How to best insulate timber frame exterior wall?

[TheCyclingProgrammer]

I'm hoping somebody could give me some advice on the best approach to insulating our bedroom wall.

The reason I'm asking is because we're in the process of refurbishing the room - we're basically gutting it and starting it from scratch. All skirting/architraves have been removed and the room is going to be re-skimmed. Because some of the plaster is in a particularly bad way, the worst of it is going to be removed entirely before re-skimming.

The front of our bedroom overhangs the downstairs bay window and is made out of a timber frame. This frame extends to the side wall for about half a metre - the left wall of our bedroom looking towards the front of the house used to be an exterior wall however the house has been extended to the front and is now an internal dividing wall. This image should make it clear:

https://www.dropbox.com/s/cdde5jrs8ctybpz/Screen_Shot_2015-09-30_at_18_22_02.png?dl=0

Internally, the timber frame is old lath and plaster and we plan to remove it entirely and have it re-lined with plasterboard. I've already moved the small side section. I also intend to fit some 18mm plywood between the studs under the window as this is where we want to fix our new radiator.

The outside of the wall seems to have wooden lath/batten, some kind of black lining/membrane (vapour barrier?) and then render. As we are removing the lath and plaster internally it seemed like an ideal opportunity to insulate it as the wall gets quite cold (we've only been in the house 6 months so no idea what its like over winter or if there are condensation issues).

What is the best way of doing this? I've done some research and its clearly not as simple as whacking some loose insulation in there and that I have to make sure whatever I do causes no issues with damp - particularly interstitial condensation - and I need to try and avoid cold bridging issues around the studs.

The final constraint is that because the wall isn't very deep, the window reveal isn't very deep and isn't plastered - its just a wooden frame. I'd like to avoid having to build out the reveal or getting the radiator pipework moved again, which limits the wall thickness from stud to the finished wall surface to about 25mm or so.

The cavity is about 100mm so it seems like I could put 40-50mm of solid insulation (celotex etc.) between the studs whilst still leaving some ventilation, then adding the thinnest layer of celotex you can get (12.5mm) over the whole lot, including the studs, taping all the joints to create a sealed vapour barrier on the inside, then line with plasterboard and skim.

So going from this:
https://www.dropbox.com/s/glg3cg89i944e2p/Wall_Cross_Section.png?dl=0

To this:
https://www.dropbox.com/s/tlv1ljps2fpmxho/Wall_Cross_Section_Insulated.png?dl=0

But I'm still not sure if this could cause interstitial condensation or other issues. Also what happens as far as the vapour control layer goes when you then screw through it (e.g. the radiator brackets)?

Any advice would be much appreciated as I'd like to get this done ASAP before we book the plasterers in! If the answer is to not bother insulating at all because of the complexities involved then so be it.

If this is a decent plan, is there any particular insulation product I should use? I was thinking Celotex GA4000 for between the studs and TB4000 for over them but there's so many I'm not really sure where to start.

Thanks.

 

[AronSearle]

Old type buildings like that lost a lot of heat through the wall, which helped keep things dry.

So you need to ensure the external rendar is waterproof.

Condensation isnt a concern as foil faced foam and vapour layers will make it better, small perforations have not shown to be a concern.

 

[TheCyclingProgrammer]

Thanks for the reply. From what I can tell the external render is weatherproof - there are no signs of penetrating dampness on the internal surfaces. I'll have a much better idea when the plaster is removed and I can inspect the timbers and vapour barrier. From up in the loft, where there is no lath/plaster finish on the inside the structure does look perfectly dry.

That said the exterior render does need some repairs, there seems to be a few cracks and loose patches. I was planning to get a quote for repair and redecoration and also for a complete re-render with something like monocouche but that may have to be a job for next year!

So assuming moisture can't penetrate from the outside, then the foil side of the foam insulation will prevent moisture from entering the cavity from the inside and potentially condensing inside the cavity? Because that's my main concern as it could be an expensive disaster!

I'm also guessing I don't actually need the foil backing on the insulation between the studs as it will be the layer over the studs that forms the vapour control layer?

Is there anything else I need to consider here? Do I need to seal around the perimeter to prevent cold bridging? If so, with what?

 

[AronSearle]

Water penetration does not typically occur through rendar, but round it. For instance seals between rendar and window frames, or pipes and such.

As long as the rendar, and what the render is on is more breathable than the vapour control layers on the inside you should avoid condensation (rule of thumb).

Not sure what you mean about by sealing the perimeter?

Perimeter of what, a window? if so = foam in a can.

 

[theprinceofdarkness]

I am not quite sure about your bedroom which is overhanging the front bay. If from the outside you can see what is the underneath of a rooms floor, then the void under that bit of the flooring should be insulated and any paneling/planking made totaly airtight.
Frank

 

[TheCyclingProgrammer]

There's no insulation in the floor and from what I can tell none in the wall either. The house is about 80 years old.

I can certainly see the benefit of putting some insulation in the floor where it overhangs the bay but not all of the floor is external as its not a box bay. This should make it clearer:

https://www.dropbox.com/s/mq2meyqidynbkrl/zh_VvIiqj6F82zEEChgc8Io5Uw__YxauFyNBsA-C0sk.png?dl=0

The green line shows the outline of the downstairs bay, which is a brick cavity wall. I'd be surprised if it is perfectly airtight but there's no visible gaps.

