How much more might an air tight new build cost than a leaky new build?

[Tony L]

I’m building a house that will look a bit like this:

 

124m2 footprint , around 114m2 GIA downstairs & 75m2 upstairs.  Block inner leaf, block outer leaf with render.  Although I’m only planning for 150mm EPS beads in the cavity, I’m intending for there to be sufficient insulation upstairs to eliminate the need for upstairs heating.  I’d like to see a 0.6 air tightness score.

 

 

[Tony L]

My partner (it’s half her money) has been speaking to someone who we approached to help us with the superstructure drawings, & he’s told her, unless we know we’re going to live in the house for 10 years, making it air tight is a waste of money because we won’t sell it at sufficient premium over a "normal" house to get our money back.

 

He says we’d have to use a specialist eco builder & they cost a lot more.  I’m thinking, if we have good construction drawings & we choose a builder that is comfortable with the concept of airtightness, we’ll be OK, if I’m on site to inspect the work at least every other day, & issue instructions, as necessary.

 

We haven’t yet got superstructure drawings that are good enough to give to QS.

 

So my question is: about how much more will it cost to build an airtight house than if I build a leaky house?

 

I see the main costs being:

   MVHR system (£6k, if I do some of the installation work?)

   Membrane, tape, purple paint

   Ledger boards & hangers where FF joists meet exterior walls

   Additional construction hours – it’s sure to take a lot more time to get the details right

   & there’ll be a small saving on radiators, pipe work, etc upstairs.

 

Obviously (to me at least), it’s more about comfort than payback times, but about how much could I expect to save on my heating bills each year (SE England & heating via ASHP)?  I was thinking the answer to this might be around 70% - that’s just a wild guess , so if anybody has proper knowledge, please speak up.

 

Of course, the easy answer is, “It depends…”, but please work with me on this & suggest some figures.

 

[SteamyTea]

I was bored yesterday and knocked up a simple spreadsheet to work out thermally losses.

As your house is a unique shape, rather than a basic cube, you really need to do the same. Then assign capital and running costs.

 

A bit boring to do, but best options should show themselves.

[Tony L]

Yes, the shape/form factor is far from ideal.  My early designs had good form factor, but due to planning constraints, I was forced to choose between a bungalow & what we've ended up with.

 

I don't have time to do the detailed mathematical modelling, you suggest, @SteamyTea.  I need to make progress before my relationship disintegrates - I'm hoping for some estimates of additional cost to build, & perhaps also reduced costs to run, based on BH experts' knowledge & experience.

[nod]

Do your house for yourselves 

Potential buyers will prioritize 

Location -Schools 

m2

Price 

Kitchen 

and what your including in the price 

 

We sold our first build 

High speck and very airtight 

All of our serious viewers Around 20 

where doctors and consultants 

and one Barrister Not one asked about airtightness or Sap EPC 

Levels of insulation 

Just one young couple asked about wifi and bills 

 

Other than on here Most people wouldn’t know what an air test or sap report is 

 

It’s not a lot more money to make your home airtight 

Dont get hung up on numbers 

My better half has the windows open most of the time complaining the house is too hot 

[Tony L]

9 minutes ago, nod said:

Do your house for yourselves 

 

 

Thanks, @nod. That's my way of thinking, but the problem is, I'm certain we'll be there for a minimum of three years, to avoid the CIL payment, & more than likely we'll be there longer, so I want it to be a good place to live, but I don't know about four years from completion onwards, so we're not building with a forever home mentality.  My partner is different from me.  She is very focused on the house being worth a good deal more than we're going to have paid for it.

 

9 minutes ago, nod said:

It’s not a lot more money to make your home airtight 

 

 

That's good to hear, but how much is not a lot?  Less than £12k?

 

16 minutes ago, nod said:

Dont get hung up on numbers 

I'm very focused on the numbers that have a "£" in front of them, & the missus is, even more so.

[nod]

5 minutes ago, Tony L said:

 

Thanks, @nod. That's my way of thinking, but the problem is, I'm certain we'll be there for a minimum of three years, to avoid the CIL payment, & more than likely we'll be there longer, so I want it to be a good place to live, but I don't know about four years from completion onwards, so we're not building with a forever home mentality.  My partner is different from me.  She is very focused on the house being worth a good deal more than we're going to have paid for it.

 

 

That's good to hear, but how much is not a lot?  Less than £12k?

 

I'm very focused on the numbers that have a "£" in front of them, & the missus is, even more so.

