External insulation and ground cold bridge

tails_naf

Hi,
We are considering using external insulation on an old 2 storey stone house. The walls are approx 600mm thick. The external insulation should bring the wall u value down to approx .28, but I'm concerned about the massive cold bridge that is the wall perimeter to the ground.

As this must be a common concern with external insulation, does anyone have any numbers or rules of thumb for how much heat would be lost to the ground via this cold bridge?

BryanF

Take Ewi to top of foundation.

tails_naf

BryanF wrote: »

Take Ewi to top of foundation.

Yes, the plan is to take the ewi as low as possible, but doesnt that still leave a cold bridge from the ground directly underneath the wall which will be directly connected to the house interior vertically. Is this a legitimate concern?

BryanF

tails_naf wrote: »

Yes, the plan is to take the ewi as low as possible, but doesnt that still leave a cold bridge from the ground directly underneath the wall which will be directly connected to the house interior vertically. Is this a legitimate concern?

Yes
But
Define as low as possible. What is the height from internal floor level to Top of ground level and ground to foundation?

Rule of thumb greater than 600mm but ideally 1m insulation overlap will reduce the thermal bypass

Calahonda52

Are u happy u have no damp issues in this old stone house before u ewi it as the ewi will make it a non breathing wall which if its lime based mortar between the stones may not be the right approach

how are you dealing with the cold from under the floor

Calahonda52

Are u happy u have no damp issues in this old stone house before u ewi it as the ewi will make it a non breathing wall which if its lime based mortar between the stones may not be the right approach

how are you dealing with the cold from under the floor

MicktheMan

Calahonda52 wrote: »

Are u happy u have no damp issues in this old stone house before u ewi it as the ewi will make it a non breathing wall which if its lime based mortar between the stones may not be the right approach

While a lot are, not all ewi systems are non-breathable.

Markcheese

MicktheMan wrote:

While a lot are, not all ewi systems are non-breathable.

What are the breathable systems based around . I heard cork boards mentioned -sounds pricey ..

MicktheMan

Markcheese wrote: »

What are the breathable systems based around . I heard cork boards mentioned -sounds pricey ..

wood fibre, rockwool

tails_naf

BryanF wrote: »

Yes
But
Define as low as possible. What is the height from internal floor level to Top of ground level and ground to foundation?

Rule of thumb greater than 600mm but ideally 1m insulation overlap will reduce the thermal bypass

Yes, I'd say we do have approx 600mm overlap - for 1m we'd have to go below the foundation which I assume is impossible/not done.

This still does not remove the cold bridge to the ground vertically/directly underneath the wall though - which is the main concern.

Assuming the wall is 600mm thick, then even if the internal floor level is 1m above this, there is a 1m x u-value of stone of 2.5.


I've attached a drawing :



Here you can see the significant cold bridge to the ground through the notional 'green line'. Assuming the wall is at 20C (same as the room), the u-value to the outside is ~0.28, as provided by the EWI. However, the part of the wall closest to the ground, will be at 20C, and will have 1m down to the 7C ground.

My calcs are: For every linear meter of the perimeter, the cross-sectional area to the ground is 1m x 0.6m = 0.6m2 at a u value of 2.5 and a temp difference of 13 = 0.6 x 2.5 x 13 = 19W per m

If we assume the room is say 5m x 5m - this means the perimeter is 20m, and 19x20 = ~400w are lost to the ground cold bridge.

For the same room, assuming a 2.5m high wall, the wall area is 20x2.5 = 50m2, and the heat loss to the exterior with a u-value of 0.28 (even at 0c outside) is: 50 * 0.28 * 20 = 280w.

This means the cold bridge to the ground is far worse in terms of heat loss than the entire wall, and in effect reduces the 'rated' u-value of the room from 0.28 to an effective 0.68

So my question is - is this a real issue with EWI - and if not, what should these calculations be?

tails_naf

MicktheMan wrote: »

wood fibre, rockwool

Yes, we've been quoted for EWI that is breathable, and I believe it is rockwool based.

tails_naf

Calahonda52 wrote: »

Are u happy u have no damp issues in this old stone house before u ewi it as the ewi will make it a non breathing wall which if its lime based mortar between the stones may not be the right approach

how are you dealing with the cold from under the floor

This house has no damp issues - we've knocked a few holes in the wall recently for new doors and its very dry, no sign of damp at all. Existing render (done in the 70s) does not appear to be breathable, but to be on the safe side, we're going to go with lime render inside and breathable EWI outside (pending feedback on this board if EWI really does/does-not have this serious cold-bridge issue)

The floor itself will be insulated - we're taking up the existing floor and laying new floor buildup with insulation and radon barrier.

BryanF

Where is the external ground level? Relative to top of foundation and internal floor level. Ground temp will remain relatively constant below the first foot or so.

tails_naf

BryanF wrote: »

Where is the external ground level? Relative to top of foundation and internal floor level. Ground temp will remain relatively constant below the first foot or so.

