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[irnbru]

Sorry if I sound dumb...I'm really new to this. But like...shouldn't there be a way of predicting/estimating the ACH without actually having the building? Otherwise...imagine that I wanna build a house with less than 0.6 ACH, after building it, I test it and it turns out to be 4, then...should I just demolish the house and build a new one? It makes sense that there are ways of estimating the ACH to avoid that kind of situation.

[Roundtuit]

I'm sure you can estimate ACH based on an airtightness strategy, but what you actually end up with depends on how well the strategy is implemented.  It's all about attention to detail, and most trades struggle to get their heads round it.  If you're on site every day and able to check every penetration and interface in your airtightness layer, spec good doors and windows and don't even consider open fire places/letter boxes/cat flaps, you'll be fine.

[Faz]

0.6 ACH is really optimistic unless £££££££££££ is spent on the way through and, even then, you will end up at 3+ probably on that sized house. Does the cost of trying to achieve that level of air tightness merit the energy saving is the first thing I would consider.

[Adrian Walker]

58 minutes ago, irnbru said:

Sorry if I sound dumb...I'm really new to this. But like...shouldn't there be a way of predicting/estimating the ACH without actually having the building? Otherwise...imagine that I wanna build a house with less than 0.6 ACH, after building it, I test it and it turns out to be 4, then...should I just demolish the house and build a new one? It makes sense that there are ways of estimating the ACH to avoid that kind of situation.

 

Yes there is a way of predicting/estimating the ACH without actually having build the building, and they are Certified Passive House Designers. That said there are both good architects and architectural designers that can also design to the  Passive House Standard. Using certified passive house components also makes this easier.  For MVHR they must be certified.

[ProDave]

23 minutes ago, Faz said:

0.6 ACH is really optimistic unless £££££££££££ is spent on the way through and, even then, you will end up at 3+ probably on that sized house. Does the cost of trying to achieve that level of air tightness merit the energy saving is the first thing I would consider.

Getting good air tightness is mostly down to attention to detail in the build, taping and sealing everything, and not leaving anything unsealed, no unwanted holes, design out "leaks" like cat flaps and letter boxes, mvhr so no need for lots of separate individual extract fans and no need for trickle vents.

 

It need not cost a great deal more than normal sloppy building.

[Gone West]

12 hours ago, Faz said:

0.6 ACH is really optimistic unless £££££££££££ is spent on the way through and, even then, you will end up at 3+ probably on that sized house. Does the cost of trying to achieve that level of air tightness merit the energy saving is the first thing I would consider.

I think it depends on how the house is constructed. I built a timber frame house and didn't use any airtightness tapes or membranes and achieved 0.47ACH. I used the fabric of the building, OSB3 and Icynene foam insulation, as the airtightness layer. I didn't have any penetrations through the walls or roof and fitted triple sealed windows and doors. It was built to PH standards and was heated with electric towel rails in the bathrooms and an EASHP warming the air from the MVHR.

[SteamyTea]

58 minutes ago, Gone West said:

think it depends on how the house is constructed

I tend to agree.

Some systems are going to be more airtight than others i.e SIP, ICF.

Brick and block, on its own, is probably the worst there is.

To a certain extent, airtightness has to be designed in from the start, not something to be put right later.

 

There is a contradiction in the UK where the vapour control later is on the warm side i.e. inside. Then, usually structural elements and insulation, then a wind tight layer.

It is too easy to think that the VCL is there to help stop air movement though the walls. Its main purpose is to stop high humidity air moving to a cold area and the water condensing out.

After the VCL, the materials need to be more vapour open, this is to allow any water, or water vapour to be released.

Here it is too easy to think that, as the VCL has made the inside of the building airtight, and the test shows it is airtight, the problem is solved.

It isn't.

Airtightness is also to do with making sure that cold air cannot bypass the the insulation, which will reduce its effectiveness. This is why buildings, especially timber frame ones, are wrapped in a windproof, but vapour permeable later. They stop the air movement but allow the moisture to escape..

 

So when thinking about mechanical ventilation, think of it as humidity control first, then how much energy it can recover.

 

As a final year project I built a model of a room, then added mass to it so that the thermal stability could be measured.

As I was studying active solar thermal, I had fans that blew air through the mass, or did not blow air though the mass, then measured the temperature variations, it was the data collection and analysis I liked, not the rocks and thermal inputs.

If I was asked to look at the performance of MVHR now, the first thing I would do would be to build a physical model and start measuring energy input, temperature, humidity and airflows. Fairly easy and cheap to do these days (the Raspberry Pi did not exist when I was at university, so used a desktop burning 300W).

 

Physical models are also essential to calibrate mathematical models.

Edited November 2, 2022 by SteamyTea

[MDC]

1 hour ago, Gone West said:

I think it depends on how the house is constructed. I built a timber frame house and didn't use any airtightness tapes or membranes and achieved 0.47ACH. I used the fabric of the building, OSB3 and Icynene foam insulation, as the airtightness layer. I didn't have any penetrations through the walls or roof and fitted triple sealed windows and doors. It was built to PH standards and was heated with electric towel rails in the bathrooms and an EASHP warming the air from the MVHR.

Is Icynene foam insulation acceptable? I've used it, but don't usually tell anyone, mainly because of the mortgage blah blah and all that. I feel like I'm coming out, just asking the question about Icynene!

[Temp]

16 hours ago, irnbru said:

Sorry if I sound dumb...I'm really new to this. But like...shouldn't there be a way of predicting/estimating the ACH without actually having the building? Otherwise...imagine that I wanna build a house with less than 0.6 ACH, after building it, I test it and it turns out to be 4, then...should I just demolish the house and build a new one? It makes sense that there are ways of estimating the ACH to avoid that kind of situation.

