Dormer bungalow - insulation under slope?

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usrbin

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Another question re insulation - I don't believe this exact permutation has been covered before and value any input.

In a new build dormer bungalow, surely the inside of the slope should be insulated with foam or something similar, even if it is "hidden" by an internal upstairs wall and inaccessible from the interior?

e.g.
a
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x y
(cross-section, where x is internal wall and y is slope of roof - obviously angle is wrong!)

y is insulated with foil-backed plastered board over foam (between the roof joists) ABOVE the junction with wall x, i.e. from point a upwards. However it isn't insulated below that, i.e. the lower portion of y, outside/right of x.

So you still have an envelope of insulation in the upstairs, I guess, but what about the room directly underneath, downstairs, which obviously reaches right up to the external wall (which terminates directly beneath the bottom of y) and therefore has zero insulation above it?

So, should the whole slope of the roof be insulated?
 
you can do it both way, you can insulate the whole sloped roof plan, or you can insulate between the floor joists, up the stud wall and up the sloped dormer roof....
 
In a dormer, the insulation follows the slope of the roof. Consideration will have to be given to ventilating the space between the internal lining and the outer roof, as well as vapour barrier above the ceiling finish (i.e. if a plasterboard finish, a vapour check goes above this).
 
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Syd, sorry, I neglected to mention - the house has hollowcore floors upstairs, so there are no insulated floor joists and the upstairs walls are made of block - therefore they're not insulated either!

Quinno, I'm not sure I understand you fully, but we do have vents in the soffit , if that's what you mean - should be okay as long as they're not blocked by insulation? Regarding the second point, "vapour barrier between above the ceiling finish" do you mean plastic sheeting of some kind behind/above the foil-backed plasterboard?

Thanks for speedy and helpful advice BTW
 
Knauf are a reputable supplier of drywall (plasterboard) and insulation. Check out this link - it probably illustrates better what I was trying to describe:

[broken link removed]

[broken link removed]
 
usrbin, you can still insulate over the hollowcore slabs, depending on your eaves detail, but the best thing to do is to insulate up the roof slope completely.

have you passive or mechanical ventilation?

quinno point about insulation up the rafters i sthat you need to retain 50mm gap between the insulation and the felt for ventilation.
 
Good links quinno. I've an interest in this post as I'm building along similar lines, though with floor joists. Should insualtion be placed between these, thus preventing heat rising from the GF to loft space or should they only be partially insulated to allow the loft space benefit from heat below? Not sure if it makes a difference but installing UFH on GF and leaving provisions for a few rads in attic. TF build, well insulated. Thanks, apple1
 
Syd, thanks again for getting back, and for clarification.
Ventilation - passive, I guess: there are three rounded rectangular raised bumps on the back of the roof, evenly spaced from side to side, plus vents in the soffit.

I don't suppose the hollowcore (which is six-inch) has any significant insulation qualities on its own? Looks like I should definitely insulate either above that on inside the slope.
 
if you insulate on the whole roof slope, you will need to ventilate the cavity spaces behind the stud walls.
 
Okay... but bentilate how?

Those three raised-bump vents and grille-style vents in the soffit wouldn't be enough, then?

(On a related note, what happens if there isn't enough ventilation? Could this by any chance produce loud thunks, like a timber creaking heavily, due to an over-accumulation of heat? Because we're already hearing this in one side which IS insulated...)

Thanks for continued advice
 
If you insulate over the floor slabs, up the stud walls, up the sloped ceiling plane and between ceiling joists.. then that cavity space im taking about would be cold (ie not within the thermal envelope)..

but if you insulate from the eaves up the slope of the roof, then this space is warm and needs to be ventilated to avoid problems due to condensation and mould. I would simply include 2 no. vent grills (probably 30 x 100 or similar) in the stud walls so that the space is ventilated back into your habitable rooms. Its imperative that these rooms have passive ventilation (ie holes in external wall with vent grill over)... do they??? because they should!!.
 
Okay, great, gotcha now. The house is already finished, actually, so we'll just do one or the other surface: probably inside the slope, I want to get that area into the envelope, mindful of the GF rooms underneath - which as far as I can tell are bleeding heat straight up through the hollowcore then out through the felt/slates.

The walls inside are block walls, BTW, not stud. Passive ventilation is actually through trickle vents in windows.

Your expertise and feedback much appreciated.
 
are the smalls wall of the dormer rooms block??? ie the wall above the x in your first post?? they are usually timber....
 
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Good question - should have been more specific originally but didn't want to bore anybody either :)

That diagram (from first post) really only shows the back of the house in cross section and yes, that wall is block. It runs the entire width of the house apart from an airing cupboard/HP (which has a low stud wall at the back, as you correctly surmise) and an ensuite at one end (ditto). SO for most of the width of the house, there;s the roof, at an angle, then a block wall inside it (terrible waste of expensive hollowore floorspace, I know). And this space is already insulated, extruded poly between the roof joists.

The front has a low stud wall/purline type thing, and two dormer windows. That stud wall is insulated in the conventional way - but the hollowcore floor beyond it ISN'T, and like I said, that's my concern, that the rooms underneath are losing heat up through here i.e. here this x is:
. / |
/x |
---
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Just to complicate things, there's then a third section - the protruding part of the house's L-shape, which is like the back of the house in miniature i.e. two sloping roofs, a room and a half between them, bordered by, yes, block walls, not stud.

What you've said about overheating is very interesting and I think you may have solved the mystery of the nighttime thunk sounds which reverberate through the cavity at the back (which is already insulated and possibly not adequeately vented, having only those little grilles in the soffit.

Bet you wish you hadn't asked now ;-) Are you an engineer BTW? You know a damn sight more than the guy we had...
 
im an architectural technician.

yeah, my head hurts after reading that :D

Im not entirely sure what happening in your build, but heres a few tips;

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf

look at page 11 in this link, it shows the methods of roof ventilation and also how it should be insulated. Your cavity space behind the low walls needs to be vented if its within the thermal envelope. You can vent this back into the habiltale room, but your window trickle vents probably wont give enough. If your house is gabled, perhaps you should incorporate passive vents directly into these cavitys at the gable walls. Your soffit vents wont do because they are effectively venting over the thermal envelope.
 
Hehe. Reminds me of this, one of my fave lawyer jokes:

A doctor finds himself chatting to a solicitor at a party. He says, I hate these kind of affairs. People are always asking me for diagnoses at parties, once they hear I'm a medical man. What's your opinion on that? Is it rude to refuse? How would you handle it?

The solicitor replies, well, I would give brief but serious attention to the question and then later send them a bill.

Two days later, the doctor gets an invoice in the post for 150 euro...
 
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