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

[JohnMo]

5 minutes ago, Tony L said:

 

Not even a little bit?  What am I not understanding here: I'd have thought that if the air outside is cooler than the air inside & my MVHR system is taking warm air from inside, bypassing the heat exchanger in summer mode, & replacing the exhaust air with cool air from outside, this will help keep the inside temperature lower.  Of course, MVHR won't be the entire answer to the summer overheating problem, but I think it must be having some effect. 

Difference is pretty small, you are only exchanging the house air every 2 to 3 hrs. So don't expect much if any heat movement. Solar gain is way more powerful.

 

I bought an ASHP for one reason - cooling. After living in house for one summer 

 

 

[TerryE]

On 15/05/2025 at 00:04, Tony L said:

The purpose of the glass is, to please my partner, who wants lots of glass, especially on the façade.  It’s half her money, so I’m compromising.

Surely you need an evidence-based conversation and come to a joint decision. Jan's comment was "it's going to cost a fortune to get the heating and cooling right: you've got to take the long view." 

 

You need to do the heat loss and solar gain calcs and make sure you both understand the implications. 2G panels have about 8-10× the U value of your walls and introduce lots of opportunity for air leeks. Do the sums; share them; own them and their consequences.

[saveasteading]

49 minutes ago, JohnMo said:

bought an ASHP for one reason - cooling. After living in house for one summer 

That can't be right? 

Let's say 24C is too hot indoors. How often is it that outdoors? Open windows or vents on opposite ends and get a draught through with the new air being whatever is ambient.

 

Trickle vents will do that too.

[Nickfromwales]

16 minutes ago, saveasteading said:

That can't be right? 

Let's say 24C is too hot indoors. How often is it that outdoors? Open windows or vents on opposite ends and get a draught through with the new air being whatever is ambient.

 

Trickle vents will do that too.

Trickle vents would barely scratch the surface!? You’re looking to purge not have a slight breeze  

Who cares what outside is doing when you’re baking to death inside, so active cooling in whatever form is a wonderful thing for sure. Doors and windows closed, cooling on max, on with the budgie-smugglers. 👊

[saveasteading]

8 minutes ago, Nickfromwales said:

on with the budgie-smugglers

Jumpers off is first.

I agree re trickle vents being small but it can be constant so it doesn't get si hot in the first place.

Opening windows or doors at opposite ends, with internal air path open too, will usually cause a strong draught unless it is dead still. Chimneys or high vents will also encourage air flow.

 

For anyone still designing, have high ceilings if possible. It can be several degrees warmer at the ceiling, so keep that well above head height.

[Crofter]

The little house we built has a lot of west facing glazing, and it's insulated to near PH levels, so on a sunny afternoon the temperature inside can fairly soar.

However we're on an exposed hillside and on such days there's always a sea breeze blowing hard. Opening doors or windows on either side of the building drops the inside temperature to ambient in a few minutes.

[Tony L]

6 hours ago, saveasteading said:

Chimneys or high vents will also encourage air flow.

Don't say that - I had to work really had to get her to agree to the chimney being deleted from the early design.

[Tony L]

7 hours ago, TerryE said:

On 14/05/2025 at 22:04, Tony L said:

 

Surely you need an evidence-based conversation and come to a joint decision.

 

Several evidence-based conversations have already happened.   I will keep going with more of these & eventually I may bring her around to our way of thinking, (“our”, referring to you & me, rather than her & me).  The design already has much less glass than she wants & no wood burner.  I’m making her sound like an idiot, so I must point out, she does have some excellent & rare qualities.

 

If she doesn’t get it then I may just go ahead & do what you & I think should be done anyway, or more likely, we’ll endure a hot summer then fork out for some a/c or fan coils (with no VAT reclaim).  I’ve got to bear in mind that her trade-off between how the house looks & how comfortable/economic it might be is different from mine.

 

& it’s difficult to influence some people’s beliefs by presenting evidence to them.  Within the build hub community, almost all of us like evidence, science, logic, & we’re open to having our views corrected by others who present persuasive evidence to us.  A lot of people aren’t like that.  There are millions of people in this country who believe in all sorts of strange ideas that aren’t supported by real evidence.  I could go on & on, but I’ll stop there, just in case I land myself in a load of trouble for upsetting one or more of the strange belief people who may stumble across our forum.

 

Thanks for your post – I’ll be quoting your, “8-10 the U Value”, when we next discuss the windows - that’s if I can persuade her to believe in U values.

[Thorfun]

3 minutes ago, Tony L said:

we’ll endure a hot summer then fork out for some a/c or fan coils (with no VAT reclaim).

