Is an airtightness membrane worth installing

kerryman1900

Hi,
I am looking for some advice on whether or not an airtight Membrane in worth installing on the ceiling in my new build bungalow. I plan to airtight seal around doors and windows. I have a 6in cavity which will be pumped and I'm going to put thermal slabs on the inside off the exterior walls. With all this and several hundred mm of insulation In the attic is the membrane actually going to be worth the investment?

Also, Is HRV worth installing still without the airtight membrane installed.
Thanks.

Calahonda52

http://www.environ.ie/en/TGD/
http://www.environ.ie/search/archived/current/category/housing/type/publications?query=acceptable%20construction%20details
https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

Do you understand the purpose of the A/T layer?

Is this a home brew design and build?

Metric Tensor

Do a search on this forum to see why "thermal slabs" on the inside of your external walls is a bad idea.

What is your target air-tightness value?

JonathonS

In the immortal words of the well-known political slogan, it sounds like you have a lot of research done, but as the two posts above indicate, you have a lot more to do.

If you need advice on airtightness there is a co in Tralee who can help. Google "air tightness advice kerry" and they are on the first page.

They supply materials, but can also advise and even supervise installation. This could be important as it sounds like either your builder or your architect are not convinced, which could make for poor installation.

sydthebeat

kerryman1900 wrote: »

Hi,
I am looking for some advice on whether or not an airtight Membrane in worth installing on the ceiling in my new build bungalow. I plan to airtight seal around doors and windows. I have a 6in cavity which will be pumped and I'm going to put thermal slabs on the inside off the exterior walls. With all this and several hundred mm of insulation In the attic is the membrane actually going to be worth the investment?

Also, Is HRV worth installing still without the airtight membrane installed.
Thanks.

Your starting point is wrong. Don't pump the cavity and don't use internal slabs.

Use full fill boards in the cavity instead, and let the internal block work act as a thermal store.

Wartburg

kerryman1900 wrote: »

Hi,
I am looking for some advice on whether or not an airtight Membrane in worth installing on the ceiling in my new build bungalow. I plan to airtight seal around doors and windows. I have a 6in cavity which will be pumped and I'm going to put thermal slabs on the inside off the exterior walls. With all this and several hundred mm of insulation In the attic is the membrane actually going to be worth the investment?

Also, Is HRV worth installing still without the airtight membrane installed.
Thanks.

Standard building physics are the same everywhere in the world, they´re even valid in the Kingdom of Kerry. Air Tightness - yes, HRV - yes, insulated slab on the inside of the perimeter walls - never!
I had so many building owners over the years, telling me that they do not want to have their new homes "too tight" and didn´t want to get any air tight membrane installed. This idea works as good as the idea of being just a little bit pregnant.

monseiur

sydthebeat wrote: »

Your starting point is wrong. Don't pump the cavity and don't use internal slabs.

Use full fill boards in the cavity instead, and let the internal block work act as a thermal store.

Not wishing to hijack the tread, but was visiting a cousin yesterday, he showed me plans of a new bungalow he's about to start building and noticed the architect has specified that the cavity be pumped with bead insulation - is this a big no no ??? should he use boards instead
M.

sydthebeat

monseiur wrote: »

Not wishing to hijack the tread, but was visiting a cousin yesterday, he showed me plans of a new bungalow he's about to start building and noticed the architect has specified that the cavity be pumped with bead insulation - is this a big no no ??? should he use boards instead
M.

if its just beads in a 150mm (6") cavity..... then yes, thats a pretty poor spec...

Metric Tensor

Haven't been able to get a Part L spec over the line with beads in a 150mm cavity in quite a while. Appropriate bonded bead in a 200mm cavity is doable though.

Effects

Metric Tensor wrote: »

Do a search on this forum to see why "thermal slabs" on the inside of your external walls is a bad idea.

Can you give a short answer? The search function isn't great and showing old/unhelpful posts.

Metric Tensor

Potential problems with:

1. Interstitial condensation - you will need to carry out a dew point calculation to make sure mould does not grow on the wall behind the boards.
2. Difficulty in making a proper air-tightness envelope - you will need to plaster the walls before applying the board.
3. Reduction in thermal store - if you are proposing underfloor heating your house will likely benefit from as large a thermal store as possible and the dry-lining board removes the inner leaf of the wall from your thermal storage.