 

If you could create an accurate computer model of the house right down to all the construction defects (holes) then I suppose you might be able to put it into a software wind tunnel (Computational Fluid Dynamics?) and try and calculate the ACH when pressurised.

 

To do it you would have to know where all those defects are. For example one pathway might go like this.. Air might escape from a room into a ceiling void through an array of down lights. From that void it might escape into the cavity wall via holes around the ends of ceiling joists. Then it might flow up the cavity in the cavity wall into the loft, then across the loft to an eaves vent and finally the outside world.

 

There might be hundreds of similarly complicated paths the air could take. There are people on this forum that worry about air getting in/out through the keyhole.

 

 

Near impossible to build a computer model that detailed let alone simulate all the airflow pathways. 

 

All you can do is adopt good design and construction techniques and hope the builder follows them.

 

 

 

 

Edited November 2, 2022 by Temp

[Temp]

45 minutes ago, MDC said:

Is Icynene foam insulation acceptable? I've used it, but don't usually tell anyone, mainly because of the mortgage blah blah and all that. I feel like I'm coming out, just asking the question about Icynene!

 

Injected foam in cavity walls is OK. There is/can be a problem if its sprayed onto the underside of roof tiles and rafters

[Triassic]

Maybe a trip to the Building Research Establishment might be in order, to look at their test homes.

 

BRE Group - World leaders in built environment research and development

[Temp]

I'd be surprised if any university that had built a house for the purposes of testing energy consumption and insulation etc hadn't done a pressure test on it at some point already.

 

 

[Gone West]

2 hours ago, MDC said:

Is Icynene foam insulation acceptable? I've used it, but don't usually tell anyone, mainly because of the mortgage blah blah and all that. I feel like I'm coming out, just asking the question about Icynene!

Yes, if it fully encapsulated, as mine was, with OSB3 racking on one side and plasterboard on the other. When I applied for the warranty and house insurance I was only asked the type of construction, timber frame and timber clad, not what type of insulation I used.

[Gone West]

1 hour ago, Temp said:

 

Injected foam in cavity walls is OK. There is/can be a problem if its sprayed onto the underside of roof tiles and rafters

Icynene isn't usually injected, and I think it's PU foam that's usually sprayed on the underside of roof tiles.

[MDC]

1 hour ago, Gone West said:

Icynene isn't usually injected, and I think it's PU foam that's usually sprayed on the underside of roof tiles.

I used it to insulate an overhang, where the floor in the room above was a block of ice in winter. I watched some videos on YouTube from Canada, which stated Icynene as the only solution to insulate an overhang. So that's what I did. It featured on a Grand Designs house too. Afterwards I read about the mortage issues, which was a drag. Still the room is warmer now.

[irnbru]

Oh wow! I really appreciate your input! Thank you all!

[Gone West]

17 hours ago, MDC said:

I watched some videos on YouTube from Canada, which stated Icynene as the only solution to insulate an overhang.

I would have thought it was an ideal use for Icynene and if it's fully encapsulated who knows how it's insulated.

[SteamyTea]

2 hours ago, Gone West said:

I would have thought it was an ideal use for Icynene and if it's fully encapsulated who knows how it's insulated.

Don't they usually put the VCL on the outside in most of Canada?

Different climate from ours in a lot of it. Most of habitual Canada is south of even me at Lat 50⁰.

[Gone West]

2 hours ago, SteamyTea said:

Don't they usually put the VCL on the outside in most of Canada?

Different climate from ours in a lot of it. Most of habitual Canada is south of even me at Lat 50⁰.

Haven't a clue. My youngest daughter lives in Canada, which is a huge country, and it was -6.0C and snowing hard yesterday, where she was.

[Marvin]

On 01/11/2022 at 19:37, irnbru said:

But like...shouldn't there be a way of predicting/estimating the ACH without actually having the building?

The bottom line is that a building can be designed to acheive a good level of air-tightness, however building to that standard is the tricky one.  As has been said there are better types of construction material that lend themselves to air-tightness, and so yes you can design to a predicted level of air-tightness, however, once the building is though of as airtight it must be tested proving in my mind that nobody takes for granted that the designed air-tightness is automatically acheived.

 

This leads to the cost comments above. As my brother says, it's not if it can be done, but how much it will cost. 

 

Good Luck

 

M 

[SteamyTea]

29 minutes ago, Gone West said:

Haven't a clue. My youngest daughter lives in Canada, which is a huge country, and it was -6.0C and snowing hard yesterday, where she was.

So the humidity in the air had fused, then solidified.

Which bit she in? This time a few years back the OAT went from 24⁰C to -7°C overnight, in Halifax.

 

[JohnMo]

So to answer the question.  I picked a number of 1 for ACH.  This then demanded I had MVHR (or balance supply and extract ventilation) to comply with Scottish building Regs etc.  Your worst case is you pick 4 and don't install MVHR and then get 1 and have to then retrofit to comply with building regs.

 

Choosing a low figure also demands attention to detail - which is a good thing, but not something people are used to on a build.  You have to set the ground rules (some times several times to the same person).

 

If you are using a builder ask them what they usually achieve, if the answer is 'I don't know', look for another builder who does know, then at least they understand the concept.

[Gone West]

14 minutes ago, SteamyTea said:

Which bit she in?

Calgary.

[SteamyTea]

6 minutes ago, Gone West said:

Calgary.

So altitude as well.

[Gone West]

Just now, SteamyTea said:

So altitude as well.

Very windy yesterday as well, -1C wind chill. Prefer the Cornish climate.