Exactly what we did We stayed three years Then moved into our current build In the grand scheme of things the extra that you spend on insulation and making sure the house is as best that you can make Is well worth it 

 
We never used the bedroom radiators 

Not even once Which indicates that we’ve done something right 

 

Your post is refreshing 

I despair when I read on here people intending to UFH the upper floors 

 

[ProDave]

The cost of extra materials for air tight is not much, for our TF house it was a few rolls of air tight membrane and a lot of air tight tape.  The BIG thing is attention to detail at EVERY stage and by EVERY trade to avoid unnecessary holes of any size, anywhere.

 

And then there are avoiding the planned holes.  No individual extract fans (which most of the time are just an unwanted 100mm diameter hole in your building) No letter box, no cat flap, no open vented fire with a chimney or a flue, no trickle vents in windows.  Instead you have MVHR that does all the ventilation with just one ballanced inlet and outlet and removes the need for all the other uncontrolled holes.  And if you fit a stove, have one with ducted air inlet or otherwise known as room sealed.

 

There are additional costs if you want good quality triple glazed windows etc.

 

Our house has typical U values of 1.3 to 1.4 in the walls roof and floor, and even up here in the Highlands we don't have heating upstairs.

 

If you are going to do it. seek detailed advice here, but it really means educating and supervising every trade.  Don't just let the electrician and plumber drill holes where they feel like it.  Plan it with them and keep an eye to make sure they stick to the plan.

[ToughButterCup]

26 minutes ago, ProDave said:

....  The BIG thing is attention to detail at EVERY stage and by EVERY trade

 

😔

Too knackered

 

[Tony L]

Thanks, @ProDave. I’m on board with all of this, but it’s good to hear it again from one of the BH experts.   With a 150 cavity & blocks either side, I can’t see that we’re going to do any better than 1.8 U value for the block walls, although I will look to fatten up the dormer walls so we do better there.  I have plenty of space for deep warm flat roofs, but I’ll be compromising on the sloped roof insulation, so it doesn’t take too much space from the rooms under the sloped roof on the back of the house.

 

I don’t think 3G everywhere will be affordable. If I can, I’ll have all the windows in the sloped roof in 3G, as there’s more loss through a window in a 40 degrees roof than if you stick the same window in a wall. & 3G in the roof windows will help mitigate solar gain too.

 

I’ve done a lot of reading here on BH, so I have a good idea of what I want.  The architectural technician who’s been doing the drawings for me took my brief,  which included detailed explanations of how I want things to be.  He claimed to have understood it & has drawn something that’s not even the same shape as my design, has a much higher ridge height, gas boiler, ME vents, cold bridges all over the place, & all sorts of other mistakes.

 

I decided to cut our losses with this guy & try to find somebody else to do our building reg.s drawings & construction drawings.  I spent a lot of time briefing a PH certified architect, who said he could help me & then another architectural technician, who claimed to have just completed an air tight project.  The PH guy’s quote came in & it was thousands of pounds more than he’d said it might be after our initial long phone conversation & face to face meeting.  The architectural technician, who was very keen when my partner spoke to him, said he no longer wanted the job, once he’d seen my brief: which he said was very detailed & a bit frightening.  I got the impression he likes to work for customers who know next to nothing about how their houses should be built. 

 

So it could be a case of going back to the arch tec who gets everything wrong & making every last decision myself, then telling him what corrections to make to his drawings.  This is going to take forever & I’ll be asking a lot of questions here – that’s why I was planning to pay more for the proper PH architect, who I expect would have known exactly how to draw all the details, without any guidance from me.

[Nickfromwales]

8 hours ago, Tony L said:

I spent a lot of time briefing a PH certified architect, who said he could help me & then another architectural technician, who claimed to have just completed an air tight project.  The PH guy’s quote came in & it was thousands of pounds more than he’d said it might be after our initial long phone conversation & face to face meeting.  The architectural technician, who was very keen when my partner spoke to him, said he no longer wanted the job, once he’d seen my brief: which he said was very detailed & a bit frightening.  I got the impression he likes to work for customers who know next to nothing about how their houses should be built. 

They're referred to by my good self as "useless, pretentious, undereducated, out-of-touch, overpaid, lazy, w4nkers". Good for you to have filtered out these 'shooting fish in a barrel'-style pr1cks.

 

I have met sooo many of them that are just staggeringly bad at their job; the only thing they seem outstanding at is spending their clients money and caveating their contracts to protect you from complaining about just how shite they are, or ever getting any money back.

