Achieving thermal continuity and airtightness
Achieving thermal continuity and airtightness
(1) General construction objectives
In designing and building for low heat loss, both good insulation and control of air infiltration/exfiltration are needed. Good attention to detailing is necessary during installation for insulation to work effectively and to ensure unwanted air infiltration is eliminated as far as practicable. This translates into "thermal continuity of the insulation" and "airtightness of the building".
It is important that, as buildings become more airtight, adequate ventilation is maintained. Technical Guidance Document F provides guidance on purpose-provided ventilation for buildings with an air permeability of 5m3/(h.m2) at 50pa, or less. Where the intended design is greater than 3m3/h.m2
and the actual construction achieves a lower value, then appropriate additional measures should be implemented to ensure adequate ventilation.
HOW TO ACHIEVE THERMAL CONTINUITY - AND WHY
For thermal insulation to be effective, it needs to be continuous. This means no gaps between the insulation sheets or batts. It also means no way for cold air to circulate freely on the warm side of the insulation.
• Insulation boards with stepped rather than flat butt joints give better continuity.
• Cut cavity insulation to suit. Butt the sheets tightly to each other, as well as tight up against cavity closers.
• Install roof insulation over the top course of blocks at the eaves, prior to felting the roof, having brought the wall insulation up to the top of the external wall.
Use of an airtight membrane as the air barrier can provide improved airtightness performance. With internal dry lining, a vapour control layer to prevent interstitial condensation is particularly important. To ensure the durability of the airtightness strategy, it may be necessary to use a reinforcing mesh in junctions subject to shrinkage, settlement or other movement in advance of plastering. Other good practice notes
are provided on the diagrams in Part 2 for specific junctions.
HOW TO ACHIEVE AIR TIGHTNESS AND WHY
Airtightness means cutting out unwanted draughts. Draughts can be so slight as to be imperceptible, but even slight draughts increase heat loss, sometimes dramatically.
The way to good airtightness is a continuous air-resistant layer all around the inside of the building. This includes under and around the ground floor, across the external walls and under the roof, to seal the inside from the outside. With masonry walls - whether concrete block or concrete - this is most easily done by using a wet plaster finish. It can also be done by using dry-lining boards and by taking extra care to seal around all gaps, all perimeters, and at windows and external doors.
Points to watch:-
• Plaster between the joists at suspended timber floors
• Ensure there's no gap between the floor and the walls
• Where pipes or wires pass through the air barrier, use suitable tape or grommets to seal around them
• Seal around window and external door frames with suitable airtightness tapes and/or sealants
(2) Ground floors
Concrete ground-bearing floors: The insulation under the floor slab must be continuous. Continuous edge insulation should then be installed at the junction of the floor slab and the external wall.
Concrete suspended floors: The insulation is usually on top of the concrete and under a screed or a floor finish. As with ground-bearing slabs, this is easily installed and checked. Concrete suspended floors are often used on sloping sites. For this reason, there can be a significant amount of exposed external wall below the slab that should also be insulated.
Timber floors are, of their nature, suspended with a ventilated air space underneath. The potential for gaps in insulation between floor joists and a quilt is large. With well-built floors and for good
thermal performance, insulation needs to be continuous between, under or over the joists, or
a combination of these. Pay particular attention to potential gaps along joist edges.
Concrete ground-bearing floors: When properly installed, a concrete floor slab gives excellent airtightness. Appropriate tape or sealant should be provided behind the skirting to seal between the floor slab and the air barrier in the wall.
With timber frame external walls, it's essential to maintain air barrier continuity between the wall construction and the ground floor slab.
Concrete suspended floors- A well-cast concrete floor slab is airtight. As with a ground-bearing slab, use a suitable tape behind the skirting to seal the slab to the air barrier in the wall.
Timber floors- Timber floor boarding is prone to shrinkage over time, whereas a continuous sealed sheeted floor can provide an effective airtightness barrier.
(3) Masonry walls
The majority of dwellings in Ireland are built with masonry external walls. These walls may be cavity walls, with inner and outer leaves of blockwork, brickwork or concrete, with a cavity that is usually insulated, and frequently with additional insulation on the inner face. Alternatively, they may be built of single-skin masonry, frequently of hollow blockwork, precast concrete or insitu concrete, with insulation applied internally, sometimes externally, or sometimes both.
A well-built blockwork inner leaf with a coat of wet-finish plaster will, if properly applied and with proper detailing, deliver high levels of airtightness. Alternatively an airtightness membrane can be used. Dry-lining boards applied to the inside face can also provide airtightness. However, the issues surrounding continuity of thermal insulation and continuity of airtightness at openings, roofs and suspended floors vary widely between these wall types. The details in Part 2 show these issues in detail.
