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Sprayed insulation between roof rafters

  • 27-11-2021 4:35pm
    Registered Users Posts: 407 ✭✭

    We have attic insulation but considering further insulating by adding sprayed insulation between rafters in roof. Have long, split level bungalow so a lot of roof surface. Previously pumped walls which was v effective. What should I look for in spray insulation companies? Based in Kerry.


  • Registered Users Posts: 4,705 ✭✭✭enricoh

    You won't know yourself with the spray foam. Some companies use a cardboard box to put between the felt and the foam, others say theirs is breathable and no need. I'd go with the cardboard.

  • Registered Users Posts: 355 ✭✭Biker1

    Don't waste your money insulating at rafter level. A total depth of 350mm uncompressed insulation at ceiling level will be far more effective and cheaper. After all why would you be heating your attic when it's not part of the habitable area of the house. Don't forget to seal around any pipes and wires going into the attic and make sure your attic hatch is draught stripped and insulated.

  • Registered Users Posts: 2,811 ✭✭✭MicktheMan

    Agree 100%.

    You will also benefit from a proper independent heat loss survey which will highlight where the heat is being lost and where/how to invest your money wisely.

  • Registered Users Posts: 33,073 ✭✭✭✭listermint

    Why not both and have usable space for storage or to convert into.

  • Registered Users Posts: 407 ✭✭xraylady

    Thanks for suggestions. Roof space not suitable for conversion just thought that sprayed ins might be effective in addition to the loose insulation at ceiling level but will think again. Must look at insulating trapdoor to attic, I presume you can buy something to do this in reg hardware shop? By the way, would flooring the attic with insulated boards for storage ( over the non compressed stuff) be better than rafter insulation?

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  • Registered Users Posts: 2,811 ✭✭✭MicktheMan

    You should insulate and airtighten the attic trapdoor. How to do it depends very much on the type of trapdoor.

    Yes, adding insulated boards designed for the purpose is far better than rafter insulation. Be sure to fill the full space between the joists first but don't overfill (i.e. if the gap is 100mm, then only put 100mm of insulation and not 150mm and compress back to 100mm. And remember not to compromise the attic ventilation.

  • Registered Users Posts: 49 abbey


    I read your advice with interest. I was trying to get my head around why so many people seem to be insulating the rafters in their attics with spray foam, I would have thought that you want to focus on the ceiling which is what I think you are suggesting alongside trying to minimise draughts as much as you can.

    We bought a 15 month old dormer bungalow 6 months ago and have experienced a draughty winter. As we don't have a huge amount of money we are looking at where to target the little we have.

    I agree completely about the attic hatch, we also have 4 large attic doors in the 2 dormer rooms that I think need to be insulated and made air tight. This may sound clueless but is it a case of sticking some type of insulated board to the back of the doors alongside putting seals at the door frames edges?

    If the dormer rooms are habitable, is spray foam a better job than just rolls of fibre glass? Especially as it is awkward to get at certain spots.

    Their seems to be fibreglass between the joists and another layer across that. Do I need more or should it be replaced?

    In regards to the gable walls in the dormer rooms, if the cavities have not been pumped is that another worthwhile element to tackle? Their are exposed block walls in the attic where I can see holes containing beads although I'm not sure if all the walls in the house are done. If they were I would think it should be warmer.

    The gable walls have 'dot and dab' plasterboard. Is their anything needed to improve them or is a case of pumping the cavity will improve their ability to retain heat?

    I had a look at the BER report and it seems to be no help, a lot of generalities, not much in the way of specifics.

    Any advise appreciated. Thanks.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    All materials on building sites must be fitted in accordance with the manufactures instructions. Many workers on building sites never read these instructions. 

    Slating and Tiling instructions state that the roofing membrane must be fitted well sagged between the rafters 

    The purpose of the roofing membrane is to prevent any ingress of rainwater blown in between the tiles or slates during very strong winds, from getting in to the attic area. 

    The membrane is sagged to form troughs between the rafters, to enable any of this rainwater to flow down in to the gutters. 

    If it is not well sagged, then the membrane will be up tight against the lower surface of the battens which will prevent the rainwater from escaping from the membrane. 

    Rainwater may build up and soak in to the battens and may cause Wet Rot. 

    if there is cracked or damaged slates or tiles allowing ingress of rainwater it could cause excessive damage. 