Calahonda52

Metric Tensor wrote: »

Haven't been able to get a Part L spec over the line with beads in a 150mm cavity in quite a while. Appropriate bonded bead in a 200mm cavity is doable though.

MT, what is the cavity width limit before we get into special ties and wall design etc?
Thanks as always

Gumbo

Calahonda52 wrote: »

MT, what is the cavity width limit before we get into special ties and wall design etc?
Thanks as always

150mm.
Anything over that requires design and specified wall ties.

Metric Tensor

Calahonda52 wrote: »

MT, what is the cavity width limit before we get into special ties and wall design etc?
Thanks as always

Yup, anything over 150mm. But as engineering designs go it's not the worst of designs. (Edited to add: It's not so much that the ties have to be special. They just have to be strong enough and frequent enough to do the job. And obviously long enough.)


Bear in mind that any building where the floor to floor height is more than 2.7m or even those with concrete floors at first level require a bespoke engineering wall design anyway!


(As Michael Caine would say: Not a lot of people know that.)

shianto

We had until now specified 180mm mineral wook EWI on our single leaf brick house, but I have been doing a bit of reading, and as best I can tell there if you ignore the environmental and fire considerations there are now better (cheaper) solutions with some of the more advanced EPS solutions approaching comparable thermal resistance results.

Can anyone point to some EPS that would be similar in performance to 180mm mineral wool?

As an aside we are going minimum 300mm below ground level so I am also now thinking XPS below ground with EPS above is probably the solution, but any comments welcome...

Thanks

Calahonda52

Who is "We" in this specification exercise?
What is the exact buildup of the mineral wool based EWI solution, from the outside layer all the way to inside face of brick wall ?
What is the detail between the EWI and the ceiling/roof insulation?
..
Its not just about thermal resistance

shianto

Calahonda52 wrote: »

Who is "We" in this specification exercise?

The architect in discussion with myself, but I get the distinct feeling the architect might have just done a copy and paste job, as well as generally being very "unavailable". Hence this posting...

Calahonda52 wrote: »

What is the exact buildup of the mineral wool based EWI solution, from the outside layer all the way to inside face of brick wall ?

Lifted directly from our tender specifications document

General: Knauff: ‘Rocksilk EWI Slab Plus
Thickness: 180mm minimum
Installation: Mechanically fix starter track every 300mm using SDK 085 fixings, use base rail connectors to fix starter tracks together. Fix Knauff: ‘Rocksilk’ EWI Slab 180mm thick to the brick substrate with 10mm TS-Bond and TS-Fixings. TS-Reinforcing Mesh to be fixed through the insulation into the substrate using TS-Fixings as recommended by manufacturer. . Embed Reinforcing Mesh in 5mm Bond reinforcing coat. Apply Primer Coat to dry reinforcing coat prior to application of Mineral render. Install EWI stops and drip beads around openings, at corners, at base of insulation and at damp proof course level. Where a structural movement joint occurs form with back-to-back starter-racks or with movement joint profiles.
U Value: Insulation Lambda of 0.036 W/mK to achieve an elemental u value of 0.18 W/m2K on solid brick walls
Areas vulnerable to moisture and thermal bridging: Cills moved forward for installation of new external insulation; windows mounted proud of the original walls for placement in the new external insulation layer, with Compacfoam thermal break at window cill.
100mm hydrophobic insulation, Kingspan ‘Styrozone’ or similar below the damp proof course inside and outside of the rising wall as per architect’s detail.

Calahonda52 wrote: »

What is the detail between the EWI and the ceiling/roof insulation?
..
Its not just about thermal resistance

Are you refering to air-tightness? The eve's detail is currently being worked up by the engineer (who is also stepping in to take up some of the "slack" left by the architect). We have an in between over rafter build-up warm roof, but I appreciate the interlock at the eves will not be straight forward and needs attention...

Back to the EWI, we are using a builder who seems reasonable at his craft, but could probably do with explicit detailed instruction and I thought the EPS/XPS route might be safer from an implementation perspective, give similar results and cheaper overall (also better performing beneath the ground level).