THERMAL CONTINUITY WITH SINGLE - SKIN MASONRY EXTERNAL WALLS
Internally applied insulation (insulated dry-lining) needs to be done carefully to achieve thermal continuity. Pay particular attention to gaps at the tops and bottoms of boards, at floors and ceilings and around opes.
Significant advantages of externally applied insulation are its ease of application and also of checking its continuity. Tightly-butted or lapped sheets deliver thermal continuity with little difficulty.
THERMAL CONTINUITY WITH CAVITY MASONRY EXTERNAL WALLS
Well-built cavity walls have clean cavities, with cavity insulation held firmly against the inner leaf. Insulation sheets with lapped or tongue-and-groove edges, can be butted tightly against each other and give reasonable thermal continuity.
• Clear all debris including mortar snots from cavity as work progresses to prevent thermal bridging between the inner and outer leaf.
• Cavity insulation boards should be carefully cut to suit and tightly butted to each other.
• Fix insulation tight to the outer face of the inner blockwork leaf, to prevent air circulation between the block and insulation reducing the performance of the insulation layer.
AIRTIGHTNESS WITH CAVITY MASONRY EXTERNAL WALLS
The simplest way to achieve good airtightness is to wet plaster the inner leaf. Dry lining with proper sealing of all perimeters and joints will also achieve reasonable levels of airtightness. The key areas to watch are junctions at opes, at floors, and at service penetrations - see sub headings 5-10 below.
(4) Timber and steel frame
THERMAL CONTINUITY WITH TIMBERAND STEEL FRAME EXTERNAL WALLS
External walls of timber frame usually have insulation fitted between the load-bearing studwork in an inner leaf, possibly with additional insulation applied to the inner face of the studs. I.S. 440 Timber frame construction, dwellings and other buildings provides guidance on the construction of timber frame structures.
Well-built timber frame walls have sole plates tight to the masonry underneath, with insulation fitting snugly into the space between each pair of studs and the sheathing board outside.
• Ensure the insulation is cut to fit snugly into the space between each pair of studs and the sheathing board outside.
• Fill the entire stud depth with insulation
• Where required, fit a second layer of insulation inside the timber studs
• An internal vapour control layer is generally required in this form of
External walls of steel frame usually have rigid insulation boards fitted outside the load bearing steel studwork of an inner leaf, possibly with additional insulation applied between the studs. With steel frame, some insulation must be placed outside the frame to provide a thermal break and avoid condensation.
Innovative construction systems should comply with Part D of the Building regulations and have independent third party certification. Works which involve systems, products, materials, techniques or equipment, for which published national standards do not yet exist, should have third party certification demonstrating compliance with Irish Building Regulations requirements.
Such certification may include, in part or in total, a European Technical Assessment or Agrément certification (e.g. NSAI Agrément) or equivalent.
• Warm frame construction is where all the insulation is outside the steel frame.
• Hybrid construction is where insulation is included both outside the steel structure and in between the steel components.
At least 33% of the total thermal resistance should be provided on the outside of the steel studs.
Where compressible insulation is installed between studs in addition to rigid board insulation, it should be tightly packed and be in direct contact with the rigid board. Take care to ensure compressible insulation is maintained above dew-point temperature.
With hybrid construction, the system manufacturer should provide a condensation risk analysis in accordance with Technical Guidance Document L, Appendix B to ensure there is no risk of interstitial condensation.
AIR TIGHTNESS WITH TIMBER AND STEEL FRAME EXTERNAL WALLS
Best results are achieved with the use of a membrane fitted on the warm side of the insulation and held in place with battens forming a services cavity between the membrane and the plasterboard finish.
In some installations the plasterboard provides the airtightness layer. In this case, care is required in sealing the plasterboard to intermediate floors, roof and ground floor, external wall opes and service penetrations. Butt joints between plasterboard edges must also be sealed and fully supported on studs or noggins, for example, suitable compressible tapes may be used behind joints to ensure an airtight seal.
(5) Intermediate floor junctions
THERMAL CONTINUITY WITH TIMBER INTERMEDIATE FLOORS
If the thermal insulation is in the cavity or is external type, thermal continuity at the junction of the intermediate floor and the outside wall happens almost of its own accord. So long as the cavity insulation is continuous across the intermediate floor zone, continuity is achieved.
If the insulation is on the inner face of the external wall, thermal continuity requires greater attention to detail. There is a potential cold bridge all along the zone of the suspended floor. Continue the wall insulation through the intermediate floor zone and seal any vapour control layer, where present, to the joist penetrations.
THERMAL CONTINUITY WITH CONCRETE INTERMEDIATE FLOORS
As with timber floors, if the thermal insulation is in the cavity or is the external type, thermal continuity at the junction of the intermediate floor and the outside wall is achieved readily. If the insulation is on the inner face of the external wall, thermal continuity is not possible.
