Candidate wall structure for Tomar

Suppose also that I:

• wish to optimise building a house near Tomar, Santarem
• think the engineering approaches espoused by the passive house movement have merit
• think the definition of 'passive' from PHI takes things further than is justifiable
• have a copy of 'Passive Houses in South West Europe'

Tomar has a micro-climate that is somewhat nearer to Lisboa than Oporto.

By 'engineering approaches espoused by the passive house movement' I mean:

• pay attention to thermal bridging
• pay attention to air tightness
• pay attention to thermal mass
• pay attention to thermal gain

The question is - what is the most cost-effective way to achieve static and dynamic thermal performance that is in line with 'Passive Houses in South West Europe', without going totally OTT with a standard PHI approach?

In particular I:

• need a static U value of 0.3 or so
• need a good dynamic U value
• need decent decrement delay and decrement fraction
• prefer good air-tightness from the system rather than from super-careful execution

To be clear: I'm not proposing to try to gain PHI certification. That is an expensive process and will also force me to buy expensive certified hardware. Rather, I want to use the engineering approach and get the most bang for my buck. I do not necessarily aim to remove the heating system completely.

Here is a straw man approach for the basic structure:

• I like the Artebel 'Thermal Blocks' range, as a whole
• BCP20 column blocks as 'lost formwork'
• LTE20 lintel blocks as 'lost formwork'
• TermicoProEtics BTE20 block to match the lintel and column blocks
• internal plastered finish

Now I need to consider how to insulate and provide further airtightness.

There are two basic approaches:

• an ETICS/capoto system
• dual wall with insulation between the inner and outer wall leaves

In the case of a cavity wall, it seems that some houses are built without the benefit of wall ties, which seems odd to me. Sometimes it is because the framing crosses the cavity and ties them together, while forming a rather large thermal bridge.

The cavity approach can be further divided:

• pumped full-fill insulation that provides an airtight layer
• rockwool
• rigid foam boards

Use of boards does not seem to match well with the use of wall-ties. Possibly the wall leaves could be tied by gluing the boards to the inner leaf, and then gluing the outer leaf to the boards, providing that the boards are rigid (EPS150 or PU, or possibly Multipor).

Using a wall-tie system does require that the inner and outer leaves use blocks that are aligned - and remain so up the wall. This implies that the blocks have to have the same height, and start from the same base.

Bizarrely, thermal blocks are typically quoted as being 19cm high, while standard ceramic tijolo are quoted as being 20cm high. This is the case even for Preceram who make both types of brick, where the thermal block and standard brick both appear to be 189mm high despite the different nominal sizes.

In order to play safe with the Artebel system, I chose to base the outerleaf in a tied system on the Artebel 'Freebloco' which are 11cm deep.

These blocks are rather inexpensive - about €5 a square metre plus mortar and placement. Assume €10 a square metre.

Teplo BF-2 ties for a 100mm cavity are about £1.45 each, and we need 2.5 per square metre. Call it €5.00 a square metre installed.

I discount as common costs:

• the cost of building the inner wall with framing and internal plaster
• the cost of the final external render layer and painting if not coloured

That leaves four scenarios to consider.

• ETICS using EPS150 (EPS100 is only fractionally cheaper)
• tied cavity wall, fully filled with ISOCasa foam
• tied cavity wall, fully filled with rockwool,extra care on airtight facade
• glued cavity wall, fully filled with with EPS150

ETICS

For the ETICS I chose the Duroplak EPS 150 Isosfer from www.liztherm.pt. It has a conductivity of 0.034 and with tonge and groove it is €9.25 per square metre for 100mm depth. It is less than €100 per cubic metre. The material cost of ETICS also needs allowance for: - bonding layer - mechanical fixings - bonding layer - fibreglass mesh - support rails

This seems to be about a further €10 - but the fitting is labour intensive and seems quite variable with the labour costing €25-€35 per metre - sometimes more.

It seems reasonable to regard a 100mm EPS150 as costing about €50.

This is much thicker than the optimisation calculation I made suggests, but the contribution of the extra 50mm depth is only €5 or so: it seems reasonable to play safe.

Note: there is some suggestion that the denser boards are a bad idea, even though they are stronger and have a better thermal resistance, because they limit the vapour diffusion and may cause damp problems.