    If spray foam is sprayed under the membrane, it will expand and will push up the sagging in the membrane, up against the roofing batters.

  • Registered Users Posts: 3,761 ✭✭✭Odelay

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    I’m sure that many lay people here do not need to purchase or read any detail recommendations.

    Many just want simple advice - simply explained.

    There is nothing wrong with the advice I have given.

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  • Registered Users Posts: 3,761 ✭✭✭Odelay

    Wasn't getting at you, just what you mentioned is as described in the standard. You did a great job of explaining part of it. I assumed with that knowledge you must have been familiar with it.

  • Registered Users Posts: 296 ✭✭Citygirl1

    Hi. I have similar question to Abbey, re insulating the attic trapdoor. My house has a Staire. In particular, what type of Airtightness tape would be best to put around the frame?

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Thanks Odelay



  • Registered Users Posts: 49 abbey

    Thanks C Eastwood for the advice.

    In regards to the spray foam does the fitting of a vent card lessen the pushing up of the membrane?

    Having read some of your and others advice, I think we would like to avoid putting in spray foam.

    We had someone out to the house who was suggesting multi-foil insulation as an alternative to spray foam. It's not something that is common in this country. Why is that?

  • Registered Users Posts: 30,993 ✭✭✭✭Lumen

    We had someone out to the house who was suggesting multi-foil insulation as an alternative to spray foam. It's not something that is common in this country. Why is that?

    Probably because it has no NSAI certification (I'm happy to be corrected on that but that's my guess). The NSAI website has a search function so you could look yourself. Always read the full cert as it has detailing required for the cert to apply. Details matter.

    Multi foil insulation is mostly snake oil. It's often sold as thin sheets which have really poor insulation value, and the reflective element of the foil is presented as a distraction from that. If there's any value to the foil element it's purely as a vapour barrier.

    IIRC the most insulating material for a given thickness, excepting exotic solutions like vacuum panels and aerogels, is phenolic, then PIR, then XPS.

  • Registered Users Posts: 1,914 ✭✭✭Andrea B.

    A topic I have been looking into myself.

    I have heard that another approach being taken is to slab and skim (or tape joint) existing ceilings with 50mm insulated plasterboard.

    Benefit being claimed is that eliminates the joist cold bridging.

    Any thoughts on this approach?

  • Registered Users Posts: 30,993 ✭✭✭✭Lumen

    Probably, with 100mm of insulation between the rafters (assuming you still have 50mm remaining above that for ventilation).

    You need to get u value at or under 0.16.

    There exists insulated plasterboard which uses PIR, that has better insulation values than XPS. Similarly with the insulation between rafters you have a choice of products with different values.

  • Registered Users Posts: 1,914 ✭✭✭Andrea B.

    Am I mixing up rafters and joists?

    I mean slabbing over existing ceilings and not considering any changes to rafters.

  • Registered Users Posts: 2,386 ✭✭✭olestoepoke

    I'd say you could easily make one of these yourself, especially if you had leftover insulation from a project and a roll of foil tape.

  • Registered Users Posts: 30,993 ✭✭✭✭Lumen

    Am I mixing up rafters and joists?

    No, I am. This thread is about rafters and I didn't read your post properly to spot the difference!

    Why are you considering insulating your ceilings? What's above them?

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  • Registered Users Posts: 2,979 ✭✭✭Stovepipe

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    The cold bridge you are talking about is a problem called Pattern Staining. I haven’t seen this since the 70’s in the old uninsulated flat roofs, with no Vapour Barrier

    See photo:-

    This pattern staining was caused by a number of factors.

    1. Lack of insulation
    2. The main cause was no Vapour Barrier, and this allow the damp vapour in the Kitchens and Bathrooms to pass up through the slabs, but caused condensation under the roof joists.

    Timber has a low (good) thermal conductivity value.

    The insulated slabbing you are suggesting is a very expensive and a disruptive process. I would not suggest this solution.

    To prevent any cold bridges from timber ceiling joists- thermal insulation is fitted between the joists ( approx 100 mm) on top of the slabs, and another layer of 200 mm insulation fitted over this insulation/ on top of the joists and transversely to the joists

  • Registered Users Posts: 1,914 ✭✭✭Andrea B.

    Hi, it is bungalow with attic and 200mm rockwool.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Andrea B

    an uninsulated plasterboard ceiling slab will give a U value of approx 2.0 watts/m2/degreeC/hr.

