Dormer roof/bedroom insulation/air leak issues.

paulcam

Hi,
I live in a 1960/70s chalet bungalow.  It has a full rear of the house, but a dormer for the master bedroom.
Last December I had loft insulation installed.  Done to building code this involved adding 6 ventilated slates to provide anti-condensation venting in the attic space.

The trouble is the dormer has an airgap deliberately installed along the window sill to ventilate the dormer and stop it rotting.  Of course this air gap vents the bedroom into the dead space in the front of the roof... which has a clear path up the rafters and out the ventilated slates.

Standing in the bedroom, even in the middle of the day, you can literally feel the cold air coming in that airgap and then running across the floor.  Any clothes on the floor are freezing cold to touch and not pleasant to try and put on.  At night the temperature can get as low as 13*C (for an ambient outdoor temp of 6*C) unless the heating intervenes.  I have set this up with a minimum of 16*C on the thermostat and it took 2.5 hours heating over night to keep that, even with me in the room.

I spoke with the insulation engineers who did my loft, but they, after a long discussion and lots of thinking came back to, "We don't know."

The issue is that the roof needs ventilation, so the vented slates have to stay.  But the bedroom is venting into the same space.  I can't seal the dormer for the same reason the attic is vented... it will collect condensation and eventually start to rot.  So it's a catch 22.  As the engineer pointed out... someone must have come up with a solution to this.

So I'm asking here if anyone has suggestions.
The best I can come up with is:
1.  Insulate the lower part of the roof the "deadspace".
2.  When putting the plasterboard back in after the above, add a vent which can be opened or shut and seal the existing window sill gap.  On cold days I can close the vent to keep the heat in but open the vent as often as possible.  There is still a risk though of sweating the dormer over winter.

paulcam

I tried to create a diagram showing what I mean as it's a bit confusing.
The part of the loft outlined in orange is insulated on it's lower side with 300mm glass wool.
The part of the loft outlined in blue is inaccessible and not insulated.
The blue line along the window shows the air gap for the dormer.
The blue arrows show the ventilation path which leads directly from my bedroom into the upper loft and out the vent.
It should be noted that the ceiling just inside the front door is stone cold to the touch, that bit of loft is COLD.

An additional bit of info, I keep thinking the bedroom window is open because the sound of the street outside is very clear and not muffled ... exactly like you had the window open.

Jeepers_Creepers

This sounds absolutely bizarre to me - there's an open vent on the inside bedroom wall?! Could you show this in a photo, please? 

The soffit along the front - above the window and door - is that ventilated with either regular round vents or - better - a continuous grill? Surely this would be enough to keep that lower roof section well draughty and dry?! If absolutely necessary, a couple of tile vents added - one just below the dormer, one to the right of the dormer roof top - would allow a through-flow (tho' the actual spec would need confirming)?

Then - once you can completely the need for internal ventilation, line the internal surface of the 'external' bedroom wall (ie, the section under that dormer window) with insulated plasterboard; even the thinnest will transform the insulation value of that room.

paulcam

Hi,
So this is the airgap on the window sill.  Excuse the dust!

And although not very clear, a thermal image of the same area.  You can see the dark line along the window.  The curtains on the left are 20*C probably warmed by the sun, the rest of the wall is 16*C and the heating is on.  Ambient temp in the room is 15.7*C, the 16*C on the window is just a reflection of the room, glass reflects IR.
By contrast I'm in the office next door at the back of the house and it's 21*C with only occasional 5 minute pulses of the radiator every few hours.

paulcam

This is the temperature in the bedroom for the past 24 hours.  You can see where the heating intervenes at bedtime and how fast the room cools without heating.  The minimum temp on the thermostat is 15*C, except when I boost it to 17*C at bedtime.  I like the bedroom cool, but not 14*C cool!

Jeepers_Creepers

Bizarrer and bizarrer.

Ok, I simply cannot accept that that air gap on your internal sill is deliberate! That's just crazy - NUTS. 

The tiled roof to the right of the dormer is continuous from the fascia board up to the ridge tiles, so why was it possible to add insulation to the upper section and not the lower? Even if access was difficult, surely it's the same rafters that run from top to bottom, so have repeated 400mm/600mm gaps between them? Ie, air entering via the soffits (you haven't told us whether there are vents there?) will be making its way under that tiled roof and out the vents added along the ridge - or wherever else these 6 'vent slates' were added.

Basically I'm saying that your roof void is almost certainly fully ventilated as it stands, from bottom to top, and no stupid gaps are required leading in to your house - I have never heard of such a thing. 

If this was my house I'd remove that sill to expose the gap below it. I'd also remove a section of wallboard below that window - a piece between two upright studs - easy to replace afterwards. 