AIRTIGHTNESS WITH INTERMEDIATE FLOORS
Airtightness at intermediate floors is a matter of extending the wall air barriers above and below the floor through the intermediate floor zone and taping up any penetrations of the air barrier by joist, joist hangers, beams, services etc. Where the intermediate floor is mass concrete this may form part of the airtight layer.
In timber floors, where joists are built into the inner leaf, airtightness is achieved by plastering the wall around the joists and taping the face of the joist to the plaster finish, see Figure 8.
Alternatively, proprietary airtight caps are available for building in. Where joist hangers are used, it is recommended that these be installed on a layer of airtight membrane which is plastered over.
With timber frame or with dry-lined masonry, carry the airtight membrane or plasterboards through the floor zone and tape around the joists.
Alternatively, an airtight membrane can be carried from the inside face of the inner leaf, around the joist ends and back inside above the floor and taped to the air barrier in the wall above and below the floor. Because of the risk of interstitial condensation, diffusion open membranes are recommended in this location.
Diagrams B18 and B19 of TGD L 2021 provide guidance for the insulation of exposed floors. The floor void should be enclosed to prevent warm air moving horizontally into any voids in the exposed floor space where moisture might condense by, for example, using full height noggins.
With concrete intermediate floors, when the floor spans onto the wall, pay attention to any gap under the slab, especially with precast concrete slabs. If a blockwork wall is built off the floor slab above, this will give an excellent basis for airtightness once the blockwork is plastered right down to the slab. In the case of concrete intermediate floors, which employ precast elements, care is required in sealing any joints in the precast components.
(6) Separating wall junctions
The concern at separating walls is the structural continuity, which is usual between the separating wall and the exterior wall. This can result in breaks both in thermal insulation and also in airtightness.
The issues which arise are similar to those with intermediate floors or with staircases. With a
masonry structure, insulation in a cavity, or exterior insulation, both deliver thermal
continuity. This is because the insulation runs uninterrupted either externally or in the cavity
and outside the junction of the walls.
With an internally insulated masonry structure, the insulated dry lining needs to be returned for at least 1 metre along the separating wall.
AIRTIGHTNESS AT SEPARATING WALLS
The airtightness layer in the external wall should be continuous with that of separating wall
(7) Windows and external door opes
THERMAL CONTINUITY AT WINDOW AND EXTERNAL DOOR OPES
Correct choice of the lintel or lintels to be used when forming an ope in an external cavity wall is a key factor in ensuring thermal continuity. The selection of the method of closing the cavity at the jambs, and the detail of the cill or threshold, are equally important. The nonrepeating cold bridges at these locations can account for a significant degree of heat loss in an otherwise well-insulated building.
For good thermal performance:-
Use separate lintels and insulate between them.
Fill all gaps around and between lintels with tightly packed insulation. Overlap the frame and this insulation by at least 15 mm.
Secure any partial fill insulation firmly against the inner leaf.
Cut cavity insulation to suit. Sheets should be tightly butted to each other and surrounding cavity closers and loose fill insulation.
AIRTIGHTNESS AT WINDOW AND EXTERNAL DOOR OPES
Air leakage often occurs between window or door frames and the surrounding construction.
Appropriate airtightness sealants are required between plaster finishes, window boards and
frames. Approved airtightness sealants and tapes are available to assist the formation of air
barrier continuity at such interfaces.
For air barrier continuity:
Apply a third party certified tape or sealant at all interfaces between the internal air barrier and the window or door frame
If forming the air barrier to the walls with a plaster scratch coat on blockwork, install an
appropriate airtightness tape. Where this tape is plastered over, the tape should provide a suitable key for the plaster.
To qualify for the NSAI Window Energy Performance (WEP) Scheme, manufacturers must first demonstrate that their window and door arrangements achieve a Class 4 airtightness rating when tested at 600 Pa to I.S. EN 12207:1999 Windows and doors - Air permeability - Classification. As a result, well-made windows should have little or no air leakage. The lower the air leakage value of the window assembly, the greater will be the overall efficiency of the window assembly.
(8) External Door Thresholds
Achieving sufficient thermal continuity to minimise the thermal bridge at door thresholds and to meet the critical surface temperature factor, fRsi, requires careful design.
Compliance with the critical surface temperature, fRsi ≥ 0.75, may entail a thermal model. However, certified details, or proprietary solutions, are available which have been thermally modelled by a registered thermal modeller.
Where thresholds are of such a length to be considered “key junctions”, the thermal bridge
associated with them should be fully accounted for, either through a Y-factor calculation, or by
adoption of the default 0.15 thermal bridging factor.
The design and construction of door thresholds should comply with Parts A to M of the Building
Each threshold should be designed to suit the components selected by the designer and take
account of accessibility, moisture and insulation. Proprietary thresholds are available that assist in
addressing these concerns.
