Passive Precast Solutions

What type of construction is best for passive houses.

Is it traditional masonry or timber frame or precast or a hybrid solution?

The answer is all are suitable but are all sustainable to an industry that will come under increasing pressure to produce better quality homes at cheaper prices.

What is a Passive house trying to achieve.

Let's focus on what a house using passive standards is trying to do.

Firstly it is trying to maximise solar gain to get free energy from the sun.

It also needs to store that energy using thermal mass elements.

That's fundamental as the greater the solar gain the more free energy you get which reduces your energy load.

It needs to retain heat.

The retention of heat is also fundamental as any heat produced in the house must be conserved in order to reduce the heating load.

The use of suitable insulation without thermal bridges is fundamental to the success of a passive home.

It must be airtight to ensure that no heat is carried away in draughts or air leaks.

The air tightness is the glue that binds the others together.All the gains from solar and insulation would be lost by poor air tightness.

Finally the house needs to recycle it's air.

To do this it must conserve and save as much heat as possible by heat recovery.

The efficiency of the heat recovery system is vital to reduce the heat load.

All the above criteria is designed purely to save heat but the added benefits are also vital and are as a consequence of the quality of insulation air tightness solar gain and heat recovery.

Beside the energy savings the first and most obvious benefit is the total comfort in the house.

This is caused by the air tightness which leads to a steady temperature throughout the house.

One of the major problems with standard houses is the amount of air which flows through the house.

The minimum standard in Ireland

is 10m3/(h.m2)(2007 building regulations)

reducing to 7m3/(m2.h) to meet 2008 building regulations.

So a bungalow 350m3 envelope area is allowed 3500m3m3 air flow per hour.

The new regulation will reduce this to 2450m3 air flow per hour.

Now a passive air tightness standard of 0.6 at n50 results in an air flow of about 0.75m3/hour.

The air flow for the above house will be 262.5m3 per hour.

Air flow results of 0.2 at n50 will reduce air leakage to 87.5m3/h.

The difference is startling.

If you have a house that meets the 2007 standard is is 92.5% less efficient than the passive standard.

it is 97.5% less efficient than a passive house meeting the 0.2 ac at the passive standard.

Air tightness is more than a steady house temperature it also eliminates draughts so no more cold feet on cold windy nights.Sound is air borne and the level of air tightness is directly correlated to noise reduction.

So an air tight house is very quiet.

In urban areas where noise pollution is sometimes a problem it is of great benefit to have increase sound reduction in your home.

The second benefit to passive homes is air quality.

As the house is replenishing its air supply in a controlled way the house gets a continuous supply of fresh air with reduced pollen or air borne mites etc.

This leads to a much healthier environment with reduced allergies etc.

The other major benefit is energy reduction.

With a passive house the need for heating energy is eliminated.

The house will operate in a very small temperature rangeand only on really cold days without sun will the house need heat which can be supplied via an electrical coil inthe heat recovery system.

So whats the best way to construct a passive house?

In any Passive house the Foundation floors and roofs are easily adaptable to meet passive standards.The focus in this page are the wall elements. They are the most complicated issue in determining the passive standard.

We will look at the various standard methods of construction and examine each.

Masonry

The use of masonry is as old as construction itself.House walls can be single skin or cavity walls.

Both can be adaptable to meet passive standards with careful design and workmanship.

If we look at a cavity wall masonry house the industry standard is a 300mm cavity with 100mm walls to the inner and outer skins with a cavity of 100mm which is usually filled or partially filled with insulation.

To achieve the Passive standard will increase the cavity to 500mm.

The installation of the insulation is of major concern in passive houses with the wall U value of crucial importance.Here the issue of installation of insulation is a major concern in building a cavity wall.

The question of who and how the insulation can be installed needs careful consideration.

A rigid board may give greater U values but would require either a perfect cavity width.

If the cavity insulation is installed incorrectly it may cause thermal looping with significant heat losses

The use of rock wools are less expensive but pose their own problems.

The thermal conductivity of rock wool is seriously affected by water ingress and may in fact become a thermal bridge if soaked.

If rock wool is used the covering of the insulation is vital to ensure it performs properly.

Rock wool will suffer from thermal looping if the laps are poor.Pumped or sprayed insulation's do not generally have the thermal conductivity values to meet the U values required.

They do however increase air tightness in the cavity area.

looking at the problem it is hard to reconcile the traditional cavity with Passive standards.

The cavity wall system can be successful if care is taken in the way the wall is build and how the insulation is installed.

One proposal is to built the cavity wall about 20mm larger the insulation thickness.The inner wall to a set height of say 2m.

The wall is propped to ensure it is stable.

Then the insulation is fixed to the inside face of the wall using low conductivity wall ties.

When the insulation is fixed the outer wall is completed.

This method allows for certainty in the insulation installation but will be slower than traditional cavity wall construction and care must be taken not to disturb the inner wall.

Eliminating thermal bridging in the block work will be necessary to meet the passive standard.

The junctions of windows,floors and walls need careful examination.

The biggest problem is the roof wall junction.

The traditional closing of the cavity is not an acceptable detail and the continuity of insulation from the wall to the attic space must be achieved to meet passive standards.

The traditional detail of closing the cavity with a block and to use that block for the wall plate must be replaced with a different way of resting the roof trusses on the block work.

Now this is achieved by altering the roof trusses and resting the trusses on the inner wall.

This is a tricky operation with block work as the inner wall gets no support from the outer leaf.

The whole roofing operation must be carefully handled to ensure that the walls are not disturbed.The trusses need to be strapped to the inner wall correctly to ensure proper connection to the roof.

The windows and doors need to be carefully placed in the cavity to minimise thermal bridging.

The use of thermal software to model the heat flows will be crucial as will the correct installation of the windows.

Concrete cills are not conducive to Passive houses.

The floor connection to the walls is vital as the insulation needs to penetrate below the ground to minimise the thermal bridging.

This is the easiest detail but ensuring the cavity below ground level is stable and able to take footpath loads is very important.

Air leakage is another problem with cavity wall construction.

Blocks are not airtight and neither is mortar and at any rate the amount of joints in a block work wall is very large and prone to failure.

Floor supports IE joists are very prone to air leaks. Great care must be taken to ensure air tightness at joists.

The use of joist hangers is useful but may not be sufficient to meet passive standards.Before joists are constructed a clear plan of action to minimise and eliminate air tightness needs to be put into place.

The design of the flooring supports may minimise the placement of joists in external walls or the use of block walls internally to eliminate joists to external walls.

Air tightness at windows and doors is vital.

In larger cavity construction the opes should be thought through carefully and as mentioned previously modelled for heat flows.

The use of liners such as 20mm plywood to accept the window or door frame can be used and this will give a good foundation for using tapes to seal the window and door opes.

Before any we plastering takes place the house should be checked to ensure that the windows and floors are taped properly.

The Plastering should be placed in one coat and extended to floors and ceilings.

Normally internal plastering is about 20mm thick and this thickness of coating is required.

From above the use of cavity walls are traditional and can be adapted to meet passive standards.

Excellent detailing and management on site as well as exceptional workmanship will be required.

The use of traditional build will be OK for one off housing but will be very difficult to maintain the standards for large passive developments.

Single skin walls with external insulation is another method of constructing passive walls.The use of external insulation plastering systems is common in new and refurb work.

Usually a 225mm block is built and an insulation of varying thicknesses are mechanically stuck to the wall which is rendered with high impact renders.

The system is successful as the insulation is attached after the wall is build giving greater certainty to the wall U value.

Windows and doors are placed to eliminate thermal bridging.Air tightness is achieved by the same wet plastering method as the cavity wall block work system.

The system is simpler than the cavity wall but is not without problems.

In particular the impact values although within standards do not inspire confidence over the lifetime of the house.The cost of this form of construction is expensive and if problems do occur it may be difficult to resolve.

The major problem is the correct installation of the insulation. If poorly fitted it will give rise to thermal bridging which will drastically reduce U values.

Typical timber Frame walls are another common way of constructing Passive walls.

Timber frame walls are used with brick finishes, block finishes, or composite finishes.

Timber frame construction is factory made systems which are constructed on site.

The success of the timber frame system for passive houses is dependent on the installation.

Insulation and air tightness.

One of the problems with timber frame construction is the fact that there is repeating thermal bridging in the wall construction.

To achieve the Passive U values the timber walls are insulated with additional insulation to the inner side or to the outer side of the timber frame.

The other problem is the interstitial movement of moisture which can be harmful to the timber.

The use of vapor barriers is essential to insure that the movement of water is prevented.

The use of timber frame will necessitate an airtight membrane which are expensive and difficult but not impossible to maintain.

Finally the lightness of the timber frame structure prevents the use of solar gain in the house.

Timber Frame wall insulation is a critical element and must be installed to prevent creep which would dramatically reduce the wall U values.

SIP or structurally Integrated Panels are another form of timber frame system which is the use of two plywood sheets with an insulated panel between.

Together they form a structural element which is used in timber frame construction.

The SIP method is better than traditional timber frames as it eliminates to a large extend the thermal bridge problem.

The available U values are not sufficient to meet Passive wall standards.

SIPs can be used with SIP roofing giving a thermal free enclosure.

The roof values are not sufficient to meet Passive values.When using SIP panels an additional insulation to inside the house or to the cavity side is necessary.

SIP panel construction is expensive but the U values are measurable.

Precast Concrete housing.

In my opinion this is the only viable option in meeting Passive standards on a large scale.

The history of concrete homes is checkered to say the least with precast housing rarely used in Ireland and the UK.

The use of no fines houses Construction companies in the fifties and sixty's and the problems with high rise precast structures as well as poor detailing and know how has tarnished the use of precast.

The use of precast in Japan USA and in Germany is very prevalent with no obvious problems.

However the need to standardize details and maintain measurable standards will force builders and developer to use precast concrete housing.

The advantages of precast housing is simple.

1. With autocad and modern casting accuracy can be to millimetres.

2. The concrete strength and uniformity is set to very fine limits

3. Erection times are in days

4. The wall panels can be formed to take plastering internally and externally or can be finished on site with a brick facing.

5. All the electrical points can be bedded in the walls.

6. Joints can be formed to be airtight.

7. Precast can be cavity or single skin

8. Precast internal walls can be used to maximise solar gain.

9. Precast walls are fireproof pest proof and termite proof.

10. Precast is impervious to driven rain.

11. Precast concrete will not rot.

12. Concrete lasts.

The use of either cavity wall precast options or single wall options are dependent on the end user but cavity wall installations will mirror the traditional industry as well as allowing the insulation to be sandwiched between two concrete walls.

The problem with air tightness in concrete walls is eliminated as the walls are air tight and all joints are treatable and known in advance.

The internal flooring can be in timber or in precast either without affecting thermal bridging.

In most cases two storey houses can be constructed with full height walls eliminating joints to first floors.

Overall the use of precast elements is essential to get repeatable results for developers and house builders alike.

The use of either timber frame or precast concrete will be decided by a range of issues such as flexibility price design ease of use and customer perception.

The traditional methods of wall construction will involve levels of workmanship which will not be repeatable to meet stringent quality demands.

Contact Passive Homes Ltd for a free consulation