Getting advice on airtightness

[Helen2]

Newbie question here. 
 

We are finalising the spec for our new build brick and block house in the south east.  We will be having an ASHP and MVHR - we want the latter for the quality of air as much as for the heat recovery.  
 

We are not looking for passive house type standards of airtightness but do want to make the most of our MVHR system.  The spec for the heating and MVHR are based on 4 air changes per hour and the general view of our architect, SAP consultant and construction company is that wet plaster plus triple glazing and Compriband will get us to where we need to be. But that seems to us to be missing the opportunity of the new build to get it more airtight.
 

We therefore want some fresh advice on what to specify but don’t know where to go for this.  There are lots of companies that do airtightness testing, and some that offer passive house consultancy but we want someone who could review our design and then advise on the most cost effective steps and, importantly, what materials we should use to improve our airtightness, without chasing after full <1 ach PH standards (which would not be affordable or achievable).  
 

Any suggestions?

[Helen2]

Correction - I meant 4m3/h.m2, not 4ach of course.  Said I was a newbie!

 

 

[Nickfromwales]

The wet plaster approach is a good start tbh, but detailing at the joist levels needs a method statement creating for the builder to follow ( adhere to! ) but you can laminate this proposed solution with tapes and membranes, including liquid membranes such as Passive Purple. We've just got a Velox ICF build in at 0.88ach including the doors and windows losses with my suggestions to the builder and the introduction of this 'hybrid' approach, which is pretty damn good afaic!! The fabric of the build is probably in the 0.5-0.6 range but as the doors and windows never got masked off we shall never know.

Good quality foams / tapes / membranes etc will be worth every penny. Everyone's attention to detail..........priceless.  

[NSS]

Interested that you quote air quality as being your priority. That was exactly the objective with our build and, as you rightly say, to aim for only 4m3/hr/m2 seems a missed opportunity that could risk that priority. We achieved 1.6 and the mvhr has provided an air quality that has dramatically improved my wife's life (she has a degenerative lung condition and severe allergies, both of which are exacerbated by airborne particles). Our build is TF so i can't advise on yours, but there are plenty on this forum who can probably point you in the right direction. Best of luck with it.

[Conor]

You'll easily get down to 1.5-2 with your current approach plus a few basic extra steps:

 

1. Air tight tape around windows and doors (from frame to inner block). Don't forget the door thresholds. NB compriband is an external weather sealing system to close the gap between the window frame and outer brick leaf, and won't do much for your airtightness.

2. Block up any conduits and penetrations through the walls / roof. E.g. conduits to external wall lights, get the spark to fill the ends with acrylic sealant when doing second fixes.

3. Foam / tape around all penetrations, e.g. waste pipes through the subfloor. Use foam for big gaps, then trim back and paint / tape. 

4. Roof structure... You've not mentioned this yet and is the most crucial one. you'll need a membrane or other taped solution. We just had our airtightness test done and it appears almost all of our leakage is through the roof structure. Unfortunately it's now impossible to rectify as we're baorded and painted.

 

5.. Most effective one... Let all your trades know you are aiming for an airtight house and not to go leaving holes everywhere! It's basic build quality.

 

6. Have rolls of airtight tape and a tub of paint on hand so you can quickly deal with anything you spot. E.g. we had to paint along the entire wall plate and under the windows. Paint is handy for things like bolts, hangers and around timbers than penetrate the wall.

 

To be honest, your architech should be the one guiding you on all of these. They should come out as the build progresses and advise.

 

One last tip, get an airtightness test done before second fixes so you've a chance rectify any issues.

 

 

I look forward to you posting here in a few months boasting about your airtightness test result 🤣

 

Edited May 2, 2022 by Conor

[joth]

It may sound silly, but remember to get triple glazed windows WITHOUT trickle vents. Some still charge more to remove(!) them from UK spec, one of the many reasons we went for Windows from a European manufacturer.

 

41 minutes ago, Conor said:

5.. Most effective one... Let all your trades know you are aiming for an airtight house and not to go leaving holes everywhere! It's basic build quality

 

Agree but I'd word this much stronger: make the airtightness goal a requirement in your contract with your main contractor, choose a contractor that has worked to a better than building regs airtightness goal before. Ensure they train and monitor all contractors work. Simple things like offering no-blame if airtightness is damaged so long as they report it immediately you have chance to fix it before it's too late.

 

We did OSB airtightness layer then a service void then plasterboard, this had 2 benefits: no need to seal around every socket/switch/light fitting etc, and also the OSB was bright green and really easy to teach the trades to never cut/drill into it without consulting the foreman.

 

Good luck!

 

 

[ProDave]

You won't know what air tightness you have until is it tested, so aim for the best you can do.

 

Most of it is detail where services e.g. pipes and cables enter or leave the building.  Seal all those up really well.

 

Build a service void round all the external walls e.f 25mm battens then plasterboard.  Then your cables and pipes can run down the wall without penetrating the already sealed building.

