| House A: | A cavity wall house built since the start of Building Regulations (1991) |
| Distribution: | Very common in Dublin in Lucan, Malahide and Portlaoise |
| Upgrade: | Cavity Fill, Re-tiled with ‘warm’ roof, Full Airtightness Upgrade & Attic Conversion |
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1) Filling the remaining void of a cavity wall with platinum, bonded, blown bead is a great idea and cost-effective. We don’t recommend filling cavity with fibres or foam insulation. It’s critical that at least 40mm of the cavity is continuously clear (50mm is a safer figure). Less than that and there is a risk of the cavity being bridged and water migrating to inside leaf, which can be difficult to resolve. Don’t assume there is a regular condition or width within the void. Remember it was common for insulation boards in partial-fill cavity walls to be installed carelessly and ‘snots’ of mortar were frequently allowed to bulge into the cavity. In many ways the ideal walls to full-fill are those built in the 1960s and 70 and left empty till now. They can have cavities of 50, 65 and even 100mm wide. The insulation installer must first carry out a survey of the suitability of the cavity for filing in accordance with BS 8208-1 & submit a short written report. We recommend you or your consultant carefully reviews that report before instructing installer to proceed.
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3) Having removed and stored tiles and properly disposed of old sarking felt and battens, install NBT’s ‘Pavatherm Plus’ (from Econstruction or Lochplace) or ‘Gutex’ board from Ecological Building Systems. Prime and then seal the top surface of the junction of each board to the next using ‘Pavatape’ or equivalent. Dense insulation boards on the outside of a roof buildup are of great value for attic rooms because they slow down the flow of heat to the outside in Winter and the baking effect of the sun in Summer.
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4) Next install a breathable sarking felt like ‘Solitex’ or ‘VP 400’ from Richmond BP: these felts are designed to have insulation right up to their underside but must be used with a properly installed airtightness membrane below. Never attempt to fill insulation up against a non-breathable membrane, such as traditional bitumen-impregnated felt or any other more modern membrane: the result could be rotting insulation and rotting structural timbers. Always follow manufacturers’ instructions in this regard. Now install battens on top of rafters and counter batten and put back original roof tiles. Note a few more will now be required as roof has got slightly bigger.