We have external blinds automated with the weather and internal temperature and I’m still extremely glad we installed AC. I wouldn’t discount it off the bat as it’s a lot easier to install while you’re building than retrofitting it!

[JohnMo]

6 hours ago, saveasteading said:

Let's say 24C is too hot indoors. How often is it that outdoors? Open windows or vents on opposite ends and get a draught through with the new air being whatever is ambient.

 

Trickle vents will do that too.

Try 27, 28 with blinds closed, worst case is during shoulder seasons, when sun is low without cooling. Living room is 6m tall, but with fully glazed end. Although we have large roof overhang when the sun is starting to fall in the west we get the sun all gun blazing. Yes we close blinds. Cooling went on at begining April this year. Heating one day, cooling the next.

 

6 hours ago, saveasteading said:

Open windows or vents on opposite ends and get a draught through with the new air being whatever is ambient

done that it's still way to hot at bedtime. UFH in cooling mode temperature drops to normal levels before bedtime. We still open windows and doors about 4 to 5pm. With cooling house recovers quite quickly. 2 degs outside the last night, was still 19.5 in the house this morning. Staying in England tonight and had the heating on where we are staying. Quite bizarre we are near the top of Scotland have cooling on and currently 20.8 in the house in Scotland and the house in England with heating on is about 19.

[Nick Laslett]

9 hours ago, Tony L said:

 

Not even a little bit?  What am I not understanding here: I'd have thought that if the air outside is cooler than the air inside & my MVHR system is taking warm air from inside, bypassing the heat exchanger in summer mode, & replacing the exhaust air with cool air from outside, this will help keep the inside temperature lower.  Of course, MVHR won't be the entire answer to the summer overheating problem, but I think it must be having some effect. 

Take a look at this long thread, where many forum regulars from back in the day have a discussion about MVHR cooling. 
 

 

Edited Friday at 21:45 by Nick Laslett

[Mike]

9 hours ago, Tony L said:

I'm planning external motorised blinds on the two small Fakros in the two south facing bedrooms.

You may want to consider adding an external wind sensor, to ensure that the blinds are automatically retracted in high winds, which could otherwise damage them.

 

9 hours ago, Tony L said:

I'd have thought that if the air outside is cooler than the air inside & my MVHR system is taking warm air from inside, bypassing the heat exchanger in summer mode, & replacing the exhaust air with cool air from outside, this will help keep the inside temperature lower.

Yes, it will. However while it can bring in cooler air, it's won't move large volumes of air, so won't be enough to counteract the solar gains - but still useful.

[SteamyTea]

Right, I had a brainstorm.

My original spreadsheet to look at the effects of ventilation strategies was too complicated.

So I have reduced it to the basic minimums of natural ventilation (the holes your builder leaves you with) and mechanical ventilation (what you choose).

The only things you really have to find out is the total treated (heated) volume of your building, the number of air changes (both natural and forced), the mean, local, air temperature (Met Office is useful here), how warm you like your house, some information about the MVHR efficiency, never trust the manufactures numbers, be sensible about it and that is about it.

I have only just knocked this up, so there may be errors, but you can see the formula, and edit them if you unlock the sheet.

Hope it helps.

 

 

ACH and OAT Thermal Losses (excel).xlsx

Edited yesterday at 19:40 by SteamyTea

[Nickfromwales]

22 hours ago, Mike said:

You may want to consider adding an external wind sensor, to ensure that the blinds are automatically retracted in high winds, which could otherwise damage them.

I've had to install these systems where we fitted full solar window blinds (French company which the name of I cannot recall as it's been a long as feck day) but for Fakro / Velux the manufacturer has made them wind / storm proof out of the box so there's no need (AFAIK) to have any 3rd party controls on those types of units; caveat is that you buy the correct external blinds from the manufacturer of the window / roof light vs a £3.99 one from Ali expiss.

[Tony L]

Thanks, everybody, especially @SteamyTea for putting the spreadsheet together.  I've not started playing with this yet, but I have learned about Bin Method.

[TerryE]

On 16/05/2025 at 23:51, Tony L said:

Several evidence-based conversations have already happened.

 

I did maths and operations research as my degree, and then spent over 5 years doing physics based modelling in a management consultancy before moving into systems / IT, so this type of approach is "in my DNA": I trust an evidence based engineering approach.  That's not the case for Jan who came up a different educational route.  The key is finding a common evidenced basis to informed agreement in your partnership.