From this you should have a good idea of what's going on in that triangular roof void. I expect it can be summed up as 'draughts', which is good. Assuming it is like this - plenty of air passing up from the soffits and gaps continuing upward to the roof apex between the rafters, then you can conclude that void is very well ventilated and requires nothing else. 

I'd then add loft insulation on to the downstairs ceiling part above the front door and the front room. Take this right to the front edge but DON'T block the airway gap along that front edge - stop short of there.

I'm assuming there's no insulation in that wall below the window either?! If not, then if there's room in that triangular void to work from, add Celotex-type insulation between the upright studs of the bedroom wall along that side. Cut this to fit tightly and neatly, and knock it in tight against the plasterboard. If it's easier, you could use 'loft' type insulation, but this would need something like mesh or netting to hold it in place between the studs. Ditto to the part you've opened, and then replace the p'board, fill sand and paint. 

If insulating that wall is not possible from behind, then simply overboard it with insulated p'board - even the thinnest stuff will make a huge difference.

Finally stuff the gap under the sill and replace it.

FreeBear

Jeepers_Creepers said: I'm assuming there's no insulation in that wall below the window either?! If not, then if there's room in that triangular void to work from, add Celotex-type insulation between the upright studs of the bedroom wall along that side. Cut this to fit tightly and neatly, and knock it in tight against the plasterboard. If it's easier, you could use 'loft' type insulation, but this would need something like mesh or netting to hold it in place between the studs.

Celotex type boards are nearly twice as effective as fibreglass/rockwool. I suspect the dormer framework is constructed from 3x2 timbers, so space would be limited. Cut the boards for a tight fit, and any gaps could be filled with expanding foam (use an applicator gun rather than a hand held canister). Seal that gap along the sill with a bit of foam while you're at it.

paulcam

My understanding was, that if you sealed the dormer window the condensation produced on the window would just add trapped humidity which can result in the dormer 'sweating' and eventually rotting.  Remember it has a fully sealed roof and walls. I read several horror stories online.  Can't confirm them or even say I fully understood.
I just assumed the gap on the window sill was to allow the condensation to flow down into the dead space where it could evaporate away.

The soffits are just white plastic slats which look like they are clicked together like floorboards.  So no grills or vents.

If it's possible/wise to seal the dormer I can certainly get someone in to insulate the dead space a bit, seal off the lower ceiling with insulation to prevent cold air running between the upstair floor and downstairs ceiling.  Oh and run a bead of silicon along that gap, or put a condensation catch sill in.

For now, in winter, I'm hiding in the Office with the thermostat on 21.  The rest of the house is set at 16 or 17 unless I'm going to use it.  This is simply to keep my heating runtime to under 5 hours per day.

paulcam

Oh and on, how possible it was to insulate the upper loft.  The appeared to just roll out glass wool across the upper attic and made no attempt to stuff any insulation down the lower roof rafters.
I am only after making a huge mess trying to seal a huge 6" and 2 foot air gap in the airing cupboard as it was an open vent into the attic... so they even missed putting insulation over that or sealing it.
(The mess by the way was trying to use expanding foam ... which people told me would stick.... it didn't.  I got in my hair, all over my clothes, had to use acetone to clean it off my arms.)

paulcam

To summarise the house.  It was well presented, the kitchen and bathroom modernised, was turnkey with most kitchen appliances coming with the house, the large red flag on it was the energy efficiency rating of an F.  I went for it.  I'm trying to raise that as much as I can without a complete renovation.  So far I have:
Converted the 1980s oil burner to gas combi.
Replaced some not all old single rads with convector rads.
Added TRVs
Automated the heating and install sensors in the rooms.
Insulated the loft.

Next on the list has to be the dormer or the dead space (eaves void?).

To give you an idea of how badly the house is sealed...  both the "hot press/airing cupboard" which now contains the boiler and the electric cupboard under the stairs... can and do get swung open occasionally in gale force winds.

paulcam

Sorry to spam, but... the sill might be a red heron caused by my assumptions.  When I read that sealing a dormer caused damp to build up in it and rot it I assumed they meant the dormer and that explained the gap....  reading back through those posts it seems they may have been referring to the flat roof alone getting completely sealed up and thus rotting the rafters/joists in the roof... not the dormer itself.  So there is probably no reason not to seal that gap.

The heat loss I'm experiencing this year in the dormer is most likely the result of the ventilated slates added causing the dead space and the dormer floor to be effectively open to the outside.

So the solution would be to just pay someone to take out some plaster board to allow access to the eaves void and insulate it and the floor joists as Jeepers_Creepers outlined.

Thanks!  Not sure when I will tackle it or how much it will cost and ... I'll have to redecorate.  But, gas isn't cheap and unnecessary CO2 emissions are something we should all avoid as much as we can.