 

8 hours ago, Tony L said:

that’s why I was planning to pay more for the proper PH architect, who I expect would have known exactly how to draw all the details, without any guidance from me.

LOL. (sorry). PM me and I'll send you a list of the ones to avoid. Also, it doesn't sound to me like you need a PH certified architect, as that isn't your issue?

 

[LiamJones]

It sounds like you already know a fair bit, grab a pen and paper, and do the plans yourself. That’s what I did! Helps you understand all the details. It’s really not that complicated. You should be able to draw a continuous airtight line through all your details. Here’s how I did mine 

 

[Thorfun]

8 hours ago, Tony L said:

So it could be a case of going back to the arch tec who gets everything wrong & making every last decision myself, then telling him what corrections to make to his drawings.

Don’t do it! You sacked them for a reason. Surely there are more than 3 people you can look at using. Search slightly further afield. 
 

8 hours ago, Tony L said:

that’s why I was planning to pay more for the proper PH architect, who I expect would have known exactly how to draw all the details, without any guidance from me.

You don’t need a PH architect, just an architect that knows what they’re doing and is open to ideas about PH principles.
 

You don’t need 0.6ACH, you don’t need PH certification. Just build a house using the PH principles and you’ll get a lovely home to live in for however many years you choose to stay. 

[Tony L]

15 hours ago, Nickfromwales said:

LOL. (sorry). PM me and I'll send you a list of the ones to avoid. Also, it doesn't sound to me like you need a PH certified architect, as that isn't your issue?

 

That's right: I don't really need a PH certified person to do these drawings for me, but I thought, if this man has drawn PH houses then he can definitely draw what I want, - let's call it BH principles, rather than PH principals.  I've come around to the view that most of the people who do architectural drawings are stuck in the past & they only update their knowledge so their work can just scrape through the latest building reg.s, & that's not what I want.

[Tony L]

7 hours ago, Thorfun said:

You sacked them for a reason. Surely there are more than 3 people you can look at using. Search slightly further afield. 

I didn't actually sack him - he's waiting for me to annotate his drawings & point out all the errors (this is a lot of work as there are so many errors).  I'd rather start again with somebody new, who is competent & will make all the difficult decisions for me - such as wall/roof build up, insulation types, membrane types, what happens at all the surface junctions, etc.  Each time I start a conversation with somebody new, it costs me a lot of time - only to find out they can't do what they said they'd do.  I'm under a lot of pressure from my partner, who is angry & impatient.  She's at the stage where she just wants a house that looks really good & can be built within our budget; she doesn't really care if it costs a fortune to heat in winter & overheats in the summer.

[Tony L]

8 hours ago, LiamJones said:

grab a pen and paper, and do the plans yourself

Thanks @LiamJones.  That's impressive work you've done there.  If I get to build a second house, I'd like to try to do what you've done.  I did the PP negotiation/drawings myself, but I just don't have the time to fill the gaps in my knowledge then set about dealing with all this myself.  I have a business to run, an elderly mother to look after (still in her own home) & other commitments (I'll spare you the details), so I just want to pay a competent person to deal with the drawings so we can get the full plans approval & start building the superstructure.  We've already built up to our B&B floor under building notice.  & we need the drawings to get a QS estimate as well - then we may need to change the spec' to fit our budget.

[SteamyTea]

Back to the original question about airtightness.

If you have some plans, I may have a bit of time to play about, be an interesting way of checking the theory against the build.

Care to post them up, with some dimensions and U-Values on them.

[Tony L]

18 minutes ago, SteamyTea said:

Back to the original question about airtightness.

If you have some plans, I may have a bit of time to play about, be an interesting way of checking the theory against the build.

Care to post them up, with some dimensions and U-Values on them.

Thanks @SteamyTea.  I don't have plans in a suitable format to upload here yet.  I have my original PP drawings, with lots of annotations hand written onto them.  I don't have U-values - I'm hoping to find a competent person to decide on the build up of the various external surfaces & work out all this for me.  Once I have drawings worthy of BH's attention, I'll upload them & invite criticism.  Thanks again for your offer. 

[SteamyTea]

There is that latex spray machine, not sure how much it costs, but a quick way to make a place airtight, well from the inside anyway.

[Thorfun]

2 hours ago, Tony L said:

She's at the stage where she just wants a house that looks really good & can be built within our budget; she doesn't really care if it costs a fortune to heat in winter & overheats in the summer.

