100mm cavity plus something else?

[epsilonGreedy]

I am close the point where the architectural technician assigned to creating my building control diagrams will ask me to confirm the thermal layer build up for my brick & block walls.

 

My options based on my own research seem to be:

 

1. 150mm cavity plus basic dot & dab dry lining or classic wet plaster on blocks.
2. 100mm cavity with exotic thermally performant blocks for the inner wall plus dry lining.
3. 100mm cavity plus commodity blocks plus 25mm insulation between battens on the inner block finished off by regular screwed on plaster board.

 

My location means I can have a fully filled cavity.

 

Option-1 will create a problem with a full brick reveal sash window frame due to the extra cavity width and I will loose an inch or two in many rooms. The house is L-shaped hence the sq footage loss is more painful.

 

Option-2. I have yet to get my head around the range of blocks available but the handling/fixing/cost characteristics seem to worsen inversely to thermal performance.

 

Option-3 is my favoured route because I feel it will offer more flexibility for a slow progressive diy 1st and 2nd fix where I will be camping out in the part finished house. I also like the idea of been able to work my way towards fully functional electrics/plumbing/heating and living with the guts exposed for 3 months. Then if nothing leaks and all electrics seem properly positioned I can then hire a machine and squirt 25mm of insulation foam between the battens, trim it back and attach the plasterboard.

 

I also think the 25mm of inner foam will be more effective than its R value contribution indicates. @JSHarris prompted this thought a few months ago when he suggested that an inner block wall can become chilled through thermal bridging and dry lining convection.

 

Has anyone done a selfbuild following my Option-3 and concluded it was a good solution?

[joe90]

No I haven’t, I had 200mm full fill cavity with rock wall, brick outer and heavy concrete block inner with full wet render and plaster inside. The reveals were  designed by me to be cottage style and they are great ( for us). I like option three to give you flexibility with your second fix etc but I do like solid walls to stop temp swings ( high thermal capacity) and also solid to hang everything on.

[Declan52]

The easy option is to put all the insulation in the cavity. Go for 150mm and get the beads blown in. Will be better for the brickies as they don't have to put the insulation in. Better for you as you will end up with no gaps in the cavity. Plus you don't need to fork out the cash to get the beads done until way later on in your build.

You will on loose 100mm from wall to wall. To get round the bigger reveal you can do a splayed window opening. Easy done with a concrete saw and blocks.

Wet plaster the inside. Seals it up much better.

Edited June 5, 2018 by Declan52

[joe90]

My reveals were done in plasterboard, the inner skin was 150mm larger each side than the outer skin ( which helped loads when fitting the windows in check reveals , windows fixed to back of external brick). I also did this with doorways and gives a much better Access ( doors open past 90’) 

[epsilonGreedy]

18 minutes ago, joe90 said:

No I haven’t, I had 200mm full fill cavity with rock wall, brick outer and heavy concrete block inner with full wet render and plaster inside. The reveals were  designed by me to be cottage style and they are great ( for us).

 

 

You have an excellent end result there and it would be compatible with the heritage rural look I am after.

 

200mm hmm, I would need to recompute internal room dimensions and judge the reaction on swmbo's face. 

[epsilonGreedy]

22 minutes ago, Declan52 said:

To get round the bigger reveal you can do a splayed window opening. Easy done with a concrete saw and blocks.

Wet plaster the inside. Seals it up much better.

 

 

I wonder if I am creating a problem that does not exist about reveals. The concern I mentioned in my OP related to attaching the frame (wooden sash) to the nothingness of a large cavity.

 

To overcome the block sealing issues I am already mentally prepped for a couple of days of sloppy mix parge coating. The other problem is that I imagine a full wet plaster job would render the house uninhabitable for a couple of months as it sheds all that water. I reckon the walls will be ready for finishing this time next year.

[Brickie]

Or could you push the external walls out to keep your internal  layout as it is planned? Never dealt with planners myself so have no idea. 

[PeterW]

Just finished option 3 but with ...

 

150mm Cavity

Graphite blown bead

25mm PIR between battens 

 

Quick and easy to do and the PIR is a much better option than spray foam as you can cut it and batten to it to trap the insulation. 

 

Have you got the window detail drawn as there sounds like a cold bridge there and a DPM will be a necessity. 

[joe90]

Everyone said I would have smaller rooms but I designed the room sizes I wanted then put the walls around them!!! (Unless you are limited on footprint size ). Our builder had no problem inserting the rock wall insulation as they went ( and it was fairly cheap) . In fairness our brickie told me he did not like full fill cavity but the rock wall had a bba cert for full fill. Our west wall gets lots of driven rain, close to the Atlantic coast, and when core drilling fir ASHP pipes I found the outer 1” of the insulation in the cavity was a little wet ( wettest year on record in Devon) but it had not soaked in any further so I am happy.

[PeterW]

3 minutes ago, epsilonGreedy said:

The other problem is that I imagine a full wet plaster job would render the house uninhabitable for a couple of months as it sheds all that water. I reckon the walls will be ready for finishing this time next year

 

Get a couple of big industrial dehumidifiers in and it will dry pretty quickly - we had to stop using them in the winter as they were sucking moisture from the plaster too fast ! 

[joe90]

1 minute ago, PeterW said:

 

Get a couple of big industrial dehumidifiers in and it will dry pretty quickly - we had to stop using them in the winter as they were sucking moisture from the plaster too fast ! 

 

Yes, exactly what I did, it was still very damp weather in fairness.

[nod]

46 minutes ago, epsilonGreedy said:

I am close the point where the architectural technician assigned to creating my building control diagrams will ask me to confirm the thermal layer build up for my brick & block walls.

 

My options based on my own research seem to be:

 

1. 150mm cavity plus basic dot & dab dry lining or classic wet plaster on blocks.
2. 100mm cavity with exotic thermally performant blocks for the inner wall plus dry lining.
3. 100mm cavity plus commodity blocks plus 25mm insulation between battens on the inner block finished off by regular screwed on plaster board.

 

My location means I can have a fully filled cavity.

 

Option-1 will create a problem with a full brick reveal sash window frame due to the extra cavity width and I will loose an inch or two in many rooms. The house is L-shaped hence the sq footage loss is more painful.

 

Option-2. I have yet to get my head around the range of blocks available but the handling/fixing/cost characteristics seem to worsen inversely to thermal performance.

 

Option-3 is my favoured route because I feel it will offer more flexibility for a slow progressive diy 1st and 2nd fix where I will be camping out in the part finished house. I also like the idea of been able to work my way towards fully functional electrics/plumbing/heating and living with the guts exposed for 3 months. Then if nothing leaks and all electrics seem properly positioned I can then hire a machine and squirt 25mm of insulation foam between the battens, trim it back and attach the plasterboard.

 

I also think the 25mm of inner foam will be more effective than its R value contribution indicates. @JSHarris prompted this thought a few months ago when he suggested that an inner block wall can become chilled through thermal bridging and dry lining convection.

 

Has anyone done a selfbuild following my Option-3 and concluded it was a good solution?

We have gone block and block with a full fill 125 cavity 

dot a dab on the inside If done correctly Just as airtight as wet plaster Sash windows throughout 

Exteria is finished with two coats of K rend Leaving a 60 mil external reveal 150 on the inside 

[joe90]

If you do angled reveals I will draw what we did if you like, there is one detail my builder talked me out of that in hindsite I wish I had done which has caused a little cracking due to wooden windows expanding/ contracting.

[epsilonGreedy]

1 hour ago, PeterW said:

Quick and easy to do and the PIR is a much better option than spray foam as you can cut it and batten to it to trap the insulation. 

 

 

Good to hear Option-3 is viable.

 

I was hoping that spray insulation would:

• Be cheaper.
• Create a better air seal.
• With the machine available I could zap some other voids in particular the blockwork face between the ceiling and first floor boarding. Some YouTube videos show that in the US they fill all nooks and crannies with the spray nozzle.
• Mean no need to shape PIR sheets around service conduits, cables or pipes.

The downside of foam could be:

• Cutting away excess.
• Noxious gases for days or weeks?
• Possible chemical argument between HEP or electric cable plastic.

The benefit of PIR panels I can see are:

• It suits a progressive approach to finishing the house rather than a final big bang spray day.
• It is reversible if I need to route something extra down a wall.

 

When you say the "batten it to trap it" is this a temporary measure prior to fixing plasterboard?

 

[PeterW]

14 minutes ago, epsilonGreedy said:

When you say the "batten it to trap it" is this a temporary measure prior to fixing plasterboard?

 

Will see if I can find a photo but essentially we put a vertical 50x25 batten up the wall and secured with concrete screws. Then took a "slice" of PIR that was 350mm wide and put it up against the first batten and the pushed another batten tight against it and screwed that back to the wall. The friction fit holds it tight and then the boards are screwed over the top. You can also tape the foam to batten joint too for added air tightness.

 

I don't think spraying foam everywhere has any benefit and its eyewatering in price compared to PIR. It also adds another job, and creates a lot of unusable waste.

[epsilonGreedy]

1 hour ago, PeterW said:

Have you got the window detail drawn as there sounds like a cold bridge there and a DPM will be a necessity. 

 

 

Only drawn in my head.

1. I have found a caststone cill design that hangs back 50mm over the cavity, it is specifically to support the bottom of a sash window frame set back by a full brick reveal.
2. The frame will be oversized horizontally so that the sash boxes will sit behind the facing bricks to allow for more glazed area and also match building standards of the late 1700's.
3. I hope that with a 100mm cavity enough of the frame will butt up against the inner blockwork to allow some fixing. This is the most fuzzy part of the design, though I have a couple of months before the blockwork needs to reflect the a final design.

Where would a cold bridge be formed?

[PeterW]

So by full brick reveal you mean a brick depth (102mm) not a brick length (215mm)..? 

 

So in your scenario, you will basically make a window "sandwich" between the brickwork, the timber of the window and the inner blockwork...? If so, you will need a DPM on the front and rear of the timber framing, and preferably some method to thermally break the sandwich to ensure that you don't create a cold spot on the frames. 

 

How are you planning on getting the frames into the cavity though if they are mounted behind the brick and in front of the block..??

[epsilonGreedy]

4 minutes ago, PeterW said:

 

Will see if I can find a photo but essentially we put a vertical 50x25 batten up the wall and secured with concrete screws. Then took a "slice" of PIR that was 350mm wide and put it up against the first batten and the pushed another batten tight against it and screwed that back to the wall. The friction fit holds it tight and then the boards are screwed over the top.

 

 

Ok I can picture the fitting process, thanks for the extra description - no need for a photo unless it also shows how you fitted around services.

 

I assume you had to keep an eye on batten centre distances to ensure this matched plasterboard sheet size?

 

8 minutes ago, PeterW said:

I don't think spraying foam everywhere has any benefit and its eyewatering in price compared to PIR.

 

 

Oh ok I just thought that mixing the raw chemicals onsite in a machine would be cheaper than ready made foam insulation sheet and folks here have complained about the cost of these. I am happy to be persuaded that insulation sheet is the answer as it is more diy friendly (if laborious to cut @recoveringacademic ). I think @PeterStarck is the forum's spray insulation expert.

[PeterW]

There is a huge difference between commercial sprayed Icynene and the DIY stuff - per cubic metre its about £400 for Froth-Pak, 25mm PIR is about £270 and is clean and pre-cut to thickness.

 

Cutting thin PIR is simple with a Stanley blade and a T-bar that slides down the side of the insulation - we got it down to about 2 mins to cut a full sheet into the slices.

[epsilonGreedy]

7 minutes ago, PeterW said:

So by full brick reveal you mean a brick depth (102mm) not a brick length (215mm)..? 

 

 

Yes 102mm.

 

8 minutes ago, PeterW said:

So in your scenario, you will basically make a window "sandwich" between the brickwork, the timber of the window and the inner blockwork...? If so, you will need a DPM on the front and rear of the timber framing, and preferably some method to thermally break the sandwich to ensure that you don't create a cold spot on the frames.

 

 

Maybe a sandwich, it depends on the depth of the frame.

 

I like the idea of a DPM for the front face of the frame, this had been troubling me and left to my own devices I might have used copious amounts of structural adhesive to both perform fixing and also provide an insulation layer. Thinking more it guess it is better to use one material for sealing i.e. a bead around the aperture and another for the membrane function.

 

16 minutes ago, PeterW said:

How are you planning on getting the frames into the cavity though if they are mounted behind the brick and in front of the block..??

 

 

From the inside. The inner blockwork would be stepped back on the window aperture verticals by say 100mm each side.

 

Is the R value of wood low enough to create cold bridge concerns? 

[PeterW]

Timber windows move and I would want something flexible between the brick and the window frame as they will expand and contract differently. You will need a cavity closure detail and also a DPM around the edge and front of the window so I would be looking to bring the blockwork back further and then either splay or box in the window from the inside as per @joe90

[Matt64]

@joe90Thanks for the pics on this thread, I'm new and this is my first post. I've been searching for angled/splayed window reveal details with little luck. My 60's semi and extension has standard cavity wall 300mm (brick,insulation,block) buildup, and wonder if this sort of reveal is possible with uPVC windows?

 

As typical details use standard cavity closers, which I can't see working at an angle, so was thinking about just using some PIR board cut at an angle as a closer instead (a bit like this http://tonyshouse.readinguk.org/air-sealing-windows-and-doors/)

 

I have butchered one of my architects drawings to show what I was thinking, a bit of timber against the brick face to support uPVC frame, then angled PIR board (red) for cavity closer with plasterboard on angle (maybe with a bit of ply underneath it?)

 

I really like your angled door reveals as well, so I suppose same question applies for uPVC composite door detail.

 

Any suggestions much appreciated!

 

 

[PeterW]

You would need to be careful with that as it will create a significant cold bridge next to the window. BCO would also need some sort of vertical DPC between the inner and outer leaf. 
 

You’ll also need to change all your lintels as they will need to be significantly wider on the internal leaf - I would expect you would have to use split lintels or something substantially longer which may show on the front face. 

[Matt64]

Thanks Peter - this detail would be on some narrow slot windows (460mm wide) and lintels are easily wide enough to cover lost blockwork.

 

Cold bridge and DPC are very good points, that's what I'm not quite clear on.

 

Also I'm not clear how the typical uPVC frame is fixed, would I even need the timber bit? Is it the frame only attached to the brick? If so i could remove the thermal bridge and add a DPC.

[joe90]

@Matt64 similar to my set up but my windows were on the back of the external brickwork, will do a little sketch in the morning.