Tuesday, 12 June 2012

Insulating the roof

When it came to the roof insulation, my original plan was to use 100mm Celotex, and have a 50mm air gap above, creating whats known as a 'Cold Roof'. Soffit vents would then be placed at each side of the shed, to allow airflow on the upper side of the celotex, preventing condensation.

BUT. I messed up. I went to fit the 100mm celotex, to find my rafters were only giving me 120mm gap, meaning my air gap could only be 20mm, which would not be enough.

Because of this I opted for a kind of 'hybrid warm roof'. A breathable membrane is attached to the roof from the inside, coating it completely. The 100mm celotex is then placed between the rafters. At this point there will be little to no gap between the Celotex and the plasterboard that'll be added later.

I sealed up any gaps with expanding foam, and used Celotex Foil Tape to completely seal the gaps around the celotex. This means no air can get above the celotex, thus there should be no condensation.

Roof insulation
The only downside to this method is that an air vent has to be placed somewhere inside the shed to avoid condensation building up on the ceiling. I'll likely place a small vent above the window or door, assuming I can find one that is small enough.

If this was a house build, this method would probably not work, however since its a one room shed conversion, it shouldn't cause any problems with thermal bridging so long as sufficient ventilation exists inside the room.

So far I've got the celotex up and secured, I've now just got to place the plasterboard on the ceiling and can then make a start on the stud work walls.

I had to wait until now to start the stud work as the building has a sloped roof, and it would have been near impossible to work out how high each wall joist needed to be without having the roof completely finished.

Once I have the studs in place, I'll be calling an electrician in to get the power fitted, as I assume this will need to be done before I add wall insulation.

The end is in sight...weather permitting I should be able to get the stud work done over the next week. That being said, given the weather over the previous few days I'm pretty doubtful. I came very close to going out and buying one of those white plastic gazebos so I could continue working!


  1. Hi there Ricky! Were you able to finish installing your roof insulation? I hope everything is working perfectly. On the other hand, I think it would be better to call an expert in roof insulation next time, so you can be assured that you are applying the right kind of insulation in your house. The type of insulation you should install will depend on your home’s structure and the climate in your place.

    1. Hi Lue,

      Yep, roof is all ok :) Not sure I agree about calling in an 'expert' as its pretty basic theory, I just messed up on my measurements and took a gamble on a custom solution. Generally all you have to do is look up the basics of a hot or cold roof system, there's plenty of info about it online. I'm guessing however your post here is due to you being a roofing contractor looking for business ;)

  2. Even though you messed up the installation of the insulation, it’s still good that you managed to fix things up. However, it’s sad to hear that your supposed ‘cold roof’ turned into a ‘hybrid warm roof.’ I just hope you can benefit from it, just like what you plan for the ‘cold roof.’

    @Chantay Smithingell

  3. Hi Chantay,

    The roof seems to have worked very well - I've never had condensation in here either which has been a great sign. Quite surprised at how well the roof turned out all things considered.

    I never actually got the vent added in (the weather got too bad before I had the chance) so expected to have problems but so far it's not been a problem.

  4. Good to hear that it turned out well despite the slight slip-up. Both cold and warm roofs have their own advantages and disadvantages. Cold roofs need ventilation and aren't recommended for extremely humid areas, whereas a hybrid warm roof won't need ventilation. Todd @ ABImprovements.com

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  6. Wonder if you could impart some advice regarding the hybrid warm roof...
    I am in process of insulating my summerhouse roof. At present i have 50mm cellotex between rafters all taped and this leaves a 15mm gap (not enough) above boards which is poorly vented if at all. Above this is a T/G roof covered in felt.
    Throughout winter summerhouse cosy, but now after a few warm days noticed condensation on underside of cellotex at highest point and on removing cellotex some condensation in the 15mm gap. Obvious that the attempt at cold roof is feable so I need to do something about it. I am having the TG roof overboarded and a rubber edpm roof installed in next few weeks so need to rectify or do something before then...do i..
    1. install a warm roof system on top of roof...membrane over t/g roof then cellotex/then osb/rubber but height then goes up or
    2. try and create a hybrid roof as you have done? how would i do this? staple a membrane on underside of T/G roof and push cellotex right up against? then ply the ceiling so the 15mm gap (rafters 65mm deep) is between ply and bottom of cellotex? or does that need filling?
    the rubber roof would then be installed above... osb on top of T/G then rubber edpm or would another membrane be worth adding between osb and T/G roof?

    any advice welcome thanks Tim

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