Thoughts?

Thanks

BryanF

Is the house detached? How old?

Calahonda52

Re:
Are you refering to air-tightness?
No, am concerned as to the construction of the existing wall
as per my question
What is the exact buildup of the mineral wool based EWI solution, from the outside layer all the way to inside face of brick wall ?

shianto

BryanF wrote: »

Is the house detached? How old?

Yes fully detached we estimate exact age 1890-1905 hence the single leaf brick. Just to add further context, we are adding an extension to to rear so really only 3 walls need wrapping (with a little on the 4th wall were the extension doesn't cover)

Calahonda52 wrote: »

Re:
Are you refering to air-tightness?
No, am concerned as to the construction of the existing wall
as per my question
What is the exact buildup of the mineral wool based EWI solution, from the outside layer all the way to inside face of brick wall ?

We are using brick slip on some parts and render on others, but no other written detail on the build-up has been worked up (other than what I pasted in the previous quote).

At the time I suggested to the architect that maybe we should be more explicit, but they said 'we will just specify type and depth and leave the rest up to the builder' and I believe the architect's intent was to leave it to the builder to engage the manufacturer to make sure the system was correctly installed.

Did I understand your question correctly?

Thanks

BryanF

Was the proposed render system also breathable, like the mineral wool?

shianto

BryanF wrote: »

Was the proposed render system also breathable, like the mineral wool?

Apologies I tell a lie, the build-up was specified (at least the render was in another part of the document). Here it has been lifted from the document:

MINERAL RENDER ON EXTERNAL WALL INSULATION:
General: Apply to external wall insulation a mineral and silicate based render as indicated on architect’s drawings.
Proprietary reference: Keim Universal Render
Location: External wall insulation externally
Background: Rock mineral Wool
Application: Embed Reinforcing Mesh in 5mm Bond reinforcing coat. Apply Primer Coat to dry reinforcing coat prior to application of Mineral render. Apply to manufacturer’s recommendations
Movement joints: Any movement joints present in the wall should be continued through the plaster finish through the use of proprietary movement beads. Adequate movement joints to be provided in the render to manufacturer’s recommendations.

As it is silicate based I believe it is breathable. That said, I am still concerned that the Mineral Wool will under-perform beneath the ground due to dampness... Is this a legitimate concern? If so, could we just place a line of XPS around leading on from the mineral wool, starting just a bit above ground level (say 150mm) running it down to 300mm below to improve this?

Calahonda52

The wool wont work below ground, you are correct about that.
I still don't know what the buildup of the existing brick wall, that will be covered in some form of EWI, is.
Clearly I am not communicating my question very well.
eg

1. Is there any render/paint on the outside of the brickwork?
2. How thick is the wall?
3. Is the mortar between the bricks lime based or not?
4. Is the brickwork rendered/skimmed on inside?
5. Is it painted/papered/etc?

shianto

Ah! Have your point now.

Answers interlace in red

Calahonda52 wrote: »

The wool wont work below ground, you are correct about that.
I still don't know what the buildup of the existing brick wall, that will be covered in some form of EWI, is.
Clearly I am not communicating my question very well.
eg

1. Is there any render/paint on the outside of the brickwork?
   On the north wall there is a cement(?) render everywhere else is just exposed 'Portmarnock' brick
2. How thick is the wall?
   1.5 bricks thick
3. Is the mortar between the bricks lime based or not?
   Lime based
4. Is the brickwork rendered/skimmed on inside?
   Plaster skim but interior being redone as well so will be re-plastered in areas missing
   
5. Is it painted/papered/etc?
   As above painted plaster

We have agreed (at this point anyhow) not to included an air-tightness barrier as the advice has been that EWI + the plaster it should be sufficient, and the joists, roof etc will most likely create greater air-tightness problems (so we are focusing our efforts on these). We are looking to bring the house down to 3sqm/hr, and while this is somewhat aspirational, any advice on this would also be appreciated...

BryanF

Is the house gutted inside? If so install the membrane or combination of internal plaster tapes and membrane. Ewi may not solve the air-tightness unless adequately detailed at all junctions and overlapped with internal air-tightness works - at eaves level for example.

