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Yet another insulation query….?

  • 07-08-2022 4:34pm
    Registered Users Posts: 1,080 ✭✭✭

    I am in the process of getting quotes to ‘pump walls’ etc of a 25 year old detached two storey house of about 200 Sq m floor area. The existing in situ insulation comprises the typical 100m cavity with 50’mm polystyrene boards and attic has approx 125mm thick of rock wool (extremely itchy stuff) between the ceiling joists. As the attic is sizeable and very handy for storage I’d rather not add the an additional 200/250mm depth of rock wool type insulation as is recommended as it is partially floored already and I’d like to fully floor it out and add a ‘stire type stairs’……..I’ve read about the pros and cons of this on various other threads. Some people suggest to put the ‘spray foam ‘ type insulation between the roof rafters but if not done correctly as in no air gap is maintained between the roof felt and the spray on insulation. Then there is a risk to the rafters ultimately rotting?…..also there is the view that Having any air circulation in the attic space is totally pointless and defeats the purpose altogether ie you might as well add/do nothing with the attic space as you can’t achieve any air tightness And therefore the heat from below that enters the attic space simply continues to escape As the attic is just like one large ‘vent/air escape pathway ?

    Does anyone Have any suggestions on how it might be possible to insulate the attic space only while still maintaining air flow/ventilation for the roof rafters…..?


  • Registered Users Posts: 32,232 ✭✭✭✭Penn

    If your roof rafters are 175mm deep (for example), you could fit 120mm rigid insulation between the rafters (leaving the 55mm gap at the back for air ventilation), and then insulated plasterboard under the rafters (say 40mm or so). This is one of the most common ways to insulate pitched roofs for attic conversions, as it provides the required levels of insulation, air circulation to the timbers, and prevents thermal bridging through the rafters.

    Air circulation to the timbers isn't for air tightness, its to prevent a buildup and increase of moisture content of the timbers which can lead to rot.

  • Registered Users Posts: 1,080 ✭✭✭Asdfgh2020

    Many thanks for your reply…..I’m not 100% sure but I think the rafters are not quite 175mm more like 150mm…in any case I’m sure there are 100mm thick rigid insulation boards available……?

    Have you any experience or thoughts on the foil type membranes that are appearing on the market…..and claim to be as good as the rock wool/rigid board….eg below…..