Self build design - airtightness

[BrianWg]

I am planning a self-build on a plot that I have approval for a two-storey detached house with basement.

The structure is a traditional tiled roof on full fill cavity blockwork walls, rendered on outside and painted, with an external footprint of 7.5 x 9m.

Looking at air tightness I can find many guides and products that relate to the sealing the internal structure but I cannot find anything that relates to sealing the externals.

My thought is to use the external render to provide the airtight seal, but I cannot find any articles that either promote or dispute this approach.

Surely it must be the best solution for as long as the doors/windows and service entries are sealed,  whatever happens with electricals/plumbing etc inside is then irrelevant.

 I would be grateful for any comment or views as to whether this approach will work or not.

[nod]

1 hour ago, BrianWg said:

I am planning a self-build on a plot that I have approval for a two-storey detached house with basement.

The structure is a traditional tiled roof on full fill cavity blockwork walls, rendered on outside and painted, with an external footprint of 7.5 x 9m.

Looking at air tightness I can find many guides and products that relate to the sealing the internal structure but I cannot find anything that relates to sealing the externals.

My thought is to use the external render to provide the airtight seal, but I cannot find any articles that either promote or dispute this approach.

Surely it must be the best solution for as long as the doors/windows and service entries are sealed,  whatever happens with electricals/plumbing etc inside is then irrelevant.

 I would be grateful for any comment or views as to whether this approach will work or not.

I’m not sure what benefits you would have by making the Tge external airtight 

None of the through coloured renders are airtight This is why planners prefer them They keep the elements out while still being breathable 

Sand and cement would be better but you will still have weep vent above all openings and even around the DPC

 

Making the internal skin airtight is a better and very easy option 

[SteamyTea]

The benefit of making the external surfaces airtight is that you don't replace warm air that is trapped in the insulation with cold air.

The disadvantage is moisture retention.

[BrianWg]

Just found "https://www.labcwarranty.co.uk/blog/weep-holes-in-rendered-walls-when-are-they-required/" which states "Whilst render potentially offers additional protection some water penetration will still occur. As a result it is important in rendered masonry walls that cavity trays are installed with stop-ends over openings and lower level abutments where the external wall becomes the internal wall. However, because of the increased protection offered by render British Standards state, “weep holes need not be provided if walls have a rendered external finish." This relaxation is based on the assumption that the small amount of water that may penetrate the outer leaf and collect in the cavity tray will dissipate by natural ventilation. This exception only applies when there is no unrendered brickwork above the rendered section."

 

So this seems to indicate that I can avoid weepholes and therefore make a continuous layer that could be airtight. But any ideas of how I make the render airtight?

[jfb]

Why not concentrate on making the internal leaf airtight since it is easier and more effective? There isn't any real advantage to having two airtight layers.

[BrianWg]

From what I am reading it appears that it is difficult to create an airtight seal on the internal wall - as there are either cable runs/switches/sockets + joists and so on that have to be effectively sealed or you line it with a dpm and create a false stud wall to enclose all of the services.

The latter will take off a precious 50mm or so of room on the outside walls and the former seems like a lot of fussy work with a great risk of leakages

 

Am I overestimating the problems with an internal approach?

[Declan52]

For sockets etc on a block wall you set the back box on a dab of bonding so it seals the back of the block. The conduit coming into the box gets sealed up as well so air can't come down from the joist space.

Remember you only need to do this on external walls so with careful planning you won't need to do that many .

Joists can get wrapped in membrane to form a tray and this gets sealed to the wall.

You can buy special tape that is like cloth so one side sticks to the frame of a window or door and the other gets glued to the blocks to form a seal here.

There are lots of methods to seal up a block house. Wet plaster is the best method.

 

[tonyshouse]

You need to decide where the air tightness layer is, what it is and how it joins or seals to the adjoining ones.

 

basically everyone’s home is a colander with draughts everywhere 

 

I worked hard to get a good level of airtightness - it is tough - easier on a self build 

 

set target u values 0.1? And have an air tightness target to aim at, test it before finishes, 0.6 or 1 Not 3, 5 or 10 

[BrianWg]

Have just seen a passivhaus article that indicates that blockwork can be made be made airtight as long as the entire surface is parged, so I think you guys are right and I am best to concentrate on the internal skin and leave the external as breathable. Many thank for the advice

[Iceverge]

Hi Brian,

 

Don't forget you'll need to have a good windtightness layer to ensure your insulation performs optimally too. 

 

Alas "breathable" is too oft used interchangeably with "ventilated" creating much confusion about the topic.

 

A build up of moisture in a wall is bad news for a structure. Breathability means that the wall can dry through the material It's composed from, or in other words it's somewhat vapour open. Ventilation means there's holes involved. Holes can carry heat out of the wall and moisture into the wall, best avoided. 

 

Our wall was externally to internally as follows:

 

1. Sand cement render with lime for flexibility and breathability. (Windtight layer)

2. Concrete blocks (not airtight)

3. EPS blown bonded beads. 

4. Concrete blocks ( again these are not airtight!)

5. Sand cement lime plaster. (Airtight layer)

 

The wall is fully breathable but in no way ventilated. 

We had an airtightness test result of 0.31 ACH.

 

Having an internal airtight layer removes the risk of interstitial condensation and importantly makes it possible to test and remedy problem areas too. 

 

Good luck and there's plenty of details here as to how best achieve your aims .

 

Jonathan

 

 

[tonyshouse]

Internally wet plaster is thousands of times better than parge coat and stuck on linings 

[Conor]

If you have big concerns over airtightness and measures required to make a non-airtight construction air-tight, you could change to an inherently airtight method, e.g. ICF.

[joe90]

My build is brick and block and airtightness very good, parge coat and wet plaster to inside, silicone to cables in sockets. Don’t see any point in airtight external skin.

[BrianWg]

All good - if Jonathan can achieve a 0.31 ACH then I am going to go for

 

1. Sand cement render with lime for flexibility and breathability. (Windtight layer)

2. Concrete blocks (not airtight)

3. 100mm Ursa 35 Full fill cavity batt (seems the cheapest at £5 sqm)

4. Concrete blocks ( again these are not airtight!)

5. Services chased into inner wall and sealed 

6. Sand cement lime render

7. Skim plaster

 

Joists set into hangers but with that section of blockwork having been parged before installing the hangers

 

and hope for a similar .3 ACH

 

[joe90]

Will 100mm cavity batt give you a good enough U value?. My cavity is 200mm full fill with batts, brick and block skins.

[Oz07]

5 hours ago, tonyshouse said:

Internally wet plaster is thousands of times better than parge coat and stuck on linings 

I got 1.19 on my air test tony with parge. I reckon both methods would be as good at sealing the blocks. Disadvantage with parge coat is if it cracks behind plasterboard stuck on you wont know whereas you can see and fill any cracks in proper plaster.

[tonyshouse]

Mine is 300mm full fill ?