# Mycelium-Based Thermal Insulation for Domestic Cooling Footprint Reduction: A Review
## 5 Conventional Insulation Materials
### Phenol Formaldehyde  
Phenol-formaldehyde thermal insulation represents a category of insulating material crafted from synthetic polymers formed through the interaction of phenol or its derivatives with formaldehyde. Its production follows two techniques: one entails a direct reaction of phenol and formaldehyde to create a thermosetting network polymer, while the other method limits formaldehyde to generate a prepolymer called novolac. This prepolymer can be molded and subsequently solidified by adding additional formaldehyde and elevated temperatures. Phenol formaldehyde insulation has advantages such as thermal and chemical stability, adhesivity, high thermal insulation, and flame retardancy. However, it also has drawbacks, such as low mechanical performance, increased flammability, and limited availability. Some researchers have tried to improve the properties of phenol formaldehyde insulation by incorporating cardanol-based flame retardants or other additives [55].  
Polyisocyanurate  
Polyisocyanurate (PIR) is a type of thermal insulation produced as a rigid foam board or a foamed-in-place material. PIR is a thermoset plastic that is derived from the reaction of polyol and methylene diphenyl diisocyanate (MDI), with a higher proportion of MDI than polyurethane (PUR) [56]. The resulting polymer has a complex structure with isocyanurate rings and high cross-link density, which gives it superior thermal stability, chemical resistance, and mechanical strength. PIR also has a low flame/smoke rating and a low thermal drift, which means it retains its R-value over time. PIR is an ideal insulation material for energy-efficient and fire-safe buildings. Although PIR is a widely used thermal insulation material, it has some drawbacks that may limit its applicability and efficiency in certain situations. For instance, PIR can deteriorate under moist conditions, lose shape and integrity due to temperature changes, react adversely with some chemicals, perform poorly at very low temperatures, and cost more than other insulation options [57].  
Urea-Formaldehyde  
Urea-formaldehyde thermal insulation (UFFI) is a foam-insulating material that combines urea and formaldehyde. This non-transparent thermosetting resin or polymer possesses notable attributes, including high tensile strength, flexural modulus, heat-distortion temperature, and surface hardness. It also has low water absorption, mold shrinkage, and volume resistance. UFFI has been used for bonding particleboard, MDF, plywood, laminating adhesive, and thermal insulation in buildings [58]. UFFI has a thermal conductivity of 0.0343 to 0.0373 W/m-K, which can reduce heat loss and save energy. However, UFFI has some limitations, such as low compression strength, high pulverization rate, low limiting oxygen index, and possible formaldehyde gas emission during curing and aging. Therefore, UFFI has been banned or restricted in some countries due to health and safety concerns [59].  
Polylactic acid (PLA) is an environmentally degradable polymer from sustainable resources like corn starch or sugar cane. It is a thermoplastic material that can be processed by melt extrusion and compression molding. PLA is used as a thermal insulation material due to its low thermal conductivity, low density, and high compressive strength. It has a thermal conductivity of 0.0643 to 0.0904 W/m-K, depending on the degree of crystallization, which can be controlled by annealing. PLA also has a low water retention of less than 0.19%, which can resist moisture and mold growth. However, PLA also has some drawbacks as a thermal insulation material, such as poor heat resistance, a low glass transition temperature, a high pulverization rate, and a low limiting oxygen index. These properties make PLA susceptible to degradation and flammability at high temperatures [60].