Rendered Insulation Systems
Generally, the design of insulation render and its installation is best left to rendering professionals. It shouldn’t be taken as a DIY job.
In most cases, insulation render systems come as proprietary products such that the design methodology and standard details are decided upon by the manufacturer.
Insulation render products are typically accredited by third-party bodies for application in various situations. They’re often guaranteed as long as they’re installed by a contractor approved by the manufacturer.
Although the construction techniques employed in making rendered insulation systems are largely similar, the systems differ in terms of render finishes and the type of insulation.
Typical insulation render systems include:
- Basecoat for render
- Finishing coat for render
- Mechanical fixings that are at times fitted through the wall with an adhesive
- A standard insulation board attached to an existing wall
- Reinforcing mesh that forms a backing for the base coat and finishing coat
Basic U-Values for Insulation
U-values for insulation measure the effectiveness of an insulation material. The basic U-values for various render insulation systems include:
Polyurethane foam slab
Mineral wool insulation slab
- 31 (100mm thickness)
- 22 (150mm thickness)
- 17 (200mm thickness)
Phenolic foam board
- 20 (100mm thickness)
- 14 (150mm thickness)
- 11 (200mm thickness)
Expanded polystyrene slab
- 33 (100mm thickness)
- 23 (150mm thickness)
- 18 (200mm thickness)
The above U-values are based on solid brick walls with a U-value of 2.2 and a thickness of 215mm.
How to Avoid Thermal Bridging
Insulation render systems typically come with various mechanical fittings made of plastic or metal.
These fittings are used in fixing the insulation onto a wall. The fittings form several thermal bridges that have a similar effect to that of the ties in a wall cavity.
You can avoid thermal bridging by:
- Fixing the insulation with a mechanical track and rail system- In this method, the insulation material is supported in place by plastic or metal tracks. The tracks are fixed to an existing wall. Although this system is advantageous in that the fixing is placed behind the insulator, a small gap is created that may cause air to move between the surface of the wall and the insulator.
- Fixing the insulation with adhesive- In this method, you should ensure the adhesive completely covers the insulator to prevent air passage behind it. However, this method isn’t recommended in circumstances where there’re doubts on the quality of adhesion such as on rendered or painted walls.
There’re several types of base renders including:
Essentially, acrylic render comprises acrylic as the bidding compound instead of cement. Also, it includes high-quality sand or aggregates.
It comprises two render coats with a total thickness of about 4 to 6mm. It’s often used in circumstances where there’s a lower likelihood of damage and a lot of exposure to light.
The reinforcement used comprises a glass mesh or mineral fiber. Acrylic render has numerous advantages including being lightweight, weather-resistant, breathable, and high elasticity.
Cement render is often mixed on-site. However, it can be supplied to the site while pre-mixed. Pre-mixed cement render often includes additives that make it easier to apply and improve curing.
It comprises two or three coats totaling 16 to 25mm. It’s mainly used in exposed weather conditions. Also, it’s ideal for application in situations that pose a higher likelihood of damage.
Reinforcement is done using galvanized or stainless steel. However, cement render tends to crack over time. Thus, there should be movement joints to prevent cracking. Also, cement render is quite heavy.
Hydraulic lime render
Hydraulic lime render is often used alongside Foamglass insulation on a 10 to 12mm coating. It’s reinforced with a glass fiber mesh and a 3mm thick decorative layer.
Polymer modified cement render (PMCR)
PMCR comprises one or two coats totaling 6 to 12mm thick. It’s mainly used in relatively exposed weather conditions and in circumstances where there’s little likelihood of damage.
It’s reinforced with glass mesh or mineral fiber. The polymer in the render mixture reduces its weight.
Also, the polymer allows it to be applied in both high and low-rise buildings.
The polymer content often varies from one PMCR system to another and the application requirements. PMCR is a medium-weight type of render.
Hemp insulation has been used for quite some time. Today, lime render is often mixed with hemp to form a blended render.
Although it’s not yet ascertained, manufacturers argue that this render enhances the insulation of solid brick walls.
It increases the U-value from 1.71 to 1 for a brick wall with a 215mm thickness when coated with a 50mm hemp-lime render.
A finishing coat of lime plaster is applied to enhance the longevity of the render.
Insulating renders often include a render system combined with an insulator. The insulator may include vermiculite, perlite, or polystyrene balls.
However, it doesn’t have a significant improvement in the render’s thermal efficiency.