External ground level is approx 600mm below intrenal floor level, as indicated by the left hand side of the image. The constant temperature of the ground is part of the problem, we're always sinking heat into the ground which is at 7c, which has a differential temp of 13 to the room at 20c. So from my calc the ewi cannot help here and we loose most of the heat that way.

BryanF

tails_naf wrote: »

External ground level is approx 600mm below intrenal floor level, as indicated by the left hand side of the image. The constant temperature of the ground is part of the problem, we're always sinking heat into the ground which is at 7c, which has a differential temp of 13 to the room at 20c. So from my calc the ewi cannot help here and we loose most of the heat that way.

what height is it from GL to Top of foundations ?

tails_naf

BryanF wrote: »

what height is it from GL to Top of foundations ?

Approx 600mm, however I don't follow how the answer really affects the result of the calcs?

Asume the outside of the house is completely wrapped in ewi, so little heat loss to the outside. However, whatever the gl to foundation height is, the stone wall sits on the ground. The stone wall is at 20c and the ground is at 7c. Even with a full meter of stone between the 20c part and the 7c ground, the heat loss is immense, by my calculations.

So to be clear, assuming no heat loss though the floor, and no heat loss through the ewi, what would the heat loss be through the wall to the foundations be, given the wall and foundations are all stone, and 1m below floor level.

If the calcs are as I posted before, it would suggest ewi is not suitable for this kind of renovation.

BryanF

BryanF wrote: »

Take Ewi to top of foundation.

BryanF wrote: »

Rule of thumb greater than 600mm but ideally 1m insulation overlap will reduce the thermal bypass

BryanF wrote: »

Where is the external ground level? Relative to top of foundation and internal floor level. Ground temp will remain relatively constant below the first foot or so.

So, 1200mm from FFL (finished floor level) to TOF(top of foundation)

tails_naf wrote: »

Approx 600mm, however I don't follow how the answer really affects the result of the calcs?

Asume the outside of the house is completely wrapped in ewi, so little heat loss to the outside. However, whatever the gl to foundation height is, the stone wall sits on the ground. The stone wall is at 20c and the ground is at 7c. Even with a full meter of stone between the 20c part and the 7c ground, the heat loss is immense, by my calculations.

So to be clear, assuming no heat loss though the floor, and no heat loss through the ewi, what would the heat loss be through the wall to the foundations be, given the wall and foundations are all stone, and 1m below floor level.

If the calcs are as I posted before, it would suggest ewi is not suitable for this kind of renovation.

https://windows.lbl.gov/software/therm/therm.html

tails_naf

BryanF wrote: »

So, 1200mm from FFL (finished floor level) to TOF(top of foundation)


https://windows.lbl.gov/software/therm/therm.html

You suggest modelling this in a thermal analysis program? Fair enough, but seems a bit extreme given there is literally 1 material (stone) and I'm only talking about the heat loss vertically down through the foundation to ground. I would have imagined a standard u value calc would have done for a first order approximation.

I think whatever the number, it looks like it will be close to 19w per meter, which means ewi is not a good overall solution as it can't address about foundation heat loss.

air

Even allowing for the foundation heat loss external insulation is still likely to give you a better overall solution than the alternative which is internal dry lining.
Internal insulation leaves you with innumerable cold bridges, is limited in it's thickness due to the risk of interstitial condensation and reduces the usable size of your rooms.
Unfortunately there are compromises to be made when living in old buildings, in your case living with the floor heat loss is going to be one of those compromises.

Assuming that you are pouring new floors you could internally insulate the ground floor rooms perhaps and make the internal wall insulation the perimeter upstand for the new slab.

air

Even allowing for the foundation heat loss external insulation is still likely to give you a better overall solution than the alternative which is internal dry lining.
Internal insulation leaves you with innumerable cold bridges, is limited in it's thickness due to the risk of interstitial condensation and reduces the usable size of your rooms.
Unfortunately there are compromises to be made when living in old buildings, in your case living with the floor/wall/ground heat loss is going to be one of those compromises.

Assuming that you are pouring new floors you could internally insulate the ground floor rooms perhaps and make the internal wall insulation the perimeter upstand for the new slab.

aghbren

Looked into this a few years ago and one option to mitigate the cold bridge was to use Calsitherm for dry lining of the ground floor external walls. Its breathable so should be fine for dampness issues. If this is a very old house with no damp course i would be wary of damp and be thinking along the lines of french drains outside if necessary, although the floor level does seem to be well above the outside ground level. Also i think extruded polystyrene (XPS) if your insulating below ground with the ewi which would be preferable. There is specialist software to work out the exact extent of the cold bridge, 'therm' i think it could be, if someone could do this for you, maybe a cost benefit analysis could be done once you have actual figures. Apologies if this has been done already

duckcfc

IMO people are to picky about cold bridges, esp the one mentioned here. Insulate externally with the insulation and forget about this cold bridge. When you've heated the house, it'll be grand!