The ewi specified is for breathability, and this should be respected, unless you are willing to take ownership of the consequences. as regards the polystyrene, this will be installed below plynth level, an ewi installer will know to install that.

Re ‘3sqm/hr’ - can you clarify if/ how this is specified, and Are you drilling 6’’ vent holes in the walls of each room or installing a ventilation system?

Calahonda52

shianto wrote: »

Yes fully detached we estimate exact age 1890-1905 hence the single leaf brick. Just to add further context, we are adding an extension to to rear so really only 3 walls need wrapping (with a little on the 4th wall were the extension doesn't cover)



We are using brick slip on some parts and render on others, but no other written detail on the build-up has been worked up (other than what I pasted in the previous quote).

This will cause a massive thermal bridge, unless your team has some sexy detailing in mind, you need to EWI all the way around

As BF has indicated, this solution is breathable, so the brick slips will give a different performance, maybe suboptimal depending on adhesive, than the render.


Re the internal plaster: is this gypsum based?

shianto

BryanF wrote: »

Is the house gutted inside? If so install the membrane or combination of internal plaster tapes and membrane. Ewi may not solve the air-tightness unless adequately detailed at all junctions and overlapped with internal air-tightness works - at eaves level for example.

There have been a couple of conversations around this. The house is being gutted but we were going to try to keep the cornicing for aesthetic but reasons. I was advised by an air-tightness tester not to be too concerned by not having a air-tight membrane on the walls as once it is re-plastered/painted that should be sufficient from an air-tightness perspective. That said, we are installing an air-tight / VCL membrane under rafters, and using taping and Gyproc Air Tight Quiet at the first floor joist zone to try improve the joist penetrations.
The eves are going to be problematic however particularly as the house has a hipped roof, but we are aware and will do out best on the detailing. Anything words to the wise on eves or is it just a matter of logically thinking it through?

The ewi specified is for breathability, and this should be respected, unless you are willing to take ownership of the consequences. as regards the polystyrene, this will be installed below plynth level, an ewi installer will know to install that.

OK, maybe that is what the architect was thinking, but isn't the EPS the most common EWI on the market? Why is breathable important in this case, (single leaf brick)? That fact that we installing MHVR system makes not difference to this I guess?

The builder is installing the EWI and polystyrene has not been specified or detailed but we can have that conversation..

Re ‘3sqm/hr’ - can you clarify if/ how this is specified, and Are you drilling 6’’ vent holes in the walls of each room or installing a ventilation system?

The 3sqm/hr was a single sentence stating the builder shall bring the building to this level, nothing more than that. On the ventilation we are installing a MHVR system. Looking at brands at the moment if you have any suggestions

Calahonda52 wrote: »

This will cause a massive thermal bridge, unless your team has some sexy detailing in mind, you need to EWI all the way around

After reading your point I started to worry but I spoke to the engineer, and as the extension is timberframe the is no reason the rendered masonry outside the timberframe needs to attach to the brick wall of the old house.

As BF has indicated, this solution is breathable, so the brick slips will give a different performance, maybe suboptimal depending on adhesive, than the render.

OK, but what can we do about this? Specify particular type of adhesive? Again to the point I reply above to BF's I understand now that the solution specified was breathable, but why is that important in this case and not other EWI situations? Does 1.5 leaf brick always require a breathable solution or is it a case of 'generally it is considered healthier for the building'?

It would be good to reduce costs by going with a cheaper type as we already have some unforseens...

Re the internal plaster: is this gypsum based?

not sure. I asked the builder suryer / Assigned Certifier and we will let me know next time he is onsite. What should I be considering here?

BryanF

your arch to detail the eaves and air-tightness

Eps is commonly used But is NOT appropriate in an old house, where a breathable ewi system has been specified by your consultant. Cost is not the only factor to be considered.

Mvhr has nothing to do with wall breathablity.

breathable : hygroscopic requirement of old walls with lime mortar pre-cement, pre circa 1920’s

https://www.cat.org.uk/info-resources/free-information-service/building/natural-building-materials/

https://www.lime.org.uk/community/lime-and-its-properties/lime-properties.html

Wartburg

shianto wrote: »

It would be good to reduce costs by going with a cheaper type as we already have some unforseens...

Why don´t you try the very common solution like pictured in the attachment? Insulated slab to the inside of the brick work and some timber battens with dpc underneath. Works perfect until the chequed is cleared. Sarcasm off

Consider to reduce the costs (temporarily) on the interior but not on your building structure. Your existing building requires a well-working solution regarding insulation and moisture protection/ vapour control. You can´t mix old structural pieces like bricks and limestone with our fancy new artificial materials, mostly made of a chemistry set.

shianto

BryanF wrote: »

your arch to detail the eaves and air-tightness

Eps is commonly used But is NOT appropriate in an old house, where a breathable ewi system has been specified by your consultant. Cost is not the only factor to be considered.

Mvhr has nothing to do with wall breathablity.

breathable : hygroscopic requirement of old walls with lime mortar pre-cement, pre circa 1920’s

https://www.cat.org.uk/info-resources/free-information-service/building/natural-building-materials/

https://www.lime.org.uk/community/lime-and-its-properties/lime-properties.html

Thanks the links a very helpful in understanding better. We will run with rockwool, however to @Calahonda52 observation, the implementation of the brick slip is now concerning me as this is something the consultant most definitely did not address.

Any advice on how to make the area covered with brickslip the most breathable?

Wartburg wrote: »

Why don´t you try the very common solution like pictured in the attachment? Insulated slab to the inside of the brick work and some timber battens with dpc underneath. Works perfect until the chequed is cleared. Sarcasm off

Consider to reduce the costs (temporarily) on the interior but not on your building structure. Your existing building requires a well-working solution regarding insulation and moisture protection/ vapour control. You can´t mix old structural pieces like bricks and limestone with our fancy new artificial materials, mostly made of a chemistry set.

Thanks for the thought, but better to stretch ourselves now and not have to revisit...

Calahonda52

Well, if you want to use fancy finishes which don't marry well with the breathability problem, then maybe go the rain screen route, where there is ventilated gap behind the rain screen, which could be cement board...
https://en.wikipedia.org/wiki/Rainscreen

The fact, IIRC, that your windows will be fixed to outside of the wall makes the rain screen detailing much simpler.

BryanF

I wouldn’t worry about the brick slips as long as the adhesive and pointing material is not n some way breathable.

shianto

Calahonda52 wrote: »

Well, if you want to use fancy finishes which don't marry well with the breathability problem, then maybe go the rain screen route, where there is ventilated gap behind the rain screen, which could be cement board...
https://en.wikipedia.org/wiki/Rainscreen

The fact, IIRC, that your windows will be fixed to outside of the wall makes the rain screen detailing much simpler.

Thanks I will have it costed.

BryanF wrote: »

I wouldn’t worry about the brick slips as long as the adhesive and pointing material is not n some way breathable.

Sorry not clear on your comment here. You are saying you wouldn't worry about using something like a rainscreen?
The total surface area is about 200sqm of this about 100sqm will be covered in brickslip in case that makes a difference.

Calahonda52

shianto wrote: »

Sorry not clear on your comment here. You are saying you wouldn't worry about using something like a rainscreen?
The total surface area is about 200sqm of this about 100sqm will be covered in brickslip in case that makes a difference.

BF's comment applies if not a rain screen

BryanF

shianto wrote: »

Thanks I will have it costed.



Sorry not clear on your comment here. You are saying you wouldn't worry about using something like a rainscreen?
The total surface area is about 200sqm of this about 100sqm will be covered in brickslip in case that makes a difference.

Sorry, predictive text

Brick slips are fine, assuming adhesive material is breathable.

shianto

BryanF wrote: »

Sorry, predictive text

Brick slips are fine, assuming adhesive material is breathable.

Sorry one last question: any brand names you know of that produce breathable adhesives for brick slip? A cursory 'Google' does not yield anything obvious.

Thanks

BryanF

shianto wrote: »

Sorry one last question: any brand names you know of that produce breathable adhesives for brick slip? A cursory 'Google' does not yield anything obvious.

Thanks

Have you contacted the ewi manufacturer specified?

shianto

BryanF wrote: »

Have you contacted the ewi manufacturer specified?

Nope... great point! I thought I used to be good at working out the obvious..

Thanks again.