It’s often used in areas like door and window reveals where the dimensions make it challenging to apply other forms of insulation.
In such situations, insulating render may reduce thermal bridging.
Finish topcoats include:
Acrylic and silicone renders
They’re ideal for application on both thick and thin coat renders. They enhance the resistance of a wall to cracking.
Also, including silicone in render enhances its water-repellent properties and allows water vapor to pass through it freely. Thus, the dry surface resists lime bloom and algae growth.
You can choose from several finishes and textures of acrylic and silicone renders.
Also, the finishes can be done using a wide selection of rollers and trowels. It’s also possible to apply the finishes using a spray.
The textured coating should be applied carefully to allow for a consistent finish.
Scraped finish or scratch plaster
It’s pre-coloured and comprises large-sized grains. It allows for a textured finish once scratched.
Also known as pebbledash or spar dash, aggregate dash is used as the background for roughcast or dry dash finishes.
It’s typically applied on trademarked base coats.
It’s a self-coloured, polymer-modified, and cement-based render. It incorporates small, even pebbles that allow for a textured render finish.
It’s typically applied on a dash receiver and base coat.
Flat render or coloured smooth
It allows for a clean and smooth finish. It’s available in several colour options.
Tyrolean finish or spray render
Tyrolean finish is a weatherproof and hardwearing render. It’s achieved by spraying repeatedly to achieve a coating with a thickness of at least 4mm.
The render is typically applied on a smooth, polymer-rendered, and coloured layer.
Cement-based renders tend to crack over time due to moisture variations and temperature changes. More flexible and thinner acrylic-based renders have a lower likelihood of cracking.
It’s recommended to maintain movement joints at a maximum of 5m intervals. The movement joints should be maintained both vertically and horizontally.
You can replicate the look of traditional brick walls by installing brick slips. In the recent past, several systems have come up as replacements for aligning brick slips and adhering them to a board backing.
Today’s advanced systems allow for the mounting of brick slips on several formats such as wire mesh and polystyrene backings.
Work Stages For Rendered Insulation and the Tasks Involved
Stage 1- Appraisal
- Surveying the existing building.
- Determining the ventilation, structural integrity, and thermal performance of the building.
- Determining the current SAP performance.
- Determining the air-tightness strategy and target.
- Determining the insulation strategy.
- Identifying any issues that may result from condensation and dampness.
- Assessing planning constraints if any.
- Establishing a performance target for the walls alongside other elements.
- Identifying any factors that can affect the durability of the cladding system such as weather conditions and the likelihood of damage.
Stage 2- Feasibility Study
- Determining the site’s exposure zone.
- Calculating the wind load.
- Identifying any defective areas on the building and determining how to fix the defects.
- Determining surface classifications according to fire safety standards.
- Determining any risks of a potential rendered insulation system.
- Identifying the procedure for reviewing and testing the potential render system.
- Establishing the durability requirements for the potential render system such as lifetime expectation when installed, reliability, and maintenance requirements.
Stage 3- Outline Proposals
- Assessing the impact that the proposed construction may have on the environment.
- Considering any aesthetic issues such as colour and texture.
- Considering performance issues with respect to the types of insulation render, reducing cold bridging, air-tightness, and suitability of the insulation.
Stage 4- Detailed Proposals
- Choosing the rendering system and using locally available materials where possible.
- Ensuring the chosen render material is suitable and certified for the required application.
- Developing and applying detailed design methods for terminations, penetrations, openings, corners, and abutments.
- Confirming that the existing structure can support the weight that the rendering system will add, including wind load and other loading details.
- Ensuring there’s compliance with local building regulations.
Stage 5- Final Proposals
- Ensuring there’s coordination between the designers.
- Ensuring there’s airtightness, minimizing penetration, and preventing cold bridging.
Stage 6- Production Information
- Selecting a sub-contractor in case one is needed for specialist tasks.
- Specifying insulation, membranes, and components.
- Ensuring the existing exterior fixings are removed before installing the render and re-attaching them thereafter.
Stage 7- Tender Documentation
- Defining the responsibilities of contractors.
Stage 8- Mobilization
- Preparing samples, QA procedures, training, and testing.
Stage 9- Site Works
- Coordinating inspection.
- Ensuring the rendered areas are inspected.
- Ensuring any changes in design won’t compromise performance.
Stage 10- Post Completion
- Carrying out any remedial tasks as needed during the defects liability period.