 

And the roof is the other difficult area, especially if you have the more common cold roof with the insulation on the floor of the loft.  Then your ceiling plasterboard layer upstairs becomes your air tight layer.  So fit an air tight membrane with all joints taped first, and then EVERY penetration in that i.e. every light fitting cable, all vent ducts, loft hatch etc have to be sealed.  That is why I much prefer a warm roof design with the insulation following the line of the roof but that really only works in a room in roof design.

[SteamyTea]

Spend a grand and keep testing.

Then rent it to others.

 

https://www.ebay.co.uk/itm/164767790472

 

 

[Iceverge]

20 hours ago, Helen2 said:

We are not looking for passive house type standards of airtightness but do want to make the most of our MVHR system.

 

 Why not? It's achievable with a little forethought.  Watch the Denby Dale videos on youtube.  Here's the first one.

 

 

20 hours ago, Helen2 said:

without chasing after full <1 ach PH standards (which would not be affordable or achievable).

 

We did all our own airtightness with no training or prior experience, wet plaster and block built. Materials were probably under €1000 including membranes for the ceiling. Key point, swere having a very exact plan from the outset and attention to detail but we got 0.31Ach. 

 

Beware the " close enough is good enough" method with this as missing just a few key details will leave you disappointed. 

 

Forget @SteamyTeas expensive suggestions. He must be made of money!!

 

 

This taped into a sheet of osb sealed into an opening, connected to your car battery with cable and crocodile clips will depressurise your house to a level that far exceeds a blower door. Then ram a candle into a length of electrical conduit and you have a £3 super sensitive leak detector. 

 

 

 

 

 

 

Edited May 2, 2022 by Iceverge

[SteamyTea]

1 minute ago, Iceverge said:

Forget @SteamyTeas expensive suggestions. He must be made of money

Why I was suggesting others but one.

There is a new 'pulse test that has been approved.

Compressed air is released, then the pressure profile measured.

Similar to farting under a light blanket, or so I am told.

[MikeSharp01]

8 minutes ago, Iceverge said:

. Key point, swere having a very exact plan from the outset and attention to detail but we got 0.31Ach. 

So where were the leaks that were still left? 

[Bramco]

10 hours ago, Conor said:

Air tight tape around windows and doors (from frame to inner block)

 

Partel do a tape specifically for these right angles - makes it much easier.  

 

9 hours ago, joth said:

It may sound silly, but remember to get triple glazed windows WITHOUT trickle vents. Some still charge more to remove(!) them from UK spec, one of the many reasons we went for Windows from a European manufacturer.

 

We didn't have an issue finding a UK company that didn't charge more.  But really important that all windows have no trickle vents. 

[Iceverge]

3 hours ago, MikeSharp01 said:

So where were the leaks that were still left? 

 

I don't know. I suspect it was an aggregate of pin holes in the scratch coat of wet plaster in the external walls. 

 

 

 

I couldn't find any more leaks myself. I'd be interested to redo it since the final layer of sand cement and skim was applied. Also one shower tray was just open to the insulation below as it hadn't been installed. The 400mm of cellulose pumped into the attic since should help also. 

 

Also there is a small leak from the corner of the window that I had the DIY blowerdoor installed as I couldn't pretest that. 

[MikeSharp01]

26 minutes ago, Iceverge said:

I couldn't find any more leaks myself. I'd be interested to redo it since the final layer of sand cement and skim was applied. Also one shower tray was just open to the insulation below as it hadn't been installed. The 400mm of cellulose pumped into the attic since should help also. 

 

That's interesting, what I was wondering then is how big a hole is .31ACH?

 

 

 

[MikeSharp01]

13 minutes ago, MikeSharp01 said:

how big a hole is .31ACH?

I guess the 0.31 is the passive house methodology eg air change rate h^-1 @ 50 Pascals.

[Iceverge]

It depends on the size and shape of the house. For us it was 50cm2, slightly more than the size of a credit card. 

 

If you look at leakage areas below you'll see the ELA F50 at 0.0000277m2/m2. This equates to a 6mm diameter hole drilled per m2 of building envelope or 1 part per 36000. 

 

If you had a less square building with more surface area but the same volume and floor space this number allowed per m2 would drop. 

 

Typically large buildings with low amount of external surfaces are easiest to get good ACH figures on mathematically (and practically). Small ones with complex forms are the worst. 

 

 

 

[MikeSharp01]

So is your building 532m³ ? Not sure how the shape of the building make any difference to the maths but I can see why it makes a big difference to the build challenges. Our build is 465m³ so next stop is to work out what our holes can add up to 

[Iceverge]

Correct. 532m³

 

I don't know if the leakage scales proportionally. If it did I guess you would be about 80cm2 for 0.6ACH 

 

 

 

 

 

 

 

[Helen2]

Thanks everyone for the super-helpful advice.  It gave me just what I needed to have a robust conversation with our architect who is now fully on board and making up for lost time rapidly!  And the contractor has confirmed that they frequently build to these standards so has also given helpful input.  

 

Works due to start on 13 June - it is getting close and scary but exciting at the same time!  I look forward to reporting our air pressure tests!