Insulating a Dormer

Redjeep!

Hi Folks,

I've got a dormer bungalow (purpose built not a conversion), thats about 5 years old.

It's generally well built, but I think it needs more insulation.

Problem is if you go upstairs in the winter it's absolutely bloody freezing - it must be at least 5 degrees cooler than the downstairs, and in the summer it's boiling. The downstais typically keeps a much more constant temperature.

The loft is insulated with about 4 inches of 'rockwool' style insulation on top of about 2-3 inches of the solid pink foamy sheet stuff. Behind the upstairs stud walls are about the same and the joists above the downstairs rooms are insulated the same way, but only outside of the upstairs rooms (hope that makes sense - what I mean is that there is no insulation between the upstairs and downstairs rooms).

Any ideas ? One website suggested that the gaps at the ends of the joists should be blocked up, so I tried it partially but it made no difference. I've also added a further 4 inches of rockwall around a couple of the bedroom stud walls (in the crawl space), and some more above the ceiling joists for the downstairs rooms. I've also floored the attic and added another 4inches of insulation under the roof joists (I left some air gap). There always seems to be a huge draft through the area and I kinda feel that this should be restricted.

I've added about 8 rolls of insulation (the 4 feet wide stuff) to the house without much of a difference). I'm trying to avoid this becoming my lifetimes work.

There's no obvious drafts from anywhere and all the windows are good quality double glazing.

rooferPete

Hi,

Gaps at the ends of the joists should not be blocked up or you could find further problems later, the roof space is supposed to be vented.

What you could try is using another layer of 100 mm insulation and basic plastic sheet behind the studs and seal any joints with silicone, an ideal product is a vapour permeable membrane similar to Starex.

I know the vapour check is supposed to be fitted behind the plaster slab but it's too late for your house now.

Add another 150 mm of insulation to the attic space, it should help to keep in the heat and prevent the dormer from overheating.

Was the slope of the roof / ceiling insulated ? it might be possible to have fibreglass blown in between the joists.

The same could be done for the floor although insulating between the floor and ceiling will possibly have a better effect on noise being transmitted than insulation values.

.

flocker

I've added about 8 rolls of insulation (the 4 feet wide stuff) to the house without much of a difference).

The radiators in these area's may also be undersized. Trying to minimise drafts and increase insulation would be the way to go.

Is their a lot of soffit vents. Pete maybe able to indicate what should be normal.

Redjeep!

Thanks for the replies, much appreciated.

The area that I blocked of was between the upsatirs floorboards and the downstairs ceilings. This was after I read that the ends should be blocked on a USA Dept. of Energy website (shows you how anal I've become over this !).
I figured that the soffit vents are causing cold air to pass between the upstairs and downsatirs rooms along the main joists. There's very little insulation between the upstairs and downstairs rooms.

The area is well ventilated.

........./
......../ Attic
......./======================================
....../.|| Bedroom
...../..||
..../...||
.../....||
../.....||
================================================
.......## <--- BLOCKED OFF HERE
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|| ||
|| || Living room


Pete - are you saying to add the insulation around whats already on the stud wall and then place the poly sheeting on top of it all (so looking from the outside in, its poly sheet, new insulation, old insulation, plasterboard? ) or the other way around so that the poly sheet is against the plasterboard ?

Would it be possible to insulate the joists in the roof, immediately behind the slates rather than laying in top of the other insulation ?

Thanks again,
Andy

rooferPete

Hi Andy,

Gotcha now, I thought you were blocking the air space at the other end, although air should still be able to pass across the ceiling it does not have to be a big gap as much as the depth of the joists.

Since you are fitting the insulation after the house is finished I would stand / hang the extra insulation along the dwarf walls, the sheet you use now is more holding the insulation in place but it should be able to breathe.

If I understand you correctly what you are thinking is to fit the insulation under the rafters ? (roof timbers sloped), You won't get much of a result for your money that way because the heat will have already passed through the walls and ceilings.

Your drawing is showing more a "Mansard" type roof than a dormer bungalow because it appears you have no ceiling area sloped in the upstairs rooms ?

I'm not trying to make fun of your drawing it's a lot better than I could do

What I was referring to about blown fibre insulation is if there are sloped ceilings they may not have been insulated properly and blown fibre may be the best way to insulate the sloped areas.

I too have a dormer style house (second owner) and I found the insulation on the dwarf walls although to regulations was not enough to keep the heat in, also some wise guy had left out the insulation on the sloping ceilings.

I didn't stay exactly to regulation when I replaced the soffit and settled for 50 mm insulation strips every 2 m because air can enter the roof space between the tiles and it does find it's way in at the overlaps of the underlay.

I do know that my own house made me a fan of the system Lastbuilders used in his house, sadly it's too late for us unless we want to start a major renovation job.

.

Redjeep!

Hey Pete,

Thanks again.

My drawing was wrong in that there is a sloping area in the upstairs rooms, behind this it is insulated with rockwool fibre.

I've started adding some more insulation on top of the old stuff behind the stud walls, but it doesn't seem to have done much. My concern is that the ventilation is too much as sometimes when I'm in there, there's a huge draught and so was thinking about blocking about half of the soffit vents. That was one reason why I was wondering about using the polythene sheet to avoid the warm trapped air in the insulation from being blown away ! I'm not sure if wrapping it over the insulation is OK though.

So what method did Lastbuilders use ?

Cheers.

Viking House

Hi Redjeep
What you are describing is a problem they discovered in Scandinavia in that the wind venting the roof was blowing all the trapped heated air out of the rockwool.
What you need to use is a strong breathable insulation material that is resistant to wind blow like softboard or Paroc hardboards. (Wind stop technology)
What they say in Scandinavia is that 1mm of softboard used as a windstop gives the same effect as 10mm of Rockwool.
Its a bit like wearing a wolly jumper on a cold windy day and the wind blows straight through your jumper and you are cold.
You have two options, put on a second jumper (20mm) or put on a tightweave windbreaker (1mm) over the first jumper. Both have the same effect.
We use windstop technology in our projects with 40mm in the roofs of our houses and 35mm in the outside walls of our timber frame houses.
This enables us to allow extra air flow through the roof by crossbattening which keeps the roof wood healthier.
There is more info on my website www.viking-house.net and there is a link in the Viking hi-tech page to the Paroc website.