# Dormer bungalow - insulation under slope?



## usrbin (8 Nov 2007)

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
   \
   |\
   | \
   |  \
   |   \
   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?


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## sydthebeat (8 Nov 2007)

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....


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## quinno (8 Nov 2007)

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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## usrbin (8 Nov 2007)

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


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## quinno (8 Nov 2007)

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]


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## usrbin (8 Nov 2007)

Very helpful diagrams at those links - cheers!


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## sydthebeat (8 Nov 2007)

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.


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## apple1 (8 Nov 2007)

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


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## usrbin (8 Nov 2007)

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.


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## sydthebeat (8 Nov 2007)

if you insulate on the whole roof slope, you will need to ventilate the cavity spaces behind the stud walls.


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## usrbin (8 Nov 2007)

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


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## sydthebeat (8 Nov 2007)

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!!.


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## usrbin (8 Nov 2007)

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.


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## sydthebeat (8 Nov 2007)

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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## usrbin (8 Nov 2007)

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 |
  ---
 |

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...


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## sydthebeat (8 Nov 2007)

im an architectural technician.

yeah, my head hurts after reading that 

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

http://www.environ.ie/en/Publicatio...ng/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.


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## usrbin (8 Nov 2007)

I'm in your debt Syd, thanks again.


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## nutty nut (9 Nov 2007)

usrbin said:


> I'm in your debt Syd, thanks again.


Wait till you get the bill


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## sydthebeat (10 Nov 2007)

usrbin said:


> I'm in your debt Syd, thanks again.


 
nae bother....

who do i send the invoice to ??





(kidding!)


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## usrbin (13 Nov 2007)

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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## usrbin (14 Dec 2007)

One question I forgot to raise - what kind of insulation should be put down here?  

I guess ideally polyboard should have been put down and screed poured over it, but failing that, would foil-backed kingspan or similar, on the flat, be the best bet?  

Or maybe attic-style wool insulation?

In either case, it is sufficient to just lay it down and spread it out or should it be overlaid with something else - maybe fixed with timber in some way?


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## sydthebeat (14 Dec 2007)

where exactly are you talking about??


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## usrbin (14 Dec 2007)

Well, I'm referring to the previous conversation (in which you were v. helpful) regarding insulating under the slope of the roof of a dormer bungalow, i.e. the inaccessible area "hidden" by a stud wall....  

... and I'm under the impression that insulating the top of the hollowcore (which is at the heart of the issue, I guess - if we had timber joists there'd already be insulation there) is the way to go.  

SO my follow up question was just, assuming I wanted to insulate the currently UNinsulated hollowcore on the flat, what's the best material to do this with?

One additional thought: would it be an option to dryline the ceilings underneath the uninsulated hollowcore ceilings?  I.e. there are habitable rooms directly underneath.

Sloping roof 
 over
Hollowcore
 over
Downstairs room

(See diagram in original post)

Thanks!


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## sydthebeat (14 Dec 2007)

to meet current regs you need to incorporate 150m of PUR (kingspan etc) or 300mm quilted insulation at the horizontal ceiling. I would personally insulate all the roof on the slope.. i dont know how practical this is for you right now, but IMHO thats the best option.
I assume the wall plate is on the hollocore slab... and that there is insulation at the edges around the conc slab...


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## usrbin (14 Dec 2007)

sydthebeat said:


> I assume the wall plate is on the hollocore slab... and that there is insulation at the edges around the conc slab...


I'll have to check that, but I think so.

Drylining from underneath wouldn't be any good, then?  
(Given that my real concern is the heat from the groundfloor living rooms underneath which is rising through the hollowcore ceiling and straight through the uninsulated cavity space and out the uninsulated sloping felt)


Cheers!


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## sydthebeat (15 Dec 2007)

i think the best option is to create the situation where the hollowcore slabs are 'inside' the insulation envelope. 

Ive drawn up a little section to describe it better....
http://www.4shared.com/dir/4960625/6cc4047/sharing.html

check out the eaves... thats the way i would do it.....
for the following reason alone....
if you plan to use the cavity space behind the stud wall, then this construction is better. You are not compromising the insulation envelope by adding doors etc, and you retain the space over the slabs...


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## usrbin (17 Dec 2007)

Syd, you've gone above and beyond the call of duty with that drawing, thanks so much.  You're a gent.

I can fully see that is the correct way to do it and how it should have been done in the first place.  Unfortunately the man who built this house seems unable to understand concept of a heat envelope and so the slope has been sealed (i.e. no access, stud walls are closed), so it would be difficult and intrusive to get in to insulate the slope now... 

... which is why I was wondering about dryling from underneath.  Even as a short term solution, would drylining be any good at all, do you think?


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## sydthebeat (17 Dec 2007)

in order for that section of ceiling to comply with building regulations you would have to 'dry line' with at least 150mm or rigid board insulation. this may be undesirable in the rooms below.
I would incorporate 150mm insulation over the slabs... and ensure you get insulation where the ends of the slabs meet the eaves.


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## usrbin (17 Dec 2007)

Fantastic Syd, thanks for the info, you've clarified everything in my mind, very much appreciated.


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## ronny78 (5 Nov 2009)

Hi !

I've read a number of threads on here about insulation, but I'd really appreciate if someone can just sanity check this for me so I don't make a costly mistake.

We have a dormer house with access to the crawl spaces via doors in each of the upstairs rooms and on the landing. They were left so we could use the area for storage. If we want to do this I think we need the crawl space to be 'warm'. We have the plasterboard short dormer wall insulated at the back between the joists with fibreglass but as we are in an exposed place regardless of what we do with the doors we end up with a gale coming down the stairs to meet us.

So, I was going to put 60mm of Kingspan between the sloping roof joists in the crawl space and then seal along the bottom (where the board will rest on the cavity wall) with expanding foam or silicone. Will probably use additional fibreglass to seal the top of the board if necessary where it disappears into the sloping wall/roof section of the dormer (this has aeroboard in it - pre stricter regulations). Does this sound right ? The roof joists are 150mm deep so plenty of room at the back of the insulation for air circulation to attic.

I was thinking we would have room for 100mm Kingspan board but thought the 60mm would be easier to fit - no point in fitting it if we can't get a good fit. I was hoping the 60mm could be cut with a good knife rather than a saw ?

Anyone done this ? Advice greatly appreciated !

Thanks is advance,
R-


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## galwaytt (6 Nov 2009)

ronny - sounds like my old house.  I insulated and slabbed BOTH sides of the dwarf wall........and it was still a wind-tunnel.

This is to be expected, really, as the wall/roof junction is.......wide open - it's not airtight.

What I would do is this:  insulate between the rafters, in the crawl space, from the top of the dwarf wall down as far as you can (usually just past the line of where your crawlspace floor meets the rafter....).

Then, get some airtight membrane (Intello, CIGA, etc), and line the back of the rafters and onto the 'mini-gables' at the ends of those spaces, back onto your dwarf wall.  Do the back of the Dwarf wall too, if you can/want to.   Then, seal all the joints in the membrane with an appropriate tape (again, Intello, CIGA etc).

Almost there !

Ooops- forgot  - is your crawl space floored ?  If not, insulate/membrane/etc, as above.   Then floor it.  I sheathed the bottom of my rafters with light ply as well, inside the crawl space, so that the whole crawl space has a solid 'roof', floor and dwarf wall.

Finally, use a good draught seal on your hatch doors.


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## ronny78 (7 Nov 2009)

Hi Galwaytt - thanks for your reply.

Yes there is a floor in the crawl space and there is insulation under it between the floor joists so although we are still no doubt losing some heat from downstairs this hasn't been an issue as downstairs is very warm. 

In your old house, did the outside wall of the house extend up into the crawl area ? We have about 2 feet of the concrete cavity wall all the way around. Will the Intello tape stick to the block when Im fitting the membrane over the rafters and kingspan or is this a job for silicone to keep the membrane in place ?

Thanks again,
R-


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## seantheman (7 Nov 2009)

ronny78 said:


> Hi !
> 
> The roof joists are 150mm deep so plenty of room at the back of the insulation for air circulation to attic.
> 
> ...


 Kingspan/Xtratherm etc. manufacture a rafterlock type product that is 100mm/150mm and is precut to suit rafter, accordion type that you squeeze,fit between rafters, and it expands to give snug fit. A bit more expensive than 8x4 sheets but a lot less messing and waste.Should cost roughly €85 vat inc for 4.44mmtr sq bale of 100mm  [broken link removed]


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## ronny78 (9 Nov 2009)

Hi Seantheman,

Thanks for your reply. That sounds like the product for me. The only thing putting me off going for the 100mm insulation was thinking that I might not get a good fit and it would end up having been a waste of money. I think I'll order a bale of that and see how I get on. I can assess any draughts at the top of the cavity wall once I've made a start on the insulation. I may end up back here again for further advice !

Thanks again,
R-


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## galwaytt (9 Nov 2009)

Ronny - there was only 1 row of blocks above the joists, at the outside of my crawlspace.   SIGA airtightness tape RISAN iirc, will stick, with a primer.  But you should the membrane down over the block, on to the crawlspace floor.  I think SIGA have a gun-gum you could use to tack the sheet to the block as well, just to stop it falling in.


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## Coolaboy (10 Nov 2009)

I have a dormer bungalow. The question is "what depth of insulation should I have in between the joices between the ground and dormer floor?


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## onq (10 Nov 2009)

Coolaboy said:


> I have a dormer bungalow. The question is "what depth of insulation should I have in between the joices between the ground and dormer floor?



I hope I'm reading this right.

You seem to be talking about the First Floor.

No insulation here, unless your're using it for sound insulation.

Thermal insulation is only required for the "outside" of the habitable rooms, not between them.

So the lowest floor requires insulation between the rooms and the ground itself and the upper floor requires insulation between the bedrooms and the air outside.

HTH

ONQ.


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## Coolaboy (10 Nov 2009)

Thats very interesting ONG. I was going to insulate between the floor joices. If I were to do this, what would be the outcome? Does it stop warm heat rising up through the ceiling into the dormer. Is that one of the reasons why?


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## galwaytt (11 Nov 2009)

It's still worth insulating the joisting out by the wall. 

If you think about it, that space is usually not insulated at all - any heat that does find it's way in there will have free access to your (cold) outside walls, for the height of the joist - anything that stops that is a + in my book.


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