    100 mm insulation between the joists and 100 mm transversely above would give a U value of approx 0.10 watts/m2/degreeC/hr. See diagram below.

    This is a heat loss of only 1/20 of the uninsulated ceiling.

    This is perfect and there is no need to add more insulation.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    Apologies for the late reply.

    Timber needs ventilation. The attic needs ventilation. The roofing membrane should be sagged between the rafters to allow any ingress of rainwater to flow down unimpeded In to the gutters.

    Spray foam interferes with all of the above.

    This is unbiased advice.

    The person advising you to fit Spray foam or multi foil insulation is a sales person - possibly on commission and looking for a few thousand euros from you.

    Furthermore, I do not understand why anyone wishes to heat an uninhabitable void in the Attic or - the voids above and at both sides of Attic rooms.

    Example - an attic with a 2.7 m rise of a pitched roof - in a house 9 m x 14 m gives a volume of space in the Attic of approx 9 x 2.7 / 2 x 14 = 170 m3 volume of air in the Attic.

    Heating this unused 170 m3 of space is ludicrous. - Just because a sales person advised insulating between the rafters. They are fantastic salespersons.

    200 mm of insulation on top of the plasterboard ceilings will give an approx U value of 0.10 watts/m2/degreeC/hr.

    In the above example a house with a floor to ceiling height of 2.4 m will have a volume of air of 14 x 9 x 2.4 = 300 m3 of air which must be heated to approx 20 degree C, which is expensive to do for many months of the year- and every year

    WHY heat up the 170 m3 of uninhibited attic space, which is approx half the volume of air in the house which must be heated and Will increase the heating costs by 33%. €€€€€€€€€€€€€

  • Registered Users Posts: 1,376 ✭✭✭dathi

    perhaps you should edit your advise to andrea B to say that they would have to remove the rockwool and replace it with the two layers of kooltherm to comply with partL of the building regs as currently the two layers of rockwool that they have gives a u value of 0.21 W/m2k which doesnt comply

  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    I‘m at a total loss as to what you are referring to here, or what you wish me to state.

    I would not under any circumstances alter the advice I have given to abbey B, with the information given.

    On the 19 April Andrea B stated as follows:-

    • I mean slabbing over existing ceilings and not considering any changes to rafters.


    • Hi, it is bungalow with attic and 200mm rockwool.

    I do not have any further details of their house

    I do not know when the house was constructed or if it was constructed before or after the Building Regulations of June 1992, or if the house was constructed before or after the Draft Building Regulations.

    Therefore if the house was constructed in 1978, the ceilings did not have to be insulated.

    Building Regulations do not have to be complied with retrospectively.

    Therefore it is axiomatic that we do not know what Regulations, if any, which are applicable to the said house.

    Here are the current BR, for works to existing building, and which states “as far as is practicable “.-

    The only logical reason that I would instruct a Builder to remove perfect Attic Insulation, would be for Health and Safety reasons - when the plasterboard slabs of a ceiling are being removed.

    andrea B has 200 mm of Rockwool insulation at ceiling level.

    The Building Regulations state inter alia, that insulation material at ceiling level Must be non-combustible. - Rockwool meets this criteria.

    We have not been given the density of the Rockwool, so we cannot ascertain it’s k-value ( thermal conductivity) . Without the k Value the U value cannot be calculated.

    It would be totally ridiculous and expensive for any person to remove perfectly good insulation material from an attic, in order to install new insulation, when they only have to add more insulation on top of the existing Rockwool.

    You requested dathi that I should advise Andrea B, to remove the 200 mm of Rockwool. I would not advise any person to do that under any circumstances as it would constitute an act of gross incompetence and negligence.

  • Registered Users Posts: 1,376 ✭✭✭dathi

    i am referring to this line in your post ,

    100 mm insulation between the joists and 100 mm transversely above would give a U value of approx 0.10 watts/m2/degreeC/hr. See diagram below.

    which you followed with a picture showing two layers of kooltherm insulation layer between and over the joists

    your first statement implies that that they have enough insulation as you say that 100 between and 100 over gives uvalue of 0.10w/m2k. as the thermal conductivity of rockwool is 0.044w/mk which would give a u value aprox 0.21 w/m2k. this is double the uvalue you stated for rockwool, so either you miss calculated your uvalue for rockwool or were advocating the use of kooltherm

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