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Floors and ceilings

for floor finishes see Internal Linings

Green priorities

LCA of materials

With timber floors and ceilings make sure all materials are woodmarked. Consider using engineered joists (such as Masonite) as these use less material and leak less heat. Plasterboard is showing up as something of a problem for eventual disposal .

Embodied energy

Particularly with floors there are two likely areas where high embodied energy  may be a problem

  • with the use of concrete floors which use large amounts of energy in the manufacture of cement (but see comments below about thermal mass). Try to use GGBS (Ground Granulated Blast Furnace Slag) cement in a ratio of 50/50 with OPC. This reduces the embodied energy very considerably.
  • with the carpeting of floors. A study in the US, where people change their carpets on average every 7 years showed that the amount of energy to make the acrylic and nylon for the carpets accounted for the highest single category of embodied energy use in the house.

Insulation values

Ceilings in roofs

continuous roof insulation The uppermost ceiling is the point where you can really increase the thickness of the insulation. Unlike wall insulation it does not reduce the volume of the house. You simply build slightly higher. It is the area where the warm air in the house rises to and therefore the place which is most effective to insulate. It also helps to prevent overheating in roof areas in summer.

In many situations, particularly older houses, there is no need to contain the insulation in any way – simply lay it there. (but remember about not causing existing electric wiring to overheat by being surrounded by insulation). It may be that by having higher insulation values in the roof you can allow more window area elsewhere. This depends upon the SAP calculations . How much insulation to have? Well at least 200mm but consider 300mm (or 400mm if you are thinking about Passivhaus standards).

Ground floors

There are two kinds of ground floors in terms of insulation:- those that rest on the ground and those that are suspended. The latter should be considered as similar to walls because air flows beneath them and they need similar levels of insulation, at least 200mm but more likely 300mm if you are considering Passivhaus standards. (air tightness is also very important).

Floors which rest on the ground are quite interesting because building technologists are still not quite sure what to make of them and research is rather limited and inconclusive. There is one thing certain which is that perimeter walls around such ground floors need insulating to prevent heat going downwards and then escaping outwards. What is not certain is what happens to heat that goes downwards from the centre of a building, because although earth is a poor insulator there is an almost infinite amount of it (infinitely thick insulation – which gets warmer the deeper you get). In fact with a building which has a large floor area compared to its perimeter there is an argument that the ground beneath it could usefully act as a heat store to stabilize temperatures within the building and that putting insulation into the floor would have a detrimental effect. In practical terms, given that houses generally have relatively small footprint areas compared with their perimeter walls then it seems sensible to use a layer of insulation beneath the floor which connects with an extension of the vertical wall insulation going down into the ground. However, opinion on this matter may change in the future in favour of having no horizontal floor insulation but extending the wall insulation much deeper to create a large thermal store.

Floor insulation which is in contact with damp ground needs to be inert and to be able to withstand the pressures of the structure above it. The high density polystyrene and polyurethane flooring insulations are ideal for this although they are high in embodied energy. Timber I beams are much better insulators than the traditional timber joist. Over the last decade or so timber I beams have gained popularity because of the way that engineered timber can create more efficient structural members. They use finger jointed timber flanges and OSB webs which make them lighter, stronger and more efficient in timber use. They generally do not need strutting and the slenderness of the web means there is much less thermal bridging.

The thermal mass of floors

Although the use of concrete in floor construction represents high embodied energy there may be an argument for it, especially with houses where the bedrooms are on the ground floor and living rooms on the first. Part of the Passivhaus principle is the capture of solar energy through glazing (either normal windows or a type of conservatory area). This warmth tends to rise and is easier to capture in a dense thermal store at first floor level than into the ground floor. In other words the living room, dining room, kitchen floors store the heat and the bedrooms below are cooler. Also the acoustic insulation of the concrete becomes beneficial to those sleeping below. See more on Passive solar design

Structural

fscFrom a green standpoint the main thing to remember is that timber should be woodmarked and sourced as locally as possible, preferably in the UK.

Structural timber has the advantage over most other structural materials of being renewable and also a relatively good thermal insulation material.

Structural timber is usually of the following types

  • normal softwood sections such as floor and ceiling joists as specified in the Building Regulations. See Approved Documents Part A – Structure
  • larger sections of softwood or hardwood forming beams, trusses etc. and usually calculated by a structural engineer.
  • laminated structural timber
  • engineered timber such as I beams.

The normal softwood sections are usually graded as C16 or C24 (respectively replacing SC3 and SC4) and this will be stated in the approved building regulation drawings. It is often available from UK plantations

Larger sections of softwood are also available from the UK and it is often possible to get them from local sawmills.

The larger hardwood sections may be more of a problem to source sustainably and in the case of tropical hardwoods, only the woodmark can be relied upon. (However it is seldom that tropical hardwoods are used structurally in housing). There is a considerable amount of timber produced sustainably in the UK which is not woodmarked mainly because it is produced in such small quantities that the certification procedure would not be warranted. E.g. there is a constant supply of hedgerow ash (although this might be threatened by the recent outbreak of Chalara) and to a lesser extent oak and other species which is not woodmarked but which gets replaced. There are also organisations such as Woodlots which may be of help in sourcing local timber.

Laminated timber (sometimes known as Glulam) is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and Lamisell

masonite-I-beams-150x111

Engineered timber such as Masonite I beams represents a huge step forward in timber technology. Compared with traditional beams and joists Masonite sections are, for the same structural strength, much lighter, much more dimensionally regular and use considerably less timber. They can span up to about 7m. The dimensional stability with traditional beams and joists can be a major problem if they are not supplied at a moisture content of 12%, as shrinkage can cause considerable movement. It is not unusual to hear of 20mm movement over a two storey timber structure in the first year . They also help with the insulation because the webs, being much thinner, cause minimal thermal bridging. They do however run at about twice the price of solid timber sections with merchants such as Arnold Laver quoting around £4.80/m for 220 x 38 I beams. (supplied in 12m lengths).

An interesting example of ground floors using engineered timber is in an AECB article on low energy houses at the Greenoak developments.

easi-joist by Wolf SystemsHybrid joists. Another engineered product is the joist with metal webs and timber flanges such as Ecojoist made by Gang-Nail or Easi-joist by Wolf Systems. The main advantage here is the ability to thread quite large service runs through the metal web not only near the ends (as is the case with traditional joists and timber I beams) but anywhere along the length.

Plywood. Ply is now graded by whether it is structural or not. See the page on plywood. Ply, usually 19mm thick, will need supporting with noggins where the ends of boards meet. Consider using OSB instead of ply for the reasons listed here.

European Standards

On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BS standard. These are –

  • BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1 General – Common rules and rules for buildings
  • BS EN 1995-1-2 Eurocode 5: Design of timber structures Part 1.2 General – Structural fire design

TRADA have published span tables in a new softback book called Eurocode 5 Span Tables: For Solid Timber Members in Floors, Ceilings and Roofs for Dwellings and various companies do online calculation software. However for practical purposes the self builder will still find the span tables in the old (archived) Approved Documents from 1992 to be useful in determining sizes for floor, ceiling and roof joists, binders, rafters and purlins. There is extremely little difference between the old span tables and the new ones. The slight discrepancy is mainly in spans shorter that 2.4m.

Radon

The Building Regulations require that radon gas cannot enter a building via the ground floor. > England and Wales. Scotland. N. Ireland. The BRE have pages on dealing with radon

Sound insulation

The acoustic insulation of floors is a mixed bag as far as green building techniques is concerned. The sound attenuation, measured in decibels, can basically be achieved in two ways, by mass or by isolation of the floor surface from the surrounding structure (or both). Each achieves a slightly different result – for instance a heavy concrete floor will absorb sounds such as speech and music but may still transmit impact sounds such as chairs being moved around. A timber floating floor, on the other hand may not transmit impact sounds so much but will not be so good with airborne sound.

Heavy, dense insulating floors

Heavy floors such as concrete beam and block can give good sound insulation and this can be beneficial in three ways (apart from simply sound insulation)

  • it can be part of the thermal mass of the building which helps with thermal stability. See Passivhaus standards and passive solar design
  • it can work well with underfloor heating by setting the heating pipes in a screed on the concrete floor
  • it can fit very well with the idea of Flexible design because if a house needs to be modified at some point to provide an upstairs flat then sound and fire insulation will be required by the building regulations and this can easily be achieved with a concrete floor.

However the heavyweight approach can have some drawbacks, depending on the design of the house.

  • concrete floors are high in embodied energy and difficult to recycle
  • they require heavy structural walls to support them which involves more embodied energy (or structural columns at the corners – which might not play well with thermal bridging). No good trying to use timber to support concrete floors. Heavy masonry walls cause extra thickness to a wall which may already be very thick due to insulation. The exception to all this might be when there is a party wall between semis or in a terrace. In that case there is already a heavyweight wall which is effectively internal and can support heavy floors.
  • the high thermal mass approach doesn’t fit well with intermittent heating

Lightweight floating floors

If you want to provide acoustic insulation with a lightweight timber floor then you have to float the floor finish on the structure. This involves placing a layer of resilient material under the floor and around its edges so that it has no solid contact with either the floor joists or the walls or the skirting boards etc. There are various systems on the market.

Concrete floors

concrete floor

Although the embodied energy of concrete is high, especially if it contains reinforcing steel, there may be a strong argument for using it in floors when high thermal mass is a priority, for instance in conjunction with passive solar design or when acoustic separation or fire resistance is required. Also a concrete slab may be the best way of providing foundations on certain types of uneven ground.

If you use concrete at first floor level or above it does of course mean that you cannot use a timber structure for the walls. You need to use a material which is stronger in compression such as concrete or steel columns or masonry walls.

If you use concrete or steel columns there is a potential problem with cold bridging if the columns are within the thickness of the walls and also where the columns have contact with the ground. Columns will need externally cladding with insulation.

If the floor spans between load bearing walls then cavity wall insulation or external insulation will be needed to cover the blockwork and the edge of the floor.

The insulation of concrete ground floors in Radon areas, both new and retrofit, has been addressed quite neatly by Eco-slab who make an EPS product which creates voids beneath the insulation. These can be vented. See detail drawing.

3513002428Particularly with in-situ concrete it is possible to create complex floor shapes with long spans and with stair openings pretty much wherever you want them simply by placing more reinforcing steel where necessary. The drawback with casting in-situ concrete is the need for shuttering and the time it has to be left in place after the concrete is poured. Beam and block is more suited to regular right angled floor plans and is fast to install. Hollow beams can allow for running services as they can be drilled and cut in certain places. There is also the potential to use the hollow space to circulate air for heating and cooling which utilises the thermal mass of the concrete.

Underfloor heating is well suited to concrete floors. It can either be layed in the screed which covers the beam and block type floor or it can be cast directly into in-situ concrete. In both cases the pipes are well protected from future accidental damage. Concrete floors provide very good fire resistance and acoustic insulation (providing they have a surface which reduces noise from impact.

Self Levelling Concrete

Although concrete is a material which should be used as little as possible because of its extremely high embodied energy, there are times when there is little alternative. One of these is laying ground floors in old buildings which have no existing floor except earth, or a floor in very poor condition. At this point, self levelling concrete, which is a recent innovation, may be very attractive to the self builder. As its name implies, it is very easy to level, requiring no levelling boards, and no tamping. In most cases the sequence is as follows

  • Dig out the old earth and floor down to a suitable level leaving it clean and firm.
  • Lay a layer of high density polyurethane or polystyrene foam board insulation.
  • Lay a polythene DPM
  • Pour and spread the concrete (which comes as a ready mix). The spreading can be done with a large rake such as is used for raking tarmac. It is then levelled with a special board with two handles. (This is not particularly hard work)

self levelling 1

Above is a picture of a barn conversion where previously there had been various broken bits of concrete floor. The DPM has been laid on the sheets of insulation and the joints taped.

self levelling 2

The floor after pouring. No further screeding should be necessary.

At present this type of concrete seems to be available only from Lafarge in their Agilia range (or Gyvlon range. There are some interesting construction details on this web site, which include acoustic insulation values)

Timber floors

The traditional way of designing and building timber floors into masonry walls tends to suffer from lack of insulation and lack of draught proofing.

joists into blockwork

Especially as timber joists dry out and shrink, gaps appear round them. Although each gap is small (maybe say only a milimeter shrinkage for timber which starts out damp and finishes at say 12%) this would add up to something like 250 sq.mm. per joist and be a total of something in the region of 130 sq. cm. for a small two storey house. That’s a hole 11cm. x 11cm.! Almost like leaving a cat flap open all the time. You can get a lot of cold air in through that sort of gap over a freezing winter. A better way is to fix the joists to the walls using joist hangers, making sure that the perpends behind the joist ends are filled with mortar.

With the ground floor thermal bridging is decreased if you use timber I beam joists as their thinner web conducts less heat.

Another method is to build in an airtight strip, probably of polythene, (shown in red) so that it carries across the floor thickness. This is a method used in timber frame construction as well as masonry. For instance in Walter Segal timber frame construction a strip of polythene can be trapped between the posts and beams as they are being bolted together. This is then connected to the internal air barrier above and below the floor. It is important that a secure fixing is formed between the polythene and the wall lining at points A and B. It should be fixed with tape or an adhesive and then mechanically trapped.

joists onto timber frame

In the case of masonry, if you want to build the joists into an internal blockwork leaf, the polythene in the form of a long strip (red) can be taken up round the back of the leaf at the floor level and then be fixed to the blockwork with a strip of expanded metal (blue) which is plastered over to form an airtight connection. The polythene can be taken right round the room on external walls including walls that are parallel to joists. This has the double advantage of sealing the ends of the joists and also the unplastered joints of blockwork between joists.

sealed joist end

Building regulations

The Building Regulations Approved document part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer).

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety).

Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc.

The degree of sound insulation required within and between houses and flats is covered in Part E of the Approved Documents.

Combustion appliances and fuel storage has a detailed section in the Approved Documents (part J) and this makes reference to several aspects of the design and construction of houses including:

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