Base load
The energy that is needed constantly for the running of your house.
Breather membrane
This is a water-resistant barrier which sheds the water off the building, and prevents it passing through into the timber kit behind. A typical timber kit building clad in brick will have this on the outer face of the timber kit. Whilst preventing water passing through it, it allows moisture in the form of water vapour in air to pass through it (hence the term breather membrane).
This ensures that if the timber gets damp, it can pass back out through the membrane over time. This is not needed in block-work construction because the block can tolerate an element of moisture penetration as long as it is allowed to dry out over time. An air-tight membrane should not be sued in this location because it would lead to moisture build-up in the timber kit, and potentially rotting the structure. There are many alternatives available, but the one manufactured by Tyvek is Supro.
Snagging tip #1: This does not need to be sealed, but ensure that all upper sheets overlap lower sheets to shed water.
Build tight, Insulate right
A strategy for making sustainable buildings which focusses on reducing air leakage from buildings to a high level, and then providing mechanical ventilation to ensure that the indoor air quality is maintained at a high standard. This will often be a part of a fabric first strategy.
Damp proof course (DPC)
This is at the base of your walls which prevents the moisture from the ground rising up through the masonry of the building. It is usually a sheet of black plastic, located 150mm above ground level to prevent water which splashes off the ground from bypassing the DPC. In traditional buildings without a DPC, the mortar can be injected with a water repellent compound, but this will need replacing over time.
Damp proof membrane (DPM)
This is underneath your floor slab and prevents moisture rising up through the slab, and is generally a sheet of plastic.
Snagging tip #1: Make sure it is laid on a sand blinding to prevent it being damaged by the hardcore below.
Snagging tip #2: Make sure it is lapped with the DPC so that moisture cannot bypass the DPM and DPC to get into the slab.
External envelope
All the external parts of the building which meed to be insulated to prevent heat loss, i.e. the walls, floors and roof.
Fabric First
A strategy for making sustainable buildings which focusses on the “fabric” or the “external envelope” of a building rather than on renewable energy sources like solar panels or the like. This involves insulation, air-tightness and reducing thermal bridging.
Floating floor
This is essentially a floor which does not need to be nailed or glued to the floor below. It most often refers to a floor laid on timber battens or on a solid sheet of insulation. These are usually used for insulation, either as acoustic isolation to prevent the transfer of sound through solid elements like nails, or for thermal isolation by preventing heat transferring through solid elements like a timber floor structure.
Hygroscopic
The ability of a material to absorbs moisture. The desiccant included in a lot of packaging for electronics is hygroscopic, i.e. it absorbs moisture in the air to protect the product. This is also packed into the metal strip around the edge of a double-glazed window to absorb any moisture and prevent condensation. A non-hygroscopic material would be used in insulation beneath a concrete slab where it is going to be located beneath the DPM, and so needs to not be affected by water. EPS products are often used in this situation, such Kingspan Styrozone.
Mechanical Ventilation with Heat Recovery (MVHR)
A whole-house ventilation system which extracts air from the Kitchen and Bathrooms and supplies it to the Living and Bedrooms. The two air paths are passed very close to each other so that up to 95% of the heat is transferred from the outgoing to the incoming air.
Peak load
The maximum energy consumption of your house, e.g. when everyone is having a shower and making coffee for breakfast.
Raised floor
This is usually used to allow services like electrics or plumbing to be run through a space without affecting the acoustic or thermal insulation of the floor, or just to bring a floor up to a certain level, i.e. so that it ties in with another floor level etc.
SAP Calculations
SAP Calculations (Standard Assessment Procedures) are required for all new-build houses in Scotland and England. It quantifies the likely energy use per unit of floor area and CO2 emissions, and can be used to estimate annual heating costs. It is based on standardised assumptions for occupancy and behaviour, so cannot be relied on to be 100% accurate, but it is very useful to give a way to do a like-for-like comparison with other houses.
Every few years the government makes the SAP gradually more and more onerous to achieve. To achieve a pass for a new-build house requires that all the above are carefully considered, with U-values around 0.21 for walls, 0.11 for roofs and 0.16 for floors, as well as a good air-tightness level (around 5) as well as careful detailing of all thermal bridging. Any unusual details if you use non-standard forms of construction will probably need to have specialist Psi-value calculations to be carried out to ensure that heat loss has been adequately prevented.
Slapping
Knocking a hole through a wall to form an opening for a window or door. Why on earth it is called this, I don’t know!
Snotter
Lumps of mortar which drop into the cavity while laying a brick wall and sit on the wall ties. These breach the cavity and allow moisture to pass through the two leafs, so should be avoided as much as possible. If the brickwork has been done badly and there are a not of snotters, you can ask the Contractor to run chains through the cavity to remove them.
Thermal conductivity
The ability of heat to pass through a material. This is the K-value of a material, which is measured in W/mK and is called the “lambda” value of a material. This is useful because it allows us to compare the insulation properties of different materials. This can be combined with the thickness of the material to give you another value (R-value) which can be added to each other. Once all the R-values have been added, the inverse of this is the U-value. Not that you need to know all that, you just need to know that it allows you to compare the insulation properties of materials.
Steel: 50
Wood: 0.12
Mineral wool: 0.04 (Rockwool or Isover)
Blown cellulose: 0.035 (recycled newspaper)
EPS: 0.03-0.038 (normal polystyrene)
XPS: 0.029-0.039 (denser cell structured polystyrene)
Polyurethane: 0.022-0.028 (PUR is interchangable with PIR)
Polyisocyanurate: 0.022-0.028 (PIR – Celotex)
Phenolic foam: 0.021-0.024 (Kingspan)
Thermal mass
The ability of a material to retain heat, and therefore the time taken to heat up and cool down, i.e. concrete and blockwork have a high thermal mass and timber kit is low.
Parge coat
A cement layer on the inside of a blockwork wall which is not vapour permeable. This is often used in masonry construction to create an airtight barrier, to replace the VCL in a timber kit construction.
SAP calculation
The calculation which is required to show the heat loss for a building. It takes into account the insulation of all parts of the external envelope, the thermal bridging, air tightness, type of heating system, extent of glazed openings as well as a lot of other factors. The standard required to achieve a pass goes up each year. This is usually done by a suitably qualified SAP assessor rather than your architect or other consultants.
Solar gain
Energy gained from the sun, seen when the room gets hot on a sunny day even though it may be cold outside. South-facing windows gain more energy from the sun each year than they lose in heat loss.
Thermal bridge / cold bridge
A gap in the insulation envelope, where heat can pass out of the building without going through insulation, e.g. at windows where the reveals are not insulated, or where timber posts occur in timber frame.
Thermal mass
The ability of a material to absorb (and then release heat). E.g. a concrete slab has a high thermal mass because it takes a long time to heat up, but once warm it radiates it’s heat for a long time and takes a long time to cool down as well. A timber frame wall has a low thermal mass and so whilst it warms up quickly it also cools down more quickly. Neither property is required for an eco-house to function, but it’s impact on the rest of the system should be understood.
U-value
The ability of a form of construction to resist the passage of heat. The units are w/m2k and take account all of the materials and their thicknesses, and the lower the better. A typical insulated cavity wall may achieve 0.25 whereas a Passivhaus wall would achieve 0.10 and an un-insulated wall around 3.0.
Vapour control layer (VCL)
As the name suggests, it controls vapour within the wall. This prevents moisture passing through into the depth of your wall, from the inside. It is already protected from the outside by the breather membrane. Moisture from your house can enter the wall construction in the form of water vapour in warm, humid air, but then as it cools down in the wall construction, the water retaining capacity of the air reduces and so it condenses in the material, i.e. within the wall. This is not a good thing in timber kit construction because if it remains in the timber kit for any length of time it can damage the timbers.
This is prevented by installing a continuous, non-vapour permeable membrane, i.e. a VCL, across the inside face of your wall. These are not needed in masonry construction because they are more tolerant of some moisture ingress which will dry out over time without damaging the wall.
A vapour control layer can also be referred to as an air-tight layer, because as well as preventing moisture from passing through, it generally prevents any air from passing through at all, which allows it to also function as the air-tightness layer.
These are made by many different manufacturers and each looks different, and their appearance varied from basic plastic sheeting to a coloured or silver finish. The silver types give an enhanced u-value because they reflect the heat ack into the building. Tyvek manufacture several, including Airguard: Control (Standard) and Airguard:Reflective (Silver).
The silver foil finish on a lot of rigid insulation boards like Celotex also acts as a VCL and taping the joints with a foil tape avoid the need for a separate VCL.
Snagging tip # 1: Make sure that all joints between sheets and with windows and doors are taped.
Vapour permeability
The ability of water vapour to pass through a material. This is measured in a variety of ways, the G-value (MSs/g) takes into account thickness.
Sheet of plasterboard: 0.24
Breather membrane: 0.5
OSB: 3.6
Brickwork / Timber: 5
Vapour control layer: 1,000
Glass: 100,000
Foil-backed plasterboard: 100,000