MVHR and proposed 'air tightness' of dwelling

[flanagaj]

Excuse my ignorance on the subject of 'air tightness', but if I am building a new brick and block house to current regs, how does that differ to a Passivhaus spec design.  If for example, the windows don't have trickle vents, there are no extractors in the kitchen, bathrooms ... (because I am fitting MVHR) and the roof is insulated with Celotex and taped, then how can the house be 'leaky'.  Eg, how is air leaking out / in?

Only asking as I am wanting to get a MVHR design done and the question below was asked.


 

[JohnMo]

Unless additional mitigating measures are taken any building can leak. Air tightness is all about details, that are added at the design stage normally.

 

Brick and block benefit from wet plaster as it seals all the small gaps everywhere in the cement joints. Dob and dab done very well is ok also. Window and door design and fitting. Floor to wall and wall to ceiling junctions can leak a lot.  How sockets and light fittings are done all need thought.

 

Lots of small gaps equal a big leak.

[IanR]

44 minutes ago, flanagaj said:

 but if I am building a new brick and block house to current regs, how does that differ to a Passivhaus spec design. 

 

Energy perfomance targets.

To meet current building regs you need to achieve < 8m³/m².h @ 50Pa Air Infiltration (leakiness), however to meet PH Spec you need to achieve 0.6 ACH @ 50 Pa. Air Changes per Hour is a different unit for measuring Air Infiltration and there's no set conversion figure that works for all houses to convert between the two, but very roughly 0.6ACH will be in the order of 1m³/m².h. So, a PH spec house roughly leaks 1/8th of the air volume of a house that just meets Building Regs.

The difference in construction, between the two is certainly attention to detail, likely the use of tapes and foams, and maybe the use of air tight barriers. Some construction methods and insulation types are easier to get air tight than others. Celotex in your roof needs mitigating with taping and likely an air-tight barrier and depending on your wall costruction, other mitigation may be required.

Take some time to consider this for your build. MVHR doesn't bring you any benfits in itself, it's the air-tightness it facillitates that brings the benefits. If you are not targetting < 3m³/m².h @ 50Pa air infiltration rate, you have no need for whole house mechanical ventilation and adding MVHR will increase your energy losses, rather than reduce them, while costing you to power and maintain the system.

 

Achieving <3m³/m².h @ 50Pa will take a conserted effort in both design and construction, it doesn't happen by accident.

If you plan to put the effort in to, say, target 1.5m³/m².h @ 50Pa or better, then you really will benefit from the air-tightness and MVHR with both a more comfortable home as well as lower heating bills, but if air-tightness is not your focus then MVHR is an unrequired expense.

Edited June 3 by IanR

[MikeSharp01]

The accepted wisdom is that MVHR is only effective in a house with good air tightness although just where the line is drawn is hazy although @IanR does give a line above. Passive house air tightness is relatively hard to achieve unless the design accommodates it because, as @JohnMo says, it is all about the details. I think that for our house we had a total number of air gaps equivalent to less than the area of one credit card (achieved 0.2 on the Passive house scale or, in our case as form factor is everything, 0.3 m³/m².h on regs scale which is about 1/26^th of regs). So if you are not going for a good air tightness then as @IanR says you should perhaps adjust your approach you can see it as either all in or all out. Halfway house - pardon the pun, is not sensible because the middle ground incurs costs both ways EG you install MVHR but is just adds to running cost. 

[Kelvin]

Not only is it a function of thoughtful design it’s a function of careful detailed build by everyone that walks through your front door to carry out work in your house. Most trades don’t really understand air tightness.

 

I had a very simple rule no one other me cuts or drills through the big white membrane that lined the interior of our house and any breach of the air tight membrane had to really be justified as in there was no other way to do whatever needed to be done. In the end we had two such breaches and I used the right products to properly seal around the breaches afterwards. Fortunately there are loads of products for every situation. 

[JohnMo]

4 minutes ago, Kelvin said:

Most trades don’t really understand air tightness.

To the point that many said our house would be green from mould within six months.