Airtightness 'improve' at practical completion?

[Gaf]

Got word the airtightness test came back just over 3. House has first fix done and airtightness work also done. Builder is saying it will go below 3 once the plasterboard / plaster work is completed.

 

It's a new build with MVHR. Building regs require under 3 with MVHR. I've read 3 isn't even particularly good anymore.

 

Since the airtightness work is completed, surely it should be under three without resorting to relying on the plasterboard / plastering??

[Kelvin]

It’s better than most new builds but poor compared to most on here. 
 

 

Plasterboard and plastering is on the wrong side of your airtightness layer. While it may or may not  improve your score it won’t help the performance of the house. If you’ve not boarded it out you should still have access to the airtight layer to improve it? 
 

Are you sure you blocked up all the obvious bits such as any ductwork that leads to outside. My first result was over 1 and I couldn’t work out why as there wasn’t anything obvious then I remembered I’d forgotten to seal a duct in the plant room that ran to the outside. Final result was 0.44.  When the airtightness test is being done it’s relatively easy to find the leaks just by walking around inspecting all the obvious hard to seal areas such as wall/roof penetrations, joint where one material meets another or wall/roof/floor junctions, around windows and doors. 

Edited Thursday at 12:37 by Kelvin

[nod]

Is it a TF with an unvented loft or traditional build 

[Russell griffiths]

Depends what you are trying to achieve 

if just above building regs insulation and double glazing then don’t panic. 
 

if your aiming for something much better then you need to look at where it’s leaking out and address that first. 
 

did they not check for air leakage when doing the test. 
where you not there. 

[Gaf]

1 hour ago, Kelvin said:

It’s better than most new builds but poor compared to most on here. 
 

Plasterboard and plastering is on the wrong side of your airtightness layer. While it may or may not  improve your score it won’t help the performance of the house. If you’ve not boarded it out you should still have access to the airtight layer to improve it? 
 

Are you sure you blocked up all the obvious bits such as any ductwork that leads to outside. My first result was over 1 and I couldn’t work out why as there wasn’t anything obvious then I remembered I’d forgotten to seal a duct in the plant room that ran to the outside. Final result was 0.44.  When the airtightness test is being done it’s relatively easy to find the leaks just by walking around inspecting all the obvious hard to seal areas such as wall/roof penetrations, joint where one material meets another or wall/roof/floor junctions, around windows and doors. 

 

It wasn't flagged the test was being done, just got informed of the above "3" score. Waiting on a report. Yeah I didn't think the PB had anything to do with airtightness.

 

 

38 minutes ago, nod said:

Is it a TF with an unvented loft or traditional build 

 

Traditional block build. Cold attic (but not usable attic either as it's 1.5 storey).

 

15 minutes ago, Russell griffiths said:

Depends what you are trying to achieve 

if just above building regs insulation and double glazing then don’t panic. 
 

if your aiming for something much better then you need to look at where it’s leaking out and address that first. 
 

did they not check for air leakage when doing the test. 
where you not there. 

 

Building regs require u-value of 0.18 on walls (just taking walls as one example), our calculated u-value based on insulation is 0.16.

 

i've requested the report and to know was there no identification of leaks to remedy. I wasn't informed the test was being done, just informed of result.

 

[Kelvin]

Is the airtight target in your contract with the builder?  (I’m assuming they fitted the airtightness layer) 

[Gaf]

20 minutes ago, Kelvin said:

Is the airtight target in your contract with the builder?  (I’m assuming they fitted the airtightness layer) 

 

Only to meet building regulations, so under 3. Our architect did not spend much of any time considering an airtightness strategy (general notes in spec about various areas being made “airtight”) and I did not know enough about it at that stage of the process to ensure it was focused on. I can see why so many knowledgeable folk here attend to these things themselves.

 

Im accepting that i can only hold the builder to the regs. My understanding is that the airtightness work should get the house < 3, and not the plaster work.

 

 

 

[Kelvin]

I’d say the vast majority of trades don’t have a clue when it comes to air tightness. All of the guys I used marvelled at our final score. They’d never been in a house so airtight before and they mostly work on new builds. 
 

 

Yes the airtightness layer and how you seal all the penetrations through it should be the focus. If you’re just over 3 then it ought to be easy to get below 3.  It would be worth even a day or two of your own time armed with some suitable products to go around the place and do it yourself to get you well below three. Are you in Scotland? 
 

Them suggesting the boarding out and plastering will get you under is either ignorance or laziness. 
 

Post up some pictures of window/door edges, any holes through the external walls/roof, floor to wall and wall to roof junctions and any ductwork you have in the floor. 

Edited Thursday at 14:20 by Kelvin

[SBMS]

@Gaf if you really want to improve your score you could get Aerobarrier in. It’s likely they would improve it significantly although wouldn’t be cheap. 
 

However, i think you’d see payback with regards heat loss due to uncontrolled ventilation. 
 

Aerobarrier can be done at first or second fix although much much better done prior to plastering. 

[Andehh]

Spent a day with expanding foam, silicone and some bitumen paint type products! Just systematically check every construction interface... Have you got a log burner?

