Insulating Roof in old cottage

Paul Kiernan · 29-08-2015 12:16pm #1

I'm planning on insulating the roof of an old cottage. The roof is pitched with part flat, part pitched ceilings as can be seen in this photo -

Willow%20sitting_10412105834_zpsoba70801.jpg

Willow%20sitting_10412105834_zpsoba70801.jpg

The structure is heavy westmoreland slate on wooden lathes on 4x2 rafters and joists with plaster on lathe ceiling. See these pictures of inside of attic -

Roof%20007_zpswsxdycli.jpg

As you can see it's a bit of a mess!

I'm thinking of working from the attic space and pushing 100mm earthwool down into the sloping section from above. For the flat section I'll put 100mm earthwool between the joists and maybe 150mm over.

My worry is will I get condensation on the underside of the slates or will the earthwool allow sufficient air movement to keep it dry? The roof is over 100 years old and is perfect so I don't want to destroy it now!

Calahonda52 · 29-08-2015 1:45pm

Serious project ahead here!
Anyway
1: Air movement through the earth-wool will negate the value of the insulation so waste of time and money: there is enough written about this issue on this forum alone to fill the National Library...
try this while Mayo trash the Dubs tmrw

https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx

2: my guess is that both ceiling and slate plaster is lime based so different considerations apply: anything you do must reflect the existing materials and how they deal with damp so no gypsum plasters.
The material in the link does not cover lime based construction: there are some on this forum who know about this stuff: go look
3: how does roof finish on left: valley or flat.
4: the power cables need air circulation around them.
5: That bakelite jbox needs replacing.

how is roof space ventilated as of now?
whats beyond the brick gable?
Is roof hipped over the fireplace?
whats the item in ceiling beyond ceiling light?

Paul Kiernan · 29-08-2015 5:32pm

Thanks for the reply, Calahonda52. Plenty of reading there, I'll tackle it tomorrow!!

The roof on the left is flat (1980's flat roof extension which we hope to demolish). That wall is actually vertical over fireplace (optical illusion!).

The original building is 100ft long with gables at each end. There are a number of intermediate gables extending into roofspace (like the brick one in the picture). There are separate hatches to access each section.

There appears to be no ventilation whatsoever at the eaves! That object beyond the light is a vent into the roofspace. Directly above that is a large vent on the ridge, see this picture -

Willow%20front_10412115527_zpsttvd4frx.jpg

Willow%20front_10412115527_zpsttvd4frx.jpg

That appears to be the only ventilation in the entire roof. I assume a certain amount will get through gaps in the slates or will the plaster backing keep that out?

Calahonda52 · 29-08-2015 6:15pm

It look in great nick after 100 years, no botox there

My guess is that
the slated roof is pretty 100% wind tight
The attic hatches are not so the roof gets ventilated: up through the stack.
there is some sign of mould, the white patches on roof rafters as well as the general blackness.

what construct are the walls?

The issue here is if you radically change the thermal and humidity profile of the house through increased insulation and airtightness respectively, than if you don't understand your house then you may have some unintended consequences.

What head room do you have inside?

Is solar gain an issue or is the heavy duty slate good at decrement delay? [ Google it ! ]
Here comes the heresy!
As the risk of incurring the wrath of someone, maybe think about removing the plaster lathe on the slope and vaulted part of the ceiling,
put say 50mm of natural wood fibre insulation between the rafters on the slope and then another layer of say 50mm across the rafters on the slope on the inside.
Then a vapour control layer and plasterboard.
the flat part you can put more insulation from above: maybe loose cellulose.
The in-slope, between rafter, insulation would need to go up far enough to tie in with the cellulose.
Cables would need to be ducted to keep them cool: certainly the power circuits and cooker/shower/hob circuits

The idea her is that you maintain the head room on the flat part but narrow the flat part by maybe 50mm each side.

There are regs for this for habitable rooms: more later.

The soffits would need to be ventilated into the 50mm air gap between the wood fibre and the slates.
Attic hatches insulated and airtight

If the idea is of interest we can develop the actual details.
You will be told that kingspin and insulated plaster board is a cheaper, easily option to my suggested wood fibre etc.

Cheaper, easier yes but not the right materials in this case.

Paul Kiernan · 30-08-2015 12:34pm

Thanks again.

The walls are 18" thick granite (presumably bonded with lime mortar). Headroom isn't an issue, almost 8' at bottom of slope, 10' under flat ceiling. The slates are up to 1" thick so I'd imagine no problems with decrement delay there!

Just to add a little perspective - I'm currently in the final stages of preparing a planning application to demolish the flat roof extension, restore the roof and stone facade and add a large standalone extension connected to the cottage by a glass link. This extension will be insulated and airtight and will house the main living, kitchen and bathrooms.

I don't plan to change much in the old cottage. The plastered chimney will be removed (structural damage) and the associated internal gable wall (18" stone) may be replaced by a stud or block partition. There will be some boarding and plastering required but not a lot. Also, I'll probably rewire.

I have considered something along the lines of what you suggest (and have by no means ruled it out!) but the issues I see are:
1. Existing ceiling is in excellent condition and it just doesn't feel right to rip out something that a first class tradesman put in over 100 years ago.
2. Cost!
3. I'd probably wait until next year and do it as part of overall whereas a simple attic insulation I could do now (saving me over a grand in heating costs this winter).

I'm also conscious of the fact that the roof may be in such good condition precisely because it has effectively been heated all it's life! If I insulate and vent the roofspace I'll be exposing it to the vagaries of the Irish weather e.g. condensation from a sudden influx of warm, moist, atlantic air. But then again, if I fit a VCL do I even need to ventilate it? Or could I use controlled manual ventilation?

Calahonda52 · 30-08-2015 2:15pm

Final contribution here:
You must ventilate behind a VCL
Good luck

dathi · 31-08-2015 9:41am

"The soffits would need to be ventilated into the 50mm air gap between the wood fibre and the slates".
this will be impossible to do as the rafters will have been bedded into the stone wall up to the top of the rafter (fairly standard construction method for old houses in Ireland)
"there is some sign of mould, the white patches on roof rafters as well as the general blackness."
the white patches are common on roofs with lime parging to the underside of slates and not a sign of mould . and the black tendrils are just years of cobwebs catching atmospheric pollution .

Paul Kiernan · 31-08-2015 5:43pm

dathi wrote: »

"The soffits would need to be ventilated into the 50mm air gap between the wood fibre and the slates".
this will be impossible to do as the rafters will have been bedded into the stone wall up to the top of the rafter (fairly standard construction method for old houses in Ireland)

Thanks dathi. I've just checked and that would appear to be the case all right! So what options do I have? Would I be mad to just shove 100mm earthwool into the gap between the rafters?

Insulating Roof in old cottage

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