Care is also required to ensure the continuity of DPC, DPM and/or radon barrier at thresholds.
Airtightness at thresholds can be achieved by sealing the threshold to the floor slab or air barrier and ensuring continuity of the air seal at the jambs.
Doors and frames tested in accordance with the WEP scheme will have good air permeability performance and will contribute to achieving a low overall air permeability test result.
(9) Service penetrations
Holes and chases are formed for many different services by different specialist contractors. They may be in roof spaces (recessed light fittings, water pipes, soil vent pipes, rainwater pipes, ventilation ducts, television cables); in external walls (soil and waste pipes, electrical cables) and in ground floors (soil and waste pipes, incoming mains). Penetrations may also be required behind bath panels, shower trays, kitchen units and into service shafts.
A key element in maintaining thermal continuity and airtightness around service penetrations is to agree standard sealing procedures with contractors and make sure the right materials and tools are available.
Specify appropriate sealing methods for services penetrations and/or structural penetrations of the thermal envelope, including:
Cooker hood extract ducts
Condensing boiler flues
Background vents in walls
Air intake/extract ducts
W.C. cistern overflow pipes
Soil vent pipes
Canopies to entrances
Electricity connections and meters
Gas connections and meters
Security alarm systems
External security lighting, security cameras, sensors
TV, broadband & cable service
THERMAL CONTINUITY AT SERVICE PENETRATIONS
For good thermal performance:
Core drill service penetrations to minimise damage to the insulation layer.
Make good damage caused to the insulation layer by filling any gaps with loose fibrous insulation or approved expanding foam.
Size drill holes to provide a snug fit, reducing oversize to a minimum.
Where ducts and pipes are insulated and have a vapour-tight outer sleeve (required to prevent condensation), it is recommended this is sealed to the vapour control layer or air barrier layer, as appropriate, in the wall/floor/roof element penetrated.
AIRTIGHTNESS AT SERVICE PENETRATIONS
For good airtightness:
All penetrations through the air barrier line should be effectively sealed following installation of the services. This can be achieved with the use of appropriate airtightness grommets, airtightness tape or airtightness sealants.
Construction of a dedicated services cavity inside the airtightness barrier will reduce the number of penetrations of the barrier.
When installing socket outlets or switch plates in an air barrier formed by a wet plaster layer, seal any chases formed behind the wet plaster layer before installing the services. Consider using proprietary gasketted socket boxes.
Where airtight membranes are used, a services cavity on the warm side will allow for the installation of services without penetrating that airtight layer and ensure that accidental breaches are avoided, especially after occupation.
Where plasterboard linings form the airtight layer, apply a continuous ribbon of bedding compound around the hole and the electrical back box prior to installing the plasterboard. This will reduce air leakage through the sockets/switches into the void behind the plasterboard.
CONTINUITY OF WALL AND ROOF INSULATION AT EAVES/VERGE
Roof insulation should be installed to minimise the effects of thermal bridging at the eaves. Attention must be paid to the sequence of installation of insulation at the eaves to ensure that it is effective, as it is difficult to install insulation after the roof has been completed. The roof insulation should be laid over any cavity barrier at the top of the wall and be continuous with the wall insulation.
THERMAL CONTINUITY UNDER THE ATTIC
For best practice, in cold roof spaces, use insulation over the ceiling joists, to eliminate the cold bridge caused by the joist.
All access hatches and doors to ventilated attic spaces should be sealed and insulated.
AIRTIGHTNESS UNDER THE ATTIC
Proprietary attic trap doors with low air permeability characteristics should be fitted in lieu of site manufactured hatches. Where site manufactured doors are installed, these should be complemented with draught stripping and a compression catch to minimise air leakage into the attic space above. The attic hatch frame should be sealed to the air barrier in the ceiling.
Cables which pass through, or are enclosed in, insulation should be adequately rated to ensure that they do not overheat. Recessed fittings and transformers should have adequate ventilation or other means to prevent overheating.
Where ceilings form part of the fire protection to structural elements, the requirements of the Supplementary Guidance to TGD B may apply to penetrations, such as downlighters, soil vent pipes or ventilation duct terminals.
Care should be taken to seal around all penetrations of pipes, ducts, wiring, etc. through the ceiling, see Figure 13 above.
The use of a vapour control layer (VCL) at ceiling level, on the warm side of the insulation, will assist in limiting vapour transfer and should therefore be used, but cannot be relied on as an alternative to ventilation of a cold attic space.
Where the roof is insulated at rafter level, an alternative construction using a breathable membrane and an effective VCL is described in TGD L in accordance with Part D of the building regulations.
Sealed airtightness membrane should extend behind the plaster linings of the dormer walls
and roof to form an air barrier and, where required, an effective VCL.