[Russell griffiths]

Now you have more knowledge, what score would you like to achieve?

if you want it better then get the tester back and find the poor areas. 
probably the best 3-4 hundred you could spend. 

[nod]

Your builders correct It will improve once plastererd

[Adrian Walker]

Tell your builder than you are not paying him if it’s greater than three. 

[Mike]

On 02/01/2025 at 14:31, Gaf said:

Traditional block build.... ...My understanding is that the airtightness work should get the house < 3, and not the plaster work.

A block-built house will have many small gaps through the mortar joints and, provided you use traditional wet plaster instead of plasterboard, that should significantly improve - your builder is correct. However it's still necessary to seal all junctions ahead of the plastering, and to attend to the ceiling & service penetrations, but it seems that you have the time to do that.

 

On 02/01/2025 at 15:01, Gaf said:

Im accepting that i can only hold the builder to the regs.

If you've not specified otherwise then that would be justified. However 3ach isn't that special* and there's nothing stopping you paying them or someone else to improve on that now. Or, for the best results, read the many topics on it here and do it yourself. For example think about adding a service void to the ceiling. And get the air-tester back either before, after, or both.

 

After plastering, make sure that nobody cuts holes through anything afterwards, unless you approve where and how.

 

*the best post-completion airtightness result in the UK is currently 0.048 ach (at Larch Corner).

[Gaf]

Four attached as max file size upload reached.

[Gaf]

Got dark on me quicker than expected so didn’t get a wall to ceiling or wall to floor junction image on ground floor. 

 

We have precast concrete first floors. These are hollow core but they are pumped with what looks like a string polystyrene-like filling for airtightness. There was also an airtightness membrane wrapped around the concrete floors where they rest on the external walls (resting on inner leaf), this membrane wrapped from ground floor ceiling junction to first floor floor junction.

 

All external walls (inner leaf) are parge coated. It doesn’t look like any particular attention went to floor junctions on either floor, and no attention to ceiling junctions on ground floor.

 

The reveals in the images don’t have the parge coat, but this is being done.

 

The air tightnes tester will be coming back. When the test was done, there:

- A ~30cm hole torn in the airtightness layer above the stairs. Was done for a pulley/winch to lift plasterboard to first floor. This is being resealed.

- One wall (1.2m x 4m) hadn’t been parge coated. This is being done.

- None of the electrical conduits running into the cold attic were sealed. These will be sealed now.

[nod]

I’ve never known airtightness testing prior to plastering Except self builders 😁

[Gaf]

On 02/01/2025 at 12:26, Kelvin said:

It’s better than most new builds but poor compared to most on here. 
 

 

Plasterboard and plastering is on the wrong side of your airtightness layer. While it may or may not  improve your score it won’t help the performance of the house. If you’ve not boarded it out you should still have access to the airtight layer to improve it? 
 

Are you sure you blocked up all the obvious bits such as any ductwork that leads to outside. My first result was over 1 and I couldn’t work out why as there wasn’t anything obvious then I remembered I’d forgotten to seal a duct in the plant room that ran to the outside. Final result was 0.44.  When the airtightness test is being done it’s relatively easy to find the leaks just by walking around inspecting all the obvious hard to seal areas such as wall/roof penetrations, joint where one material meets another or wall/roof/floor junctions, around windows and doors. 

 

There were gaps as it turns out. They’ll be sealed and retested. Plan to be there for the retest.

 

On 02/01/2025 at 14:14, Kelvin said:

I’d say the vast majority of trades don’t have a clue when it comes to air tightness. All of the guys I used marvelled at our final score. They’d never been in a house so airtight before and they mostly work on new builds. 
 

 

Yes the airtightness layer and how you seal all the penetrations through it should be the focus. If you’re just over 3 then it ought to be easy to get below 3.  It would be worth even a day or two of your own time armed with some suitable products to go around the place and do it yourself to get you well below three. Are you in Scotland? 
 

Them suggesting the boarding out and plastering will get you under is either ignorance or laziness. 
 

Post up some pictures of window/door edges, any holes through the external walls/roof, floor to wall and wall to roof junctions and any ductwork you have in the floor. 

 

In Ireland. Pics attached to last two posts.

 

On 02/01/2025 at 17:56, Andehh said:

Spent a day with expanding foam, silicone and some bitumen paint type products! Just systematically check every construction interface... Have you got a log burner?

 

No log burner.

 

On 02/01/2025 at 18:08, Russell griffiths said:

Now you have more knowledge, what score would you like to achieve?

if you want it better then get the tester back and find the poor areas. 
probably the best 3-4 hundred you could spend. 

 

There were holes in overall airtightness approach and one wall hadn’t been parge coated (all being rectified).

[Gaf]

2 minutes ago, nod said:

I’ve never known airtightness testing prior to plastering Except self builders 😁

 

Yeah the various opinions on building is difficult to disentangle for a novice doing their first (and bloody last) build 🫣😃

[nod]

3 minutes ago, Gaf said:

 

Yeah the various opinions on building is difficult to disentangle for a novice doing their first (and bloody last) build 🫣😃

This is why most self builders choose the easier more expensive TF option 

Great for airtightness and very simple to insulate and attach a membrane to