I'm guessing it would probably be good to insulate under the floorboards near the front wall and bring the wall insulation down to that and tape it up to create a continuous vapour control layer around front wall of the room so vapour can't get in from below the wall insulation.

 

[TheCyclingProgrammer]

Water penetration does not typically occur through rendar, but round it. For instance seals between rendar and window frames, or pipes and such.

Like I said, the render certainly needs some repairs but there's no signs of any water penetration internally. Once the wall is opened up I'll be sure to thoroughly inspect the timbers.

Here's a photo of the front of the house, you can see a few patches of render that need repairing and how it overhangs the bay (bedroom on left as you are looking at it is the room in question, right hand bedroom is part of two-storey extension).

https://www.dropbox.com/s/6ws4lapo091ksjq/IMG_1987.jpg?dl=0

As long as the rendar, and what the render is on is more breathable than the vapour control layers on the inside you should avoid condensation (rule of thumb).

I have no idea if the black membrane between the render and outside laths is breathable. Is there any way to tell? Its hard to say how old it is - there's a strong possibility it dates to when the front extension was built and the front was rendered which would be around 1989.

Not sure what you mean about by sealing the perimeter?

Perimeter of what, a window? if so = foam in a can.

Not just around windows, but the entire wall - my understanding is that the foil backing of the insulation needs to be taped at all joins to form a continuous vapour control layer, including where it meets the ceiling, floor and other walls.

 

[TheCyclingProgrammer]

Made some progress removing the plaster today, which came off in fairly big chunks. Removing the laths was not fun. I've still got the clearing up and nail removal to do!

Here's a picture:

From what I can tell, I think at some point in the past the outside must have been rendered onto the laths you see on the outside of the studs. I could be wrong but there were still plaster snots attached in places.

I think the outside was re rendered about 25 years ago. There is a black membrane outside the laths but I don't know what it is made of.

The studs are 4x2s. Playing around with the Celotex u value calculator, to get the building regs target of 0.30 I'd need 70mm between the studs with 12mm inside them. That would leave about a 30mm cavity on the cold side.

I've got a builder coming on Monday to give me a quote for the insulation and plastering work so I will see what they say.

 

[AronSearle]

That would leave about a 30mm cavity on the cold side.

that would be a 30mm airspace rather than a cavity per say (it is not vented or drained anywhere, therefore it does nothing usefull).

You need to find out what that black membrane is, as said, if it is breathable then you should not have a condensation problem, may as well full fill. (if it is not breathable then you have a problem). I don't know for certain what it is, I suspect it is a breathable type felt product, but I'm not familiar with the products of that age.

Again I emphasis the issue of water penetration, currently heat loss through the wall will have a benificial effect in keeping things dry.

Your external weatherproofing must be top notch, if you use mastic (urg) to seal window to rendar junctions, remember the mastic bond does not necessarly last long.

Not just around windows, but the entire wall - my understanding is that the foil backing of the insulation needs to be taped at all joins to form a continuous vapour control layer, including where it meets the ceiling, floor and other walls.

Fit the rigid foam between the studs and squirty can foam around gaps (don't even try and cut them to fit tight). Then put a VCL (plastic sheet) over the whole lot which you can lap at floors and ceilings etc.

Don't bother with tapes.

 

[TheCyclingProgrammer]

Thanks, I guess I will need to see if the plasterers or builder can identify the membrane.

I'm going to have a second layer of 12mm celotex inside the studs to minimise cold bridging and take up the extra depth so I'm not sure why I'd need a separate VCL as the taped joints between the celotex will form the VCL?

 

[TheCyclingProgrammer]

So we finished the job of removing the laths and nails and clearing up today and now have a clearer picture of the state of the timber.

Probably 95% of it is sound. The black membrane looks to me like some kind of bituminous roofing felt and looks the same as the roofing felt in our roof which makes sense, as I'm fairly certain the roof was redone around the same time as the front extension and re-cladding. I suspect this isn't breathable.

The other 5%...there are some small patches of damp timber, some of it rotten (mainly the tops of the 6x2 floor joist and the old laths). My original thought was that this was due to the damaged render but the vapour barrier seems intact. On further inspection it, it looks like it's more likely to be water penetration around the window sill - definitely in both bottom corners and possibly the top right too looking at the staining on the wood. On both sides of the sill you can see daylight. This has nothing to do with the render as the window is surrounded by timber fascia boards on the outside.

And the other side:

So it looks like AronSearle was spot on about checking the window junctions. We need to get this sorted before we proceed further.

What would be the most effective solution? Mastic on the outside to fill the gaps and possibly expanding foam on the inside? I'm just guessing as I'm not an expert!

 

[TheCyclingProgrammer]

Interesting discussion here that sounds like it might be relevant:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6165&Focus=89740

It seems safest to assume the membrane under the render is not breathable. So the suggestion above is to fix battens against the felt, fit insulation up to battens, leaving 25mm cavity then find a way of introducing ventilation from the outside?

If I then proceed next year to have the cladding removed and re-rendered, the cavity could be topped up from the outside and then have a breathable membrane added.

I would still need to fix the damp problem first and make sure I fit a good VCL inside.