Innovative Insulation Solutions

Sympathetic Hempcrete Insulation

The infill and internal backing of the timber frame posed a few considerations in terms of vapour permeability, vapour control, water ingress and insulation values. A modern version could have involved a combination of non-breathable insulation and internal vapour barrier, but this felt at odds with the traditional approach with this element. After considering various different versions and designing each option by performance and materials build up, consulting with suppliers and specialist contractors we settled on a simple solution with careful detailing to provide full-fill and thick backing of hempcrete, as this would be lightweight, insulative and moderately thermally massive, helping with all year round comfort and maximum longevity, with modern standards of energy performance.

After hand casting hempcrete into the oak frame, the hempcrete was then sprayed on from the inside to form a solid mass of hempcrete on and around the new gable end timber frame, providing compliance with current regs in a fully vapour permeable form for this new thermal element. Hempcrete sits somewhere between cob and straw bale, striking the right balance between thermal mass (inertia) and thermal insulation (resistance). A good example of modern methods and standards combining sensitively with traditional construction details.

The hemp lime contractors, Hemp Lime Spray (hemplimespray.co.uk) led by Graham Durrant, commented that our site was probably the most organised and well-prepared they had encountered in their work. We thoroughly enjoyed the days working with them to collaborate on this project, which included some creative solutions to ensure that all robust detailing was suitable and enabled us to deliver the best outcome for the client. Overall, 450mm thickness here with a combination of hand casting and sprayed application, prior to finishing with hemp lime plaster inside and limecote outside, made this a healthy, Part L compliant and sustainable improvement upon the tired, uninsulated and part-rotten frame we started with. This should easily last more than 100 years.

After the team worked together to hand cast the mix in between timbers, we were ready for spraying to commence by lunchtime
At this stage, we handed over to Graham and his excellent team to fill out the rest of the wall thickness. A combination of spray adhesion and careful layering was needed here to achieve a fully contiguous mass.
Wood wool board adhered to the side of the intumescent coated steel frame provides suitable separation between the very different construction materials.
The delivery hose from front garden to 2nd floor window was just long enough to reach. Graham Durrant MSc (Hemp-Lime Spray Ltd) was the skilled hand with this work but was kind enough to show us the ropes (Mr Thorne MCIOB learning on the job here)
Every one of the junction details was carefully considered, by sketches, discussion over the phone and subsequent review with the architect, structural engineer, Building Control Officer and our on site team, with all final details checked on site. We had absolute confidence this was the best option for the building, and the truly collaborative process was rewarding too.
The oak frame was shuttered from the outside, with internal stud facings clad in wood wool board, setting the thickness for final sprayed hemp build-up.
After initial drying, striking the shuttering allows for any shrinkage to be address. Remarkably little work was needed as there was negligible slump.
The hempcrete fill was complete to all areas, even right up alongside the penultimate rafter. The nature of the material is that it is more hygroscopic (absorbs and releases moisture) than timber, so cannot cause timbers to rot in its presence. As such it is perfectly suited to this application, whether softwood or hardwood is employed.
In order to reduce clean up after lime plaster, the frame was taped. Limecote plaster provides a highly flexible and durable finish. Also, the oak has now been darkened with a semi-traditional method - white vinegar and wire wool, to bring a mellow brown richness out. The original was painted black, but softening that slightly for a dark brown was the client’s preference. Test pieces were done to conclude the best look.
Doug enjoyed using the Limecote externally. His previous experience with lime mortar/render is extensive, but this relatively new product is based on a medieval recipe from the East coast. Excellent support from the manufacturers and a product that we will certainly use again.

Underfloor Heating

Removing suspended floors and replacing with foamed glass insulating aggregate and a solid screed floor provided the opportunity to improve energy performance and enable low temperature heating via wet underfloor heating pipework. The insulating aggregate is vapour permeable and provides a capillary break to prevent rising damp without a plastic damp proof membrane being needed.

Roof Insulation

Wood fibre insulation as part of a two storey extension to provide vapour permeable roof insulation with all natural materials, and a hemp lime finish. Vaulted ceiling on a cropped hip roof design was the result of collaboration with client, builder and engineer. Glulam beams provide a robust and simple way of reducing the span on the rafters while enabling collars to be placed high up in the room for a feeling of ‘volume’ in the new space