Green Building Forum - AIR TIGHT vs air tight - where does all the air come from in a standard house?

AIR TIGHT vs air tight - where does all the air come from in a standard house?

greenfinger — Wed, 06 Jan 2021 12:02:08 +0000

Hi

This is a bit of a thinking aloud type thread... Following my recent flurry of posts in the Stoves, Fires & Fireplaces forum, I was reading last night about direct air supplies for wood stoves, and it reminded me of stuff I've read in the past RE air tightness in houses, MVHR, etc. Our house is old so I think a lot of that doesn't apply. But being warm(ish) and reducing humidity are important factors which is why I guess I keep coming across these things.

So if we broadly say there are 1. passive houses, 2. modern new builds, 3. older houses from the last ??? years, 4. very old houses - I'm interested to know how they differ. For example, if in a very old house you replace the windows for double glazing, add some insulation and address a few draughts, does it move up a notch? Or two? I've read that it is very hard to make a house passive retrospectively, but what elements are hard to achieve if not implemented from the outset?

To put it another more awkward (but perhaps more relevant) way, I live in an old house with some extensions from the 1970s and some extensions from around 2000. Where there are old single glazed windows we'll be changing them for modern double glazing. There's lots of decorating to be done, so the caulk gun will be out and gaps around windows and doors (and pipes etc) will be sealed. Likewise where walls meet, ceilings, etc. And we'll be adding insulation to the loft (although not to walls). Once that's all done, I'm guessing the house would still be far from air tight, but where would the air then be coming from? Would there still be enough for the 5kW wood stove without an air brick? Does air come through the fabric of the house itself (solid walls)?

Sorry it's all a bit rambling, but I know people around here know their stuff and could shed light on at least some of my thoughts...

Cheers

AIR TIGHT vs air tight - where does all the air come from in a standard house?

greenfinger — Wed, 06 Jan 2021 13:16:02 +0000

Just to add, we have solid floors; seemingly all concrete and at least some with a DPM beneath. No trickle vents on windows, although some need replacing so that will be a decision to make for those.

Cheers

AIR TIGHT vs air tight - where does all the air come from in a standard house?

Mike1 — Wed, 06 Jan 2021 13:24:17 +0000

First, be aware that failure to provide sufficient airflow to a wood stove is potentially life-threatening. Building Regs Part J details ventilation requirements.

But yes, you are still likely to have many leaks. The best way of identifying those that are not obvious would be using a smoke pen (or incense stick), either alone on a windy day, or during a blower door test.

AIR TIGHT vs air tight - where does all the air come from in a standard house?

tony — Wed, 06 Jan 2021 15:26:08 +0000

I swap 2 and 3, a lot of modern new builds are very air leaky, lot of bypassed insulation

Re MVHR not working unless air tight, discussed before but it works , probably should operate slightly pressurising the house, will be less efficient if house is leaky but will still function.

AIR TIGHT vs air tight - where does all the air come from in a standard house?

djh — Wed, 06 Jan 2021 15:31:29 +0000

Posted By: greenfingerI've read that it is very hard to make a house passive retrospectively, but what elements are hard to achieve if not implemented from the outset?

Airtightness is one piece that is difficult to retrofit to PH standard. It's a lot easier if you build from new, because you have control over how everything is built and especially over how every junction is constructed. And you have access before finishing layers are added to apply whatever membranes, tapes, paint etc are required to make it airtight. Hopefully you will have design drawings with a continuous red line on them showing exactly where the airtightness layer is supposed to be and detailed drawings of any complicated junction that needs special attention. Even some people that set out to build a PH fail at the airtightness.

Thermal bridging is another area that's difficult to eliminate in a retrofit. Junctions and corners in the structural components are typically the most difficult to design. Floor-wall, wall-roof, wