Airtightness requirement ??

[Post and beam]

Hi Guys

Todays question is, the frame build and weathertight package quote from my frame builder has this line in it.....

 

Air tightness requirement: You have advised a target of 1.5-3 is required.

 

Apart from the fact that i dont believe i have actually advised this value, what is the consequence of a higher or lower figure and why should i care?

Of course, i know a more airtight house is a good idea in general but why this value.

 

Thanks in advance again

keith

[JohnMo]

3 or better needs MVHR, certain Scotland it's mandatory.

 

Better airtightness means less heat loss. Coupled with MVHR even less heat loss.

[Post and beam]

I get that better airtightness means less heat loss.

What are the min and max values range.

Is there a value that planning or building regs require? I did not specify the value attributed to me and therefore am intrigued as to what the consequence might be. I have also been told that increased insulation thickness and Triple glazing might be required. Am i being led towards spending money to achieve a value that was not my chosing ?

[ProDave]

Minimum is 0 but I doubt anyone has achieved that yet.  0.5 is very good, anything under 3 means it must have MVHR (and also does not need trickle ventilators on windows etc)

 

What are you hoping to achieve?  A really well built house that is comfortable to live in and costs little to heat?  Or the cheapest build you can get away with that scrapes through building regs?

[JohnMo]

41 minutes ago, Post and beam said:

have also been told that increased insulation thickness and Triple glazing might be required

Neither contribute directly to airtightness.  Both will help heat losses but not airtightness.

[Iceverge]

57 minutes ago, Post and beam said:

. I have also been told that increased insulation thickness and Triple glazing might be required. Am i being led towards spending money to achieve a value that was not my chosing ?

 

Exactly the converse. If you are aiming for a minimum cost house you may be able to spare extra insulation and 3G windows if you achieve excellent airtightness. 

 

Airtightness is the single cheapest and best thing you can do to reduce bills and improve comfort. 

 

Passivhaus is 0.6ACH and probably the best standard although many on here have beaten that by quite a margin. 

 

Good design and about €900 of tape and membrane got us to 0.31ACH. The execution took thought however. 

[Thorfun]

if the TF company wish to specify an airtightness target then I would suggest you negotiate a maximum result of 2ACH and no minimum. for them to state a range between 1.5 - 3 means that they probably won't try very hard to get to the 1.5 level but if you specify a max of 2ACH then they have to achieve that and, in doing so, will probably end up at a much lower than 2ACH level.

 

there is a spreadsheet on here that @Jeremy Harris created that will allow you to see the potential benefit of reducing your airtightness and also increasing U-values. as @Iceverge says, airtightness has a massive effect on energy usage and insulation has diminishing returns. 

[Post and beam]

20 hours ago, JohnMo said:

Neither contribute directly to airtightness.  Both will help heat losses but not airtightness.

Agree. The point i am trying to get across is that i did not specify the value for air tightness. It is in the quote but it has nothing to do with me. Hence my subsequent question, am i being led towards an airtightness value and a U value that i did not specify?

I am not trying to build the cheapest house that will scrape through the regs nor am i trying to achive passivhaus. I am trying to get a house from the frame builders catalogue. I was, until i noticed this value in their quote, confident that they would deliver a comfortable and well built home to the budget i nominated.

[JohnMo]

When they reopen ask them where the figure came from, possibly a cut and paste from their last quote, that wasn't deleted.

[Thorfun]

30 minutes ago, Post and beam said:

Agree. The point i am trying to get across is that i did not specify the value for air tightness. It is in the quote but it has nothing to do with me. Hence my subsequent question, am i being led towards an airtightness value and a U value that i did not specify?

I am not trying to build the cheapest house that will scrape through the regs nor am i trying to achive passivhaus. I am trying to get a house from the frame builders catalogue. I was, until i noticed this value in their quote, confident that they would deliver a comfortable and well built home to the budget i nominated.

I'd take it as a positive sign that the company actually care about airtightness! I know MBC specify a max 2ACH for their standard timber frame product so maybe this company has taken a leaf out of their books and is now offering it as standard. an airtight house is a good thing but it will mean that you will need MVHR which might be an expense you weren't planning on. but if you want to reduce your bills during your occupancy of the property then airtightness and MVHR are a step in the right direction.

[Post and beam]

MVHR is indeed on the quote and yes it is an expense i was not expecting. However, reading about the benefits i am content with its inclusion.

My only real concern is the potential increase in spec of the insulation, liberally spread throughout the quote, towards a target not of my choosing. If that makes sense.

I live in a house that was built in the mid 1980's. Even a developers mass produced estate house would be much better insulated that what i live in now. And to be honest i have never previously thought of it as a cold or draughty house.

[Nickfromwales]

58 minutes ago, Post and beam said:

MVHR is indeed on the quote and yes it is an expense i was not expecting. However, reading about the benefits i am content with its inclusion.

My only real concern is the potential increase in spec of the insulation, liberally spread throughout the quote, towards a target not of my choosing. If that makes sense.

I live in a house that was built in the mid 1980's. Even a developers mass produced estate house would be much better insulated that what i live in now. And to be honest i have never previously thought of it as a cold or draughty house.

Fabric heat loss is dealt with by insulation / doors & windows etc, and ventilation heat loss is dealt with by arresting a) infiltration ( drafts ) and b) convection heat loss; loads of house builders / frame suppliers seem to not give this their full attention when detailing this at the roof level forgetting that heat rises . If you have poor ventilation heat loss measures ( eg not going to a very good / excellent ( not just 'good' )) level of airtightness, then all the insulation in the world will amount to zilch tbh as the saved heat will just escape to the clouds, whilst pulling lots of fresh, damp and cold air back in to replace it  

 

'Excellent' airtightness will mean you could install lesser amounts of insulation ( eg build to our current and shitty building regs equivalent ) and you'll still have a very high-performance dwelling as a result. I would focus all my energy and efforts in the pursuit of the best AT score you can get you mits on, and once you are already 'in to' the installation of AT tapes and membranes, it will ONLY need you keeping a close eye on attention to detail and no stone being left unturned to take you to the 'excellent' result.

 

Do not dismiss the value of putting in just a little more effort with this, as, if you're staying in the house long-term, the rewards ( lower running costs and personal comfort ) will be significant.

 

You absolutely need a definitive statement regarding method of execution / target ACH score etc ( before parting with any money ) as a lot of lesser interested TF suppliers pay this lip-service only, and that's on a good day...... Ask for details for sections where tapes / membranes start / finish etc and what happens at wall / slab junction plus doors and windows etc. If they start squirming, it's time to move on to another frame supplier.

Edited January 2, 2023 by Nickfromwales