Internal Wall Insulation

Q: I'm Thinking of Insulating My Solid Stone-built House Internally. What Are the Pros and Cons?

A: There is no one solution for your situation and there are risks and variable expenses attached to each option. However, great care is needed because what we don't want is to cause condensation problems and subsequent mould behind the insulation. So, in your home you're going to have three different external walls: you've got the original solid rubble-filled wall, the random stone and block cavity wall and the new extension, which will all need to be treated differently when considering an internal wall insulation strategy.

As a belt and braces approach, we would recommend having WUFI hygrothermal modelling undertaken before any work is undertaken.

Q: I'm Planning a Big Extension Which Will Wrap Around Two Sides of My House. Is This a Good Time to Think About Insulation?

A: This obviously offers great opportunities for improving the energy efficiency of your home. The bigger the extension around the existing building the better because you're actually surrounding the building more.

Q: What Are the Options for the Original Solid Rubble-filled Wall?

A: To minimise the risk from moisture and interstitial condensation from this type of internal wall, specialist natural insulation boards can tolerate a certain amount of moisture movement within the wall without problems building up. Examples are Gutex Thermoroom or Pavatex Pavadentro. This could be up to 100mm thick. You need to make sure that the plasters on the wall beneath the insulation is lime or sand and cement render not the modern gypsum plasters (because these can be a breeding ground for mould). It would be a case of knocking this off and rendering the walls before applying the natural internal wall insulation.

Q: What About the Random Stone and Block Cavity Wall?

A: For this wall type, as you've got the cavity there is less danger of moisture build up behind the internal wall insulation from rain penetration, so a cheaper man-made insulation e.g. polyurethane-backed plasterboard could be used – although using the natural insulation again would be a safer bet. Whatever insulation material you are going to use – all the insulation boards will need a continuous adhesive layer behind. You've got to stop moist warm air from inside the house getting behind the insulation which could cause condensation and mould.

Q: Are There Real-world Examples of These Internal Wall Insulation Strategies?

A: Yes, we have documented our approach on complex retrofit projects. Each building presents unique challenges, requiring a tailored internal wall insulation strategy.

• For a detailed look at how different wall types were tackled in one project, see our Cumberworth Retrofit Case Study. We used innovative materials to manage moisture risk in the solid stone sections of the building.
• To understand how to manage details like window reveals, which are critical for a successful insulation project, read about our Holmfirth Farmhouse Retrofit Strategy.

Q: How Can I Prevent Rain Penetration Through an External Wall?

A: Inhibiting rain moisture coming in through the wall cuts down the danger of mould growth and possible rot in timber joists within the masonry wall. Ensure the wall pointing, preferably lime based, is sound with guttering and eaves overhangs functioning properly. There are products you can use to coat outside masonry which inhibits rain penetration such as Belzona Clear Cladding that inhibits rain penetration but allows the wall to breath.

Q: I Get Condensation on My Walls in Winter. I Know That Really This is a Ventilation Problem, but if We Insulated the Walls, Could This Solve Both Issues?

A: Our construction division 21 Degrees, has been insulating internally with 50 or 75mm polyurethane board for many years but have tried to cut down air movement 'behind'. The thicker the insulation the greater the danger. Natural materials such as wood fibre board systems offer greater security due to their ability to 'take up' moisture and release again in more favourable weather conditions giving better protection to the building's fabric.

In answer to the question about MVHR, I would say that it is not a solution to your problem and that should focus first on sorting out the continuity of insulation in your home. Only once that is sorted and stringent airtightness strategies are in place, would I recommend an MVHR system.

Q: What Products Do You Recommend for an Internal Wall Insulation Project?

A: A successful internal wall insulation system relies on more than just the insulation boards themselves. Managing airtightness and thermal bridging is critical. Based on our experience, we recommend a systems-based approach using high-performance components. You can find a range of specialist products on our online shop.

Here are some key product suggestions for your project:

• Airtightness Tapes & Membranes: Use products from the Pro Clima range to create a continuous, robust air and vapour control layer. This is essential to prevent warm, moist air from getting behind the insulation and causing condensation.
• Specialist Rigid Insulation (Compacfoam): To prevent thermal bridging around windows and doors, a specialist high-density insulation like Compacfoam is ideal. It provides structural support for window installation while minimising heat loss.
• Airtight Loft Hatches: If you are insulating rooms with a loft space above, ensure you use an airtight and insulated loft hatch. This completes the thermal envelope and prevents heat from escaping into the roof space.
• Sealants and Adhesives: Use high-quality adhesives like Orcon F to bond airtightness membranes and ensure durable, long-lasting seals at all junctions.