nickfromwales]

9 hours ago, Iceverge said:

There's almost no extra labour cost increasing flat ceiling insulation to 400mm ( 2x200 mm loft roll)

The industry tends to agree that anything over 300mm of roll /wool insulation has more weight than it can support, so the bottom layers gets compacted. Compaction kills off the insulation value of wool so 300mm is the most I've every installed.

I also go with 600mm long x 200mm - 50mm thick PIR angled 'wedges' between joists / rafters at the perimeter ( where there are angled eaves ) to make the perimeter; a) more robust for the diminishing thickness, and b) to engineer the solution at the eaves so as to be 100% sure that there is zero contact between roof membrane / felt and the insulation where access is difficult / impossible. Very difficult to detail that with any discernible results with a wool so I gave up on that and went to this 'hybrid' solution a long time ago.

[OldSpot]

As @Nickfromwales suggested, if you put a 25mm EPS sheet down first and then your DPM on top you instantly reduce the likelyhood of getting holes in it. Then do what you like with PIR or EPS.

If you're installing UFH, the pipe clips struggle in EPS so having an EPS build up with a 25mm - 50mm PIR on top aswell really does help (alot)!

 

We've got 325mm (300mm EPS100 & 25mm PIR) and then a Cemex Cemfloor @ nominal 45mm which is great. With hindsight I would have gone with 50mm PIR for the reason above as 25mm is okay for the clips but a bit thicker would have been easier.

[Iceverge]

10 hours ago, Nickfromwales said:

The industry tends to agree that anything over 300mm

Didn't know that. Regularly see houses with 400mm specced here. 

 

10 hours ago, Nickfromwales said:

Very difficult to detail that with any discernible results

 

Another good reason to batten, counterbatten and ventilate above the breather membrane IMO. 

[saveasteading]

22 hours ago, Iceverge said:

It only depends on two variables

A very interesting table thanks.

 

looking at 20p/unit as an example it is shown that upgrading

from........... 100mm to 200 costs £4,000 and saves £ 920 per annum. An impressive return

BUT from ...500mm to 600 costs £4,000 and saves £ 48. per annum A payback of 80 years. 

 

In normal times you could invest (or not borrow) that cash at say 3% and earn £120.

Guessing what energy will cost in 50 years ....well I can't. Maybe it will be cheap or maybe humanity will be dressed in pelts.

 

Also, it is simplistic to only allow the cost of the insulation purchase. In the floor, the structure is being heightened by that amount. In the walls the lengths of the walls are being extended or the inner space reduced,. In the roof, 

11 hours ago, Nickfromwales said:

anything over 300mm of roll /wool insulation has more weight than it can support

this creates a limit, OR we apply additional structure to support the extra.

 

What this table does show is that the calculation assumes that the outer thickness of insulation is not doing anything like as much as the inner insulation.

Which is what I speculated (no more than a hunch) many  messages ago.

Do you know what the assumptions are in these calculations? They appear to be more realistic than the manufacturers' or Building regulations  imply/ assume.

 

What an interesting table though.

 

 

[Iceverge]

1 hour ago, saveasteading said:

Guessing what energy will cost in 50 years ....well I can't.

  Energy like food, will probably be cheap enough that average users on average wages will be just about able to afford it. 

 

1 hour ago, saveasteading said:

simplistic

  

It's an extremely simplistic comparison but I hope enough to get the idea. 

 

1 hour ago, saveasteading said:

this creates a limit, OR we apply additional structure to support the extra

I think the issue Nick is alluding to is the worse K value of insulation when compressed.