Q I am building an extension on to the back of my house and converting the garage to a bedroom. The existing house, built in the 1960s, has suspended timber floors, and the add-on area will have concrete floors. Some of the vents will be blocked by the extension. Please advise me how best to insulate the floor area or should I concrete the lot. Will the lack of ventilation rot the timber floors? The new floors will be on a raft.I hope this makes sense, as I am over 70 and know nothing about building. I read your article every Thursday, and find it interesting. I believe my problem is common and look forward to your advice.
A I assume the extension and garage conversion is permitted development and that planning permission is not required. Permission is not usually required for the conversion of a garage that is attached to the house. An application can be made to the planning authority for an exemption certificate which will be beneficial if the house is ever sold. Your designer should advise you in these matters.
The question of venting is important and cross ventilation is advisable for timber sub-floors and indeed any concealed space where timber is present. The house should have vent grilles at low level front and back which facilitate the cross venting.
Traditionally, the method was to lay pipes within the new concrete floor to allow air to pass through from the existing void to the outside. However, I have rarely seen this done when inspecting properties of this age. It does depend on the space between the internal finished floor and the outside ground level.
If there is sufficient depth then you can lay a reinforced screed on insulation directly on top of the concrete raft. The vent pipes can be laid within the raft and the steel reinforcement will have to be designed accordingly.
This will require careful setting out and monitoring of the work to ensure the design requirements are achieved. The existing timber floor boards can be removed and insulated from the underside.
Refer to Sustainable Energy Authority Ireland (seai.ie) for guidance on methods for insulating existing timber floors.
Alternatively, and as you infer, you could remove the timber sub-floors and provide a solid concrete insulated floor to negate the need for ventilation. This method may even be more practical and cost effective.
Whatever method is adopted, I would advise that the void is inspected beforehand to check the condition and for any cables and pipes that may be present.
It is likely a building of this age was plumbed with lead pipes. Exposure to lead has a health risk where low concentrations of lead can dissolve into the drinking water. Contact Irish Water for advice.
I would recommend that you seek the services of a chartered building surveyor to evaluate your options and consider any other implications including maintaining existing damp proof course levels, before you commit to employing a building contractor.
James Drew is a Chartered Building Surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie
