Do I Need Even more Insulation? Decision needed within next 24hrs

[peekay]

 

 

Hello. 

 

We are nearly finished with a renovation (that is an almost new build).

 

Large dormer bungalow shaped building. Ground floor 7.3kn blocks with Isover filled cavity and 90mm EPS covered in render.

 

First floor gable walls and low stub walls below the pitch of skeiling starts, working outward: 50mm insulated plaster board, 150mm PIR board set within studwork, OSB, membrane, 100mm cavity, then external wall studwork on to which 90mm EPS will be fixed and rendered.

 

We had a late design change from tile hanging the gables to render, hence bringing the gables slightly further out to allow a continuous render line. This has created the additional cavity at first floor level in the gables and stub walls.

 

These will have the EPS board fitted by the builder on Monday. 

 

I have the weekend free. It would be about £250 in materials for me to stick some Isover 100mm Spacesaver within this extra cavity/within the secondary studwork line. Money is tight at this stage of the build, but it would be a lot more expensive to add more insulation at a later date.

 

Is this overkill? Our SAP calcs don't require it, but is more always better

[SteamyTea]

5 minutes ago, peekay said:

50mm insulated plaster board, 150mm PIR board set within studwork, OSB, membrane, 100mm cavity, then external wall studwork on to which 90mm EPS will be fixed and rendered.

Have you done a condensation risk analysis.

 

Generally more insulation is better, but below U = 0.1, there is not a great deal to be gained.

[Roundtuit]

I'm not familiar with that sort of wall build-up, but the 50mm plasterboard and 150mm pir sound fairly decent. If it's timber frame, I suspect any cavity should be ventilated, but doesn't that render the external 90mm eps redundant?

[Nickfromwales]

4 minutes ago, Roundtuit said:

but doesn't that render the external 90mm eps redundant?

Quite often so, when a lot of work on IWI terminates at the inner leaf of the cavity. Will probably help more with summer overheating, but won't do much for heating afaic, vs the IWI.

 

All boils down to managing cold air infiltration to the dwelling interior, from atmosphere.

[Redbeard]

If the space between the OSB is *completely* unventilated the EPS may be doing something. If it is at all ventilated - intentionally or unintentionally - then at best the effect of the EPS may be lessened and at worst, negated. Similar applies if there is any air movement in the masonry cavity. Mineral wool may help 'still' any air movement a bit, but bonded EPS beads would be better.

 

It would be good to do a condensation risk assessment. I don't do them, but someone on here may show you how to use Ubakus.

[Nickfromwales]

1 minute ago, Redbeard said:

but bonded EPS beads would be better.

As long as they're installed A1, as with anything. These are the types of jobs started out with the very best of intentions, but fail due to employing useless cocks to actually do the install. Buyer beware etc, so do your diligence on the contractors and tell them you want FLIR evidence from before and after. The cocks won't come back, so will be quickly identified and omitted  

[Redbeard]

I think the bonded bead boat has sailed, as the OP intends (or not) to install mineral wool this weekend. It will be better than a poke in the eye with a sharp stick, but check for and block any air-leakage and sort it as you go along. I see no reference to vapour control layer (VCL), except that the insulated plasterboard may have one, but only where the joints aren't. Worth doing that condensation risk assessment. 'Feels' like it may be OK (except that air leakage within the 'sandwich' isn't) but I am not CRA software.

[Redbeard]

58 minutes ago, peekay said:

Ground floor 7.3kn blocks with Isover filled cavity

How wide is that cavity?

[peekay]

1 hour ago, Redbeard said:

How wide is that cavity?

Haha. Good spot. It should have read 7.3N blocks! 

 

1 hour ago, Roundtuit said:

I'm not familiar with that sort of wall build-up, but the 50mm plasterboard and 150mm pir sound fairly decent. If it's timber frame, I suspect any cavity should be ventilated, but doesn't that render the external 90mm eps redundant?

 

There is no detail on the drawings to say whether the cavity should be ventilated or not. 

 

I had similar thoughts, that if the cavity was open at the top under the roof, then the external EPS is not adding any value, therefore I have asked the builders to make sure that there is a good seal at the top. Maybe I should not have asked them to do this.

 

[Iceverge]

3 hours ago, peekay said:

Ground floor 7.3kn blocks with Isover filled cavity and 90mm EPS covered in render.

 

Fine

 

3 hours ago, peekay said:

then external wall studwork on to which 90mm EPS will be fixed and rendered.

 

You've lost me. Sketch please. 

 

3 hours ago, peekay said:

We had a late design change from tile hanging the gables to render, hence bringing the gables slightly further out to allow a continuous render line. This has created the additional cavity at first floor level in the gables and stub walls.

 

These will have the EPS board fitted by the builder on Monday. 

 

STOP!!!!!!!!!!

 

STOP!!!!!!!!!!

 

STOP!!!!!!!!!!

 

 

Render over EWI onto timber frame isn't a good idea.

 

 @ProDave has some unfortunate issues but they look minor compared to the problems of the "leaky condo crisis" in British Colombia in the 80's and 90s.

 

Often a component of these buildings failures was the lack of a proper rainscreen and a simple rendered EPS over timber frame finish. Exactly what you are proposing.

 

Get some pictures inside and outside of the building as it sits. Specifically of the gable areas and cheeks to the dormers. 

 

It may be best to go back to the hanging tiles or a rendered cement board. Some pics will tell all. 

 

 

 

 

 

 

 

 

[peekay]

5 hours ago, Iceverge said:

 

 

Get some pictures inside and outside of the building as it sits. Specifically of the gable areas and cheeks to the dormers. 

 

It may be best to go back to the hanging tiles or a rendered cement board. Some pics will tell all. 

 

 

 

 

 

 

 

 

 

Here is a photo of the gable.

 

You can see the external line of studwork.

 

VCL laid over 22mm OSB set behind this. 

Behind that is the 150mm PIR set within the internal stud wall, and faced with 50mm insulated plaster board. 

 

 

The cheeks of the dormers are tile hung. You can just make one out in the photo. 

 

The external insulation has already begun being fitted to the ground floor and can be seen in the photo.

 

The plan was for a silicone render over all of this.

[Redbeard]

Who did your insulation and air-tightness strategy? Do you know for certain that there is no chance of air movement in the 'sandwich'? If the air-tightness within the depth of the upper wall is not really good then the EPS could be just a fat expensive render board, potentially doing nothing to the sum total of insulation. How deep is the cavity in the gr floor walls? Then we can maybe roughly work out relative GF and FF insulation values. Without more info I think it's too early to make any call on additional insulation, or not.

Edited October 24 by Redbeard

[Iceverge]

Ok so not EPS direct onto the OSB in that case. Not catastrophic but.....

 

 

However this detail I wouldn't be too chuffed about. 

 

That membrane is the barrier between the water and you. Any drips that get in will run down and disappear into the old cavity by the looks of things. Not ideal and may well make itself known inside your house at a later date and you'll have no idea where the leak is coming from. 

 

EPS over this ventilated gap  like @Redbeard mentioned will be just a render board. 

 

I think this needs to be sorted on paper before any further work is done. Never mind insulation for now. Keeping water out is far more important. I have some suggestions but I'd like to get more photos and drawing if you have them. 

 

Can you get on the scaffolding please and take lots of pics. Pics from the inside too. 

 

Call the EWI contractor and delay them. This needs sorting beforehand. 

 

 

[Iceverge]

 

Ok a little detective work here. Your existing wall if i'm correct?

Whats's in the cavity as indicated?

 

 

 

This is what I think you have today???

 

What happens at the bottom of the studword with the membrane?

 

 

 

 

 

[Iceverge]

 

Here is your proposal as I see it today. 

 

Not ideal at all as the ventilated cavities will make the EPS insulation almost useless as cold air will circulate behind it. 

[Iceverge]

 

Similarly any drips that might get into the wall behind the insulation will make their way into the cavity and make the existing cavity wall damp. 

 

 

[peekay]

@Iceverge (and others)thanks for the replies.

 

I'll upload some photos and annotate your sketch over the next couple of hours 

 

The membrane that can be seen from my photo is fixed underneath the studwork that can also be seen in the photo. Unless there are any tears that I haven't seen, whist it has been exposed any rain water hitting it has not entered the cavity below. 

 

More info to follow....

[Iceverge]

Here's my proposed solution. 

 

 

 

Fill the existing cavity with beads. 

 

Apply a flashing as shown to direct any drips from behing the caldding outwards.

 

Batten and renderboard the studs as shown. 

 

 

 

 

 

Apply a bug mesh and leave a ventilated gap as shown above the EPS to allow the back of the studs to ventilate. 

 

 

[Iceverge]

As an option you could infill the external studwork with some mineral wool. Not the stuff that is used in attics but the stuff that is designed for EWI or cavities. 

 

 

 

 

 

[Iceverge]

9 minutes ago, peekay said:

whist it has been exposed any rain water hitting it has not entered the cavity below. 

 

 

run a hose on it for an hour and see where the water ends up. 

 

Water must always drain DOWN And OUT. 

 

At the moment it has a path IN. 

 

I know it'a not much fun to reverse a decision but it'll be orders of multitiude cheaper to do it now than in a few years time. 

[peekay]

Here are some photos of the gables taken at various stages over the last few months.

 

I'm not around until tomorrow morning to take new photos of specific details 

 

 

Edited October 24 by peekay

[peekay]

30 minutes ago, Iceverge said:

 

run a hose on it for an hour and see where the water ends up. 

 

Water must always drain DOWN And OUT. 

 

At the moment it has a path IN. 

 

I know it'a not much fun to reverse a decision but it'll be orders of multitiude cheaper to do it now than in a few years time. 

If the top of the EWI is under the soffit (regardless of whether there is an attempt to close the cavity behind it or not), where could water come from? Are you thinking of just condensation?

 

Thanks 

[Iceverge]

1 minute ago, peekay said:

where could water come from?

 

Unfortunately water tends to find a way in time. 

 

Cracks in the render, gaps between boards, differential expansion and contraction of timber, foam and masonry will all stress the external weather proof layer. Eventually it will  fail.

 

EWI over masonry it isn't such an issue as it's very very moisture tolerant. A small leak may do little or no damage over years. 

 

The same can't be said of timber which is a brilliant material but needs to be treated with more respect. Read about the leaky condo crisis to see what happens when you build in a damp environment without proper ventilated rain screens and appropriate drainage paths for timber frame buildings. 

 

What I have proposed I think I'd very inexpensive, easily buildable and won't require and dismantling of what is already there. If anyone else has an improved suggestion please chip in. 

[peekay]

Thanks

 

Will have a read about the BC Condo Crisis.

 

I've just checked with the builders, and the breather membrane definitely overlaps the external masonry wall on the ground floor.

 

Their plan was to fix a concrete board to the external studwork, and then fix the EPS/EWI on top of this. Going right up to the soffit and sealing at the top.

 

I'll discuss your proposal with them, thanks for the detailed knowledgeable responses.

[Iceverge]

4 hours ago, peekay said:

I've just checked with the builders, and the breather membrane definitely overlaps the external masonry wall on the ground floor.

 

 

It really should go outside the EWI too. Draining into the bricks behind the EPS isn't robust solution. 

 

4 hours ago, peekay said:

4 hours ago, peekay said:

Their plan was to fix a concrete board to the external studwork, and then fix the EPS/EWI on top of this. Going right up to the soffit and sealing at the top.

 

 

That EPS won't be achieving any insulative benefit with the open cavity to the rear. Just move the cement board out a d render that instead. 

 

4 hours ago, peekay said:

I'll discuss your proposal with them, thanks for the detailed knowledgeable responses

 

Beware they'll want to tell you "it's fine". 

 

It probably will be initially but not long term. 10-20-30 years from now you will have issues. 

 

Who specced this build-up? There's  lack of knowledge of how to create a robust water barrier, air barrier and thermal barrier.