hello i'm roger brisby from skill
builder and i just want to talk to you
about the difference between warm roofs
and cold roofs because this does confuse
a lot of people and it's really very
very simple now we're just doing this
orangery here and our intention was to
have a warm roof because a warm roof is
a lot easier in situations like this but
then we had a little word with the
neighbor and the neighbor wasn't too
happy about the height so we've reduced
the height because everyone wants to get
on with their neighbors and that means
that when we reduced the height we had
to take the insulation from off the top
of the roof and put it on the inside of
the roof thereby changing it from a wall
roof to a cold roof here we have a flat
roof these are the joists which you'll
see going through here number two joist
there so we've got a void and then we're
going to have plasterboard here on the
top we've got osb decking and on top of
that we're going to have our waterproof
cover all the way across there which in
this case is grp now
if we put the insulation on top of the
roof insulation being over there eco
therm we will put say 120 millimeters of
insulation on top of that roof
so we've got the insulation there 120 ml
of insulation there we've got a sheet of
osb going across the top of that
and then we're going to put our grp on
the top now
as the water the moisture the airborne
moisture in the house your
family of four a lot of moisture coming
out every day from the house and a lot
of it is migrating up through the
plasterboard
into this void here now if you've got a
warm roof you've got the insulation
there
it means that all this area in here
is warm it's it's within the heated area
of the house now that's the important
thing so as your airborne moisture is
coming up through here and it's hitting
the underside of the
insulation it's not condensing because
the underside of the insulation is warm
so most importantly what we put here is
a vapor barrier and that vapor barrier
goes all the way along the underside of
the insulation so that's on top of our
piece of decking that we've got here in
this case osb so the vapor barrier goes
over the top of the decking and the
vapor barrier has to wrap up the sides
and it has to be perfectly sealed all
around so in the case here where we've
got an opening we have to go around with
the vapor barrier tape it up the sides
make sure all the joints because any
moisture that comes through here
airborne moisture that's the vapor we
don't want that vapor migrating through
that board through the insulation and
sweating on the top side of that
insulation because if we do that we get
rot we get rot on this piece of deck in
here which is underneath the grp so if
you're doing a warm roof you put the
vapor barrier on the warm side of the
insulation but we're not doing a warm
roof because we have to lower it down
and that's very often the case people
haven't got unlimited height to to work
with so they've got to put their
insulation within the joists now this is
great because we've got a 9x2 joist here
we could pack that void full of
insulation except for one problem we're
still dealing with the same physics
nothing's has changed we've got airborne
moisture coming up here if it gets
through the insulation as soon as it
gets through the insulation it's going
to be on the cold side of the insulation
we haven't got this insulation that's
gone it's on the cold side of the
insulation because there's no insulation
on top there's our insulation 120 mil
say
and therefore our dew point this is the
point where the airborne vapor the
moisture condenses into droplets of
water our dew point is here our dew
point is always on the cold side of the
insulation wherever you see the cold
side of insulation what you've got to
think about is that is where the
moisture is going to form so what we
would end up with is a lot of moisture
forming here trapped under that osb at
the top trapped under that grp layer
with nowhere to go so in that situation
it is absolutely essential
no two ways about it essential that you
have ventilation
all the way through that void there so
if we've got say a 225 millimeter joist
there and we're going to put 100 and say
220 millimeters in there you can see
that we've still got ourselves a clear
100 millimeter space here to ventilate
all the way through now we need a
minimum of 50 millimeters really so 100
millimeters is good but we still need to
get a through flow of air
into that space and if we do that we can
clear that air we can clear the the
moisture as it comes in and condensing
we've got a nice draught going through
there and it will blow that moisture
away and on a warm day that will
evaporate as well so that will keep that
nice and dry i have a friend who bought
a house four years ago and he had a cold
roof in this situation and they didn't
ventilate the void and within four years
the whole thing had rotted that includes
the joists that includes the osb the
whole thing had just turned to mush so
the whole roof had to be stripped off a
big canopy pulled over and the whole
thing had to be stripped off and renewed
and this time they put ventilation in so
this is our insulation and in an ideal
world we would have put that on top of
the roof making a warm roof but we're
going to have to cut it and put it in
between these joists here and that means
that we're going to finish up 100
millimeters of insulation between each
joist
and then we're going to have to
ventilate across the front now we're
going to put what they call a soffit
ventilator in the front here which has
got those tiny little holes in it so the
air will come up and it will go through
this void here it will go above the
insulation here and we need that
draft to come all the way through
you can see that when we put our noggins
up we've left a nice gap at the top for
them and we need air to come out at this
end we can do that in several ways we
can build an up stand and put a little
ventilated top on it what we call an
over tile vent or in this case we're
probably going to end up doing it with
grp mushrooms which are going to be all
the way along in every single joist so
that we've got a nice through flow of
air through there and out of there now
when you look here you see another
problem and this is joyce going the
other way whichever way we had these if
we carried these joists on all the way
to there and put joist hangers in we
still would have finished up with this
dead spot here
where although we've got a soffit
ventilator at the front the air was
coming through here and it couldn't go
anywhere because it's on this trimmer so
you get that with roof windows if you've
got a roof window or in this case you've
got a lantern going there and you've got
these dead spots all the way around then
you have to deal with that now some
people would say okay just drill holes
through the joists and therefore you've
got a bit of ventilation going through
which you can do but we don't really
want to drill through these triple
joists with a load of holes so in this
case the grp guy again ben has got to
come up with something really clever and
he's probably going to end up putting
ventilators all the way along here
mushrooms just to ventilate this front
piece and again at the back because
we've got a dead spot at the back he's
going to have to put mushrooms in there
so this whole roof is going to be
covered with all his little pots all
these little mushrooms to ventilate it
whereas what we could have done in an
ideal world is a warm roof but that warm
roof would have taken us up another 120
millimeters high and unfortunately in
this case we can't do it so i hope you
understand that the difference between a
warm roof and a cold roof the warm roof
has the insulation on the top the cold
roof has the insulation underneath and
you need ventilation if you've got a
cold roof
one more thing you must have even if
you've got a cold roof is a vapor
barrier under here so you put your
insulation up you attach your vapor
barrier or you can use what they call
vapor check plasterboard which has a
silver foil backing on it and you screw
that plaster board up and that stops a
lot of that moisture from migrating up
into the roof so that's a very good
thing because then it leaves the
ventilation less of a job to do so any
moisture you can stop migrating up
through the roof you can then not have
to bother about removing a lot of people
will be very surprised to find how much
moisture will migrate through a sheet of
plasterboard it's a lot and then the
worst thing is that people then go and
put down lighters in there so if they've
got a vapor barrier they go and cut
loads and loads of holes in it put down
lighters in the down lighters get warm
they suck all the air up and they suck
all the moisture up so you're back to
square one so if you're going to have
down lighters make sure that a kind
where you can put the vapor checked
barrier over the top of the down lighter
so you formed a complete seal there
there are those kind of down lighters
around all you've got to do is look for
them so i hope you found that
interesting bit of a whistle stop tour
but come back and watch this project
because it will be coming up on school
builder soon we're going to be doing
this whole orangery you'll see it from
start to finish in all its glory
[Music]