Retrofit flat roof insulation

[cye]

here i set out to address a freezing living room in our 1955 house. the suspected culprit is a 1966 extension using a flat roof.

traditional tar felt waterproof deck with a slight fall (4-6") away from the back wall of the house towards the rainwater guttering.

the best practice or textbook approach to dealing with a cold roof with an old flat roof is to upgrade with a warm roof, i.e. insulation installed on top of the waterproof deck. in such cases ceilings are not interferred with & insulation installed above the waterproof deck. this is also known as a 'warm roof' or 'warm deck'.

in my ignorance some 7 or 8 years back, as the felt was intact and still waterproof, i simply stacked 2" of polystyrene on top of the old felt and added an additional layer of pond liner to the top. pleased with myself initially having devised such a low-effort / zen solution, my self back-patting subsided in winter as it became apparent that this had made little or no difference to our barely habitable icebox. yes, it reduced the risk of a leak, but no obvious improvement to the heat loss problem. knowledgeable folks told me the issue was with having used only 2" of polystyrene, which is only half as good as polyiso, and that furthermore, even 2" of polyiso would not have been enough, & that i should have used 3-4" of polyiso foam board.

in my case, i now know that this would have made little difference either due to unwanted ventilation. For retro flat roof improvements, i believe there is strong case for either removing the ceiling for thorough investigation of the ventilation paths, or very thorough investigation with a boroscope/endoscope before determining the best way to insulate.

this first pic shows the ceiling removed sufficiently to expose the outer of two RSJs used to support the opening in the (original) back wall of the house.

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[cye]

here's one of the whole ceiling removed

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[cye]

and here's a pic which shows (not very well though) why i feel vindicated in having removed the ceiling for inspection.

the pic shows the inner and outer RSJs. The broken edge of plasterboard ceiling remaining is half covering the inner RSJ.
The silver/grey material between the two RSJs is bonded bead insulation which has fallen down through the cavity (it's a cavity walled house BTW).

what the photo doesn't show is that the two RSJs are at different levels, with the outer RSJ being lower and fitting snugly to the plasterboard. the inner RSJ has been set higher, so there is a 1" gap between the plasterboard and the bottom of the inner RSJ. the builder who constructed this had created an enormous ventilation gap, whereby the area above the ceiling of the old (pre extension) living room and below the floor of the room above had been ventilating into the cavity. this has undoubtly contributed to the heat loss problem.

had i removed the decking from above to install a flat roof, and even if i had thoroughly studied and blocked off all unwanted visible ventilation underneath, i would never have seen this huge weakness in the building's airtightness.

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[cye]

here i am insulating in traditional 'cold roof' or 'cold deck' fashion. i decided to go with a cold deck because the flat roof is bounded on three sides by other awkward roofs and walls which would make installation and weatherproofing of additional insulation above the deck very tricky. had i had a roof with three free sides i would have applied the insulation on the outside above the deck to make a warm roof. to use insulation at the thickness i wanted, the only space for it was below the deck, hence the cold deck approach.

you will see that i have left a 2" gap above the insulation and below the roof deck for ventilation, and i am now fitting as much polyiso between the rafters as i can. i used treated roof tile lath tacked to the sides of the rafters, immediately underneath the underside of the roof deck, to help maintain the 2" gap. the lath also makes for a good ledge on to which foam can be squirted, and then when the foam board is offered up this gives an extra seal.

the polyiso foam board is cut so that it is a tight fit, and any gaps remaining are filled with 'low expansion type' squirty foam. i use insta-stik by dow corning, a low expansion foam which is a very good adhesive. other folk use soudal. i use a screw-type professional gun for the squirty foam rather than the throw-away cans pre-fitted with a straw. The pro guns are a hundred times better than single-use cans, as the former offer far more control than the latter. Also, unlike the DIY cans with the straw, you don't have to use all the can in one go, and the gun can be left uncleaned for months, provided there is some pressure left in the can, without seizing up the works.

So, top tip#1 for expanding foam is to use a professional type gun and the screw type cans instead of the DIY/straw cans. Top tip #2 for expanding foam : if you have females in the house, nail varnish remover (acetone) is great for clearing up the inevitable expanding foam accidents.

as the 'flat' roof has a slight fall, i am only able to get 150mm board in the area closest to the RSJs, then this will drop to 125mm, and then finally 100mm at the guttering end. i will then apply more poliso below the rafters to avoid the cold bridging which would otherwise be caused by plasterboard being tacked directly to the underside of the rafters.

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[cye]

and here's one of the mid section of the roof, where i can manage only 125mm below the 50mm air gap. i am using a 25mm sheet and a 100mm sheet to make up the 125mm.

caveman had mentioned that a better job of sealing foam board against rafters could be made when building the thickness up using a sandwich of several layers of thinner board rather than a single thick board, and there is definitely a case for this. the side walls of rafters are never vertical, and foam boards can never be cut very accurately, so the sandwich approach allows for more opportunity to seal the insulation against the rafter side wall than the use of a single thicker board would allow.

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[cye]

Ventilation - This requires careful consideration when insulating a flat roof.

Firstly I am ensuring that there are no unwanted ventilation paths from inside the house to the flat roof.

Equally importantly, for a cold roof like this, whilst I had built in the required 2" gap above the rigid foam insulation, i now needed to ensure that this gap was adequately ventilated.

UK building control, apart from Scotland (which mandates all new flat roofs to be warm decks), requires a 2" minimum gap above the insulation and below the deck, to be ventilated on two opposite sides of the roof. given that three sides of my flat roof are bounded by walls/roofs etc this presented a headache. standard soffit ventilation only being possible on one side (guttering side), i needed to find of means of ventilating the roof on the side which joins the back wall of the original house.

with the usual heath robinson bravado, i was about to lash something up myself using lead, flashband, grill mesh, chewing gum, obtainium, etc., when thankfully Stu persuaded me to look for a manufactured vent and do a 'pro job'.

After much searching, I found just what i needed. Manufactured by CavityTrays.Co.UK the RAV-FL vent is waterproof and large insect proof, albeit still requiring some minor woodwork against the wall -
See attached diagram...

for the vertical wood shown in the attached i used a length of tannalised fence board (~ 6"X 3/4" in section), and this was held off the wall to create the vent space by some more roof tile lath (short 6" vertical battens every 2'). first i had to drill vertical ventilation holes in the old cant strip (the triangular sectioned strip used by the original roofs to chamfer the angle between the flat roof and the wall).

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