Hello All,

After lurking and reading around the forum for a few weeks, I'm looking for advice on my proposed plan for insulating attic rooms in our Victorian house. Built 1877 it's one half of a semi that then had another house built directly adjacent, but at a lower level, a few years later.

- External Wall 1 - 4.2m x 2.75m high (with a corner taken out due to slope of roof)

This is a gable end wall, with the roof and attic of next door about half way up the wall. Currently brick covered with original (lime?) plaster which is very sound.

I plan to dot and dab Cellotex PL4000 65mm+12.5mm over that wall.

- External Wall / Ceiling Slope - 6m x 4.3m (3.3m of sloping ceiling and 1m of vertical solid wall)

Spanning the width of the front of house this is about 1m of solid wall before the ceiling slope which extends 3.3m to the 'flat' part of the ceiling.

The 1m height of solid wall

Plan -> Dot & Dab Cellotex PL4000 65mm+12.5mm

*Sloping ceiling wall* has approx 1.5" of latch and plaster over 64mmx85mm rafters with approx 380mm centres. Beyond that are slate tiles on battens. Slate tiles are 99% sound.

Plan -> Remove lath & plaster. Insert 80mm Cellotex GA4000 board between rathers. Tape joints with metallic tape. Fix Cellotex PL4000 65+12.5mm plasterboard over the GA4000 & rafters.

*Flat ceiling*

This is approx 3m x 6m and extends over the other attic room and a landing/stair space. Lath & plaster (very sound) onto 3" x 2" joists with 15" centres. Above that, rafters with slate tiles on battons. Very drafty. Signs of a few missing tiles. There was no access to this loft space, when I cut a hole to inspect, the big surprise was that the loft space ran the full length of both our house and our mirror image 'twin' next door. There is no dividing wall.

Plan -> Roll out 380mm width strips of Knauf Earthwool 100mm between joists. Overlay at 90 degrees with 200mm Knauf Earthwool, for 300mm total thickness.

A few questions.

1. Is there any point in removing the (sound) plaster from the solid external wall before dot & dabbing the Cellotex PL4000 on top?

2. I plan to push the Cellotex between the rafter on the sloping wall up into the loft space to abut the 300 mm of mineral wool. Is that a good idea?

3. Am I going to have problems with condensation? I plan to have a shower in the room (with an extractor venting outside).

4. Will thermal bridging be a big problem from the external gable wall to the internal solid wall that forms the rear wall of the room? Anything I can do about that?

Happy to hear thoughts on more effective / cheaper alternatives!

I have some photos here https://www.icloud.com/sharedalbum/#B0N5ON9t3FY1xg

Regards.

Greg.

''A few questions.

1. Is there any point in removing the (sound) plaster from the solid external wall before dot & dabbing the Cellotex PL4000 on top?''

* No. As long as it is lime. If it is gypsum - yes! Celotex's blurb specifically talks about maintaining a breathable 'exit' (so get rid of any impermeable paint). You also *want* the plaster as an air-tightness layer - extend it where it stops at skirting level. Please do not dot and dab! This invites thermal by-pass via air-movement behind the board, and effectively disconnects the R values of the insulation and the wall, potentially leaving the insulation value of the old wall, such as it is, 'out in the cold'. At very least use a full perimeter bead and cross-'stripes' of adhesive. (which could be adhesive grade Pu foam which does not have a tendency to 'suck' as gypsum does, in case any WV does get through.

''2. I plan to push the Cellotex between the rafter on the sloping wall up into the loft space to abut the 300 mm of mineral wool. Is that a good idea?''

Yes, but you will presumably have to notch it in each rafter bay as the ties/joists are nailed to the side of the rafters.

''3. Am I going to have problems with condensation? I plan to have a shower in the room (with an extractor venting outside).''

You might risk interstitial condensation. A WUFI calc will give you more comfort than the Glaser method offered by most manufs.

Also, using a composite board, how will you ensure the VCL is perfect at the joints and perimeters? I can't, so I never use composite boards now. I usually use Pro Clima air-tightness tapes at joints and particularly at perimeters as there is little risk of it tearing, whereas foil tape carries such a risk.

''4. Will thermal bridging be a big problem from the external gable wall to the internal solid wall that forms the rear wall of the room? Anything I can do about that?''

Yes, return the internal insulation of the external walls (IWI) onto that wall by about 300- 400mm.

Happy to hear thoughts on more effective / cheaper alternatives!

Not cheaper, but if you can use something like wood-fibre board (and lime plaster) instead of impermeable plastic, and reliance on a VCL that may not be 100% (or probably never will be!), you reduce the risks associated with IWI.

Posted By: BauwerWe supplied insulated plaster for a large internal insulation project in central London, about 300sqm at 40mm thickness, and the insulated plaster was received very well.

is this advertising, or what ?

(If so, then I think I shall start advertising my VERY WELL REGARDED ********ion, *********ing and *****ing services on here !).
(NOT...)

gg

I think this may breach the 'no advertising' rule, but I am interested nevertheless to hear a bit about the product. What was the 'before' U value in the example you refer to, and the 'after' value? Did BC give a dispensation, or were they not aware? It seems that Part L1B would have applied (you would, I guess, have been 'adding or replacing a layer').

More information would be good.

Hello Keith,

Sure, no problem, we already received very good feedback on the plasterers forum.

I'll also ask our customer to leave their feedbacks at the Green Building forum as well.

Also I just noticed this posts, after 3 days...any chance I could get a copy of a post on my email?

I agree that virtual face to face conversation helps :)


Best regards, Alexander.

<blockquote><cite>Posted By: restorationcouple</cite>These are friction fitted, we then have 50mm whole sheets under the rafters fitted with drywall screws and small insulation washers<div class="Attachments" id="Attachments_236537"><ul><div><img src="/newforum/extensions/InlineImages/image.php?AttachmentID=6189" alt="Between and below rafter insualtion.jpg"></img></div></ul></div></blockquote>

Looks like you used a heap of insulation washers, can you advise if you felt these were essential? Could you have just used one or two for each board to hold in place then use the longer screws to hold the PB and insulation at the same time?