 

Take an example Jan wanted a fire in LR, and I knew from the modelling that this would a disaster, but the argument that swayed Jan was my point that she really looked forward to spring, when we could clean out the hearth and decommission the fire over summer; the new house would be at this temperature all year round inside, so when would we ever want a fire?  The final nail was a tale from someone who was talking about his passive house, and they'd put in a fire.  They decided to light it one Christmas when the extended family was visiting.  An hour later, everyone was standing in the garden with the doors and windows open waiting for the fire to out and the house cool off because the open plan downstairs was about 35°C.  That was the one and only time they used the fire. 🤣

 

How many houses of the class that plan to build have you both visited and talked to the owners about the strengths, weaknesses and realities of living in their house? We must have visited ½doz or so.  You expressing your views and research is one thing, but listening to others talk of their personal experience can be more compelling.  Maybe your architect can link you up with other clients?  If you are willing to share you post code area or nearest town, then you might be able to hook up with other members on the forum who would host a visit.

[ToughButterCup]

16 minutes ago, TerryE said:

...

How  many houses of the class that plan to build have you both visited and talked to the owners about the strengths, weaknesses and realities of living in their house?

... 

 

@TerryE made this point to me - in person at a meeting -  years ago. (5?)

I took his advice.

Now, thanks in part to us visiting a series of local PH houses and the absence of money we ;

• made our windows bigger : less worried about heat loss
• installed £50 worth of pure joy : an oil-filled radiator from Aldi ( after reading  few posts by @SteamyTea
• bought the cheapest 'solid' little radiator for one bedroom
• installed electric underfloor heating in both bathrooms

and until recently had no MVHR. That is for the last 4 years no MVHR.

 

The key is as @TerryE says : visit.  PM me if you're driving up the M6 - visit anytime.

 

One much ignored aspect of heating  and warmth in a house is  -- the perception of warmth -- . It matters. SWMBO experiences warmth differently - and I understand that is a common phenomenon. And I expect that we will both feel the cold more acutely as we age.

[JohnMo]

1 hour ago, TerryE said:

when we could clean out the hearth and decommission the fire

We had our fire (stove on) when temperature dropped very low (about -6 and below). Although the room temperature was still about 20 to 21, it just felt cooler. Lit the stove and moved the house temperature up a little. Secret is don't overload with logs, heat output is proportional to how much fuel you add. Other factor is get soap stone covered stove, this slows down the output curve considerably. And use the air regulator.

 

Decommissioning - empty ash bucket, done. 5 minutes later complete.

 

Low energy how and stove works, just a matter of sizing correctly, specify correctly and use sensibly. Our first use of the fire caused overheating, but I fully loaded with logs, so no wonder really.

[Crofter]

My experience of a WBS in a low energy house is not especially positive. 

It's a very small house (43m² internal) built to approx twice building regs minimum u values, and as airtight as I could get it (I was building regs exempt so didn't do a test). I have MVHR.

 

The stove is a 3kw unit with external air feed, via a vent in the north elevation. 

 

In certain conditions it can be an absolute pain. I learned early on that to stand any chance of getting it to draw, I had to leave a window or door on the house open, because the airtight house just can't support the draw of the fire when lighting it. If you don't get it to light first time, you're best to let it go completely out before you open the WBS door, otherwise you'll get all the smoke blowing back in to the room.

 

I think the intrinsic problem is that it's a 90% efficient stove and it's at the lower limit of what is actually possible in terms of size. There just isn't enough heat going up the flue to get it to draw strongly.

 

I may try adding an extension to the flue to bring it higher above the house ridge, and I could try repositioning the intake, as i do think that in strong southerly winds there might be some sort of vacuum effect going on. But it's all a lot of hassle for something that is really just there for aesthetics.

 

The next house will not have a WBS.

[saveasteading]

2 hours ago, Crofter said:

to stand any chance of getting it to draw, I had to leave a window

Isn't it obligatory now to have an external air source and closed circuit?

It is best anyway and easy to plumb in a new build.

The one I know is very efficient and gets no blowback.

[Mike]

5 hours ago, ToughButterCup said:

The key is as @TerryE says : visit.

There are official UK & International PassivHaus Open Days too. There's a UK one coming up from 27 - 29 June, though I can't spot any detail yet - https://www.passivhaustrust.org.uk/events.php

[SteamyTea]

6 hours ago, ToughButterCup said:

installed £50 worth of pure joy : an oil-filled radiator from Aldi ( after reading  few posts by @SteamyTea

I have never said that. My recommendation is always the cheapest fan heater.

[JohnMo]

7 hours ago, Crofter said:

In certain conditions it can be an absolute pain. I learned early on that to stand any chance of getting it to draw, I had to leave a window or door on the house open, because the airtight house just can't support the draw of the fire when lighting it.

Only once had an issue, but tried to light it in 60mph winds it wasn't happy. Now just use a fire lighter and it starts every time even with 60mph winds