Hi all,
Looking for some advice/feedback on the following situation.
I've just air tested a house where we achieved a result of 3.18 @ q50.
Not bad you might say but the target value was only 6 and the house has NOT been fitted with any mechanical ventilation system. It is fitted with standard trickle vents etc.
The building code in Scotland basically says that any house that achieves a result of less than 5 @ q50 should be fitted with a mechanical ventilation system or similar.
I haven't spoken with BC yet but I'm wondering if there is any other guidance out there on air tightness and when additional ventilation is required. The ACH rate @ 50PA is 4.226. Do you think a MV is really required with this result??
Any pointers would be greatly appreciated.

Ben

I spoke with Stroma and they advised that a result of less than 3 requires additional ventilation. The code basically advises that MV is installed where you are designing to achieve a result between 3- 5 (in case you achieve a result less than 3) etc..It would appear air tightness is a pretty hard thing to quantify at the design stage if you are aiming for anything other than very good or very poor. Another thing that may interest a few folk is that this house had no additional vapour barriers installed. The basic constrcution was; concrete floor, 6inch timber frame with 25mm rigid insulation over inside face, plain PB to ceilings and that's it. It was however a very basic single storey box with 4 corners..

Ben,

interested to know if this is a new house or refurbished and what the construction method and or measures taken to get the house airtight.

I am nearly at the point of getting mine tested as it will be wind and water tight and I want to check it before I start dry lining!

I'm on Skye so not quite as far North. Are there any testers up there in Shetland? Give me a shout if not.
The house was a bog standard Norscot bungalow. 4 corners and an ideal layout for achieving a good result.
Little effort to make the house air tight as the aim was to achieve a q50 result of around 6.
The walls are 6inch timber frame with 140mm glass between studs. 25mm rigid insulation over the inside face fixed via 50mm battens into the stud. The walls hadn't been lined yet. The joints in the insulation were NOT taped or sealed and there were no additional vapour barriers used anywhere. Concrete floor. The ceiling looked pretty tight but was not taped and filled and again, no vapour barriers. The windows had been ‘foamed’ into the kit as had most of the ducting inlets.
From the few houses I have tested my opinion is that a well built timber framed house can easily achieve a q50 result of less than 5 with no additional membranes and/or care and attention to air tightness. If you are going down the route of a super air tight house then you should easily achieve a result of less than 3. Where you don’t make additional provision for ventilation I’d watch out that your house isn’t built too tight.

Posted By: SteamyTeaAs you have an airtight house would you not just fit an MVHR unit and get the benefit or warmed clean air?

You need less than 3ACH@50Pa before an MHRV shows any benefit. Above that, and you're just adding to the ventilation losses (even with the heat recovery), plus there's the running cost of the fans.

To know if it's too airtight or not, you need to know that actual air change rate in litres/second and compare that with recommended values based on the occupancy level.

For example, when I did the analysis for Tony's house using hot2000, this is what it came up with for ventilation requirements:

F326 VENTILATION REQUIREMENTS

Kitchen, Living Room, Dining Room 3 rooms @ 5.0 L/s: 15.0 L/s
Utility Room 1 rooms @ 5.0 L/s: 5.0 L/s
Bedroom 1 rooms @ 10.0 L/s: 10.0 L/s
Bedroom 2 rooms @ 5.0 L/s: 10.0 L/s
Bathroom 2 rooms @ 5.0 L/s: 10.0 L/s
Other 2 rooms @ 5.0 L/s: 10.0 L/s

AIR LEAKAGE AND VENTILATION SUMMARY

F326 Required continous ventilation: 60.000 L/s (0.43 ACH)
Central Ventilation Supply Rate (): 60.000 L/s (0.43 ACH)
Total house ventilation is Balanced
Gross Air Leakage and Ventilation Energy Load: 22733.789 MJ
Seasonal Heat Recovery Ventilator Efficiency: 81.565 %
Estimated Ventilation Electrical Load: Heating Hours: 1635.585 MJ
Estimated Ventilation Electrical Load: Non-Heating Hours: 98.895 MJ
Net Air Leakage and Ventilation Load: 6191.642 MJ


See if your airleakage test results give the express it in litres/second, then you'll see if you need an HRV or not (I suspect not)

Paul in Montreal.

Posted By: Paul in Montreal you need to know that actual air change rate in litres/second and compare that with recommended values based on the occupancy level.

Paul in Montreal.

So how do you calculate it if the level of occupancy changes from two to four or visa versa?

Jonti

Posted By: jms452Do you have a reference for the 3ACH figure? I see it quoted a lot but have never seen the original quantitative justification - surely this depends on lots of factors SFP, efficiency etc...

I think it's based on fresh air requirements (like the F326 ones posted above). Of course, it depends on the volume of the building, so maybe it's based on some kind of average. Of course, what's "average" in the UK may be quite different than here in Canada, say.

Posted By: JontiSo how do you calculate it if the level of occupancy changes from two to four or visa versa?

The occupancy figures are sort of included in the F326 figures shown above - so much per bedroom etc.

This document http://archive.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n15_eng.pdf has some more information on F326.

Paul in Montreal.

Paul in Montreal,

I probably didn't explain my thoughts clearly enough. I think it has been said on this thread that number of occupants has to be included in the calculations for the size ventilation. Is the number of occupants really relevant? If so, how does PH allow for the household numbers to change?

Jonti