Tied Cavity Wall Structures

These structures use:

• outer wall leaf
• wall ties (assumed Teplo for 100mm, 2.5 per square metre)
• insulation

In the case of the rockwool I will add a further 'parge coat' on the external wall as an aid to airtightness, and allow €5 per square metre.

Rockwool is about the same price as EPS - €100 per cubic metre. The high density batts need some mechanical fixing, although not as much as an ETICS system; allow €5 per square metre.

ISOCasa blown foam is a little over twice as much - €220 per cubic metre. However, that effectively includes installation.

In practice, the costs for the two insulation systems are close for 100mm:

• €20 for high density rockwool, lambda 0.035
• €22 for ISOCasa, lambda 0.038

In both cases the outer leaf adds about €15.

While ISOCasa is clearly more expensive and slightly poorer performance:

• it probably speeds the construction process
• it is probably a more reliable airtightness solution
• it is probably more reliable in terms of performance around intrusions like windows

The total wall costs are:

• rockwool €35 (lambda 0.035)
• ISOCasa €37 (lambda 0.038)

Glued Cavity Wall Structure

In this case we:

• glue and mechaniscally fix the T&G Isodur EPS150 to the inner leaf
• use a thinner outer leaf
• glue the outer leaf to the EPS
• use the longest ETICS mechanical fixings that we can find (240mm at liztherm) to augment the glue

If we ensure that glue is applied over the T&G board edges as well as dabs in the board centres then we can make a good case for reasonable air tightness behaviour.

We can offset the glue and mechanical fixings against the costs of the wall ties.

Unlike a tie system, there is less pressure to match the size of the blocks on the inner and outer leaves, and we can use cheaper ceramic tojolo that are also slightly thinner and larger, such as the Prelis UNIKO, or merely thinner (and very, very cheap) such as the 7cm or 9cm blocks from Preceram - it's hard to argue against blocks that cost €2 a square metre.

Thus the wall costs are:

• outer leaf €12
• inner glue and fixings €5
• outer glue and fixings €10
• 100mm EPS150 €10

The total cost is thus €37 (lambda 0.034).

Comparisons

On these estimations, the ETICS system costs more - by about €15 per square metre. This is entirely a result of the labour, which needs more care to achieve a robust finish.

The performance is broadly similar for all the four wall structures.

The worst performer is the most expensive - the ETICS system. In its favour, it is thinner by 11cm, with a total thickness of about 33cm.

The best performer is actually the cheapest - the tied dual wall with rockwool. The wall thickness is 44cm for this scenarios. However, we have a very resilient outer leaf structure containing hollow columns, which we could conceivably fill.

The performance difference is modest:

• the static U value is 0.245 for ETICS, and 0.221 for the rockwool dual wall
• the dynamic U value is 0.045 for ETICS, and 0.028 for the rockwool dual wall
• the decrement delay is 10.8 hours for ETICS, and 14.7 hours for rockwool dual wall

The joker in the pack is the ISOCasa system:

• the static U value is 0.234
• the dynamic U value is 0.032
• the decrement delay is 14.3 hours

So - the performance is certainly good enough and the basic cost disadvantage is marginal. The question is whether the installation method can have additional benefits: - basic dual wall construction proceeds without insulation installation but both walls must progress together so that ties can be installed easily - filling the cavity from the top does not take long and should reliably insulate around intrusions or awkward corners - the additional speed may reduce costs related to scaffolding hire - while we need to match the block heights, it is less important to match the depth of blocks on the inner leaf to the depth of formwork, or for the outer face of the inner leaf to be flat or free of pipe or cable conduits.

The glued and fixed dual wall with EPS150 performs slightly worse, at the same cost:

• the static U value is 0.234
• the dynamic U value is 0.044
• the decrement delay is 12.0 hours

The outer tijolo - reduced to 7cm to allow better mechanical fixing through the insulation - is not as good a performer as the Freebloco.

However, there are other potential advantages in terms of speed and ease of construction: - we do not need to align the external and internal leaves - we do not need to build the two leaves at the same time: we can build the inner, then install the insulation, then build and fix the outer - it can be used with existing wall structures as a retro-fit