If she’s at that stage already then she won’t last the stresses and time sink of a self-build!

[SBMS]

5 hours ago, Tony L said:

Thanks @SteamyTea.  I don't have plans in a suitable format to upload here yet.  I have my original PP drawings, with lots of annotations hand written onto them.  I don't have U-values - I'm hoping to find a competent person to decide on the build up of the various external surfaces & work out all this for me.  Once I have drawings worthy of BH's attention, I'll upload them & invite criticism.  Thanks again for your offer. 

My airtightness strategy (on a brick and block build no less):

 

Tony tray for the floor joists

No external service penetrations (all soil pipes internal) except for mvhr

Smartply propassiv for roof inner structure taped at joints, lapped onto walls

and then we are going to use aerobarrier once wind and watertight to seal up

 

There’s another post on here why I decided this route instead of membranes, tapes, parging etc

 

Then keeping on top of builders is 100% more important than what architect draws. Especially if they’re not familiar with airtight principles.  

I modelled the heat loss and uncontrolled ventilation was a huge contributor so am

hoping it is money and effort well spent. If it all works out!!

[Tony L]

1 hour ago, SBMS said:

 

There’s another post on here why I decided this route instead of membranes, tapes, parging etc

Thanks.  I'll have a look for this (& come back to you, if I can't find it).

 

1 hour ago, SBMS said:

Tony tray for the floor joists

 

I was thinking to put a stripe of purple paint or flexible (lime?) parge onto the walls, then fix a ledger board with bolts into resin in the blocks.  This seems more straight forward than Tony trays to me (although I could easily be wrong about this). Also, the ledger board approach means my joist ends won't be inside my (only 150mm) cavity, sucking heat out from the house.  I could be persuaded on the Tony trays - I should think this would give a saving on materials.

[Nickfromwales]

Not penetrating the internal leaf of the B&B structure, multiples of times, and then not having to be pre-disposed to correcting each of these penetrations, is a no brainer . 
 

Pick your battles! Life’s short. 

[TerryE]

We have a passive class TF build with external stone cladding 2 storeys with 2 rooms and en-suite in warm loft, so really 3 floors. We have UFH on the ground floor in the poured slab and nothing on the two upper floors.  Not having to stick rads on any walls is a real boon for laying out your rooms and planning built-in cupboards and furniture.  BTW, all of our wardrobe space is in built-ins / walk-ins, so no VAT, and again keeps living space uncluttered.  What we saved on CH costs easily paid for our (self-installed) MVHR.

 

IMO, air-tightness isn't really a major cost as long as you've factored into your design and build rules.  It's attention to detail and making sure everyone understands the rules and follows them.

 

When you say "I'm building" this can mean a spectrum from "I am really paying a builder to build it for me" to "I am building it all myself". Where are you on this scale? In our case, we used a specialist sub for the slab and TF, another for the fenestration, and a good local builder for all of the ground-works and stone skin.  We also used a couple of his regular and recommended tradesmen for electrics, tiling, slate roof and plaster boarding.  I also got a local architect technician to do the plans for the planning application for a few £K.  Other than that we did the overall design, project management, procurement, quality assurance, and various trades like plumbing, woodwork, MVHR, CH, so pretty much all internal second fit; also planning and BCont docs and sign off.   TBH neither Jan or I realised just how much work this would really entail, and we were exhausted by the time we moved in.  Still 8 years later, and we live in a pretty maintenance free house with pretty much no post move in problems or things needing fixing.  We have a high quality house and saved a lot on our build costs by doing a lot of the attention-to-detail stuff ourselves.

 

One Q I would ask: Why so much glass?  It costs a lot more than TF/blockwork and dumps heat like crazy.  I know that it can occasionally help you connect to a stunning view, but if you are overlooking the street or neighbours, then do you really want to connect to them or have them looking into you private space?  There's a 1970s/80s estate close to us where the architects loved having full 2-storey wall to ceiling window panels. Maybe 95% of the houses have since bricked in / insulated clad the bottom halves and fitted  conventional 2G/3G windows instead.  Architects might love it, but IMO occupants usually end up hating it.

[Crofter]

As others have said, the extra cost depends on design and labour factors. 

 

On my very simple build it really didn't add any cost at all, other than the MVHR itself, was was about £700 plus ducting. I had a simple box with a warm roof, so minimal jobs in the membrane. You need a VCL membrane in a TF build anyway, I just made this my airtight barrier too. I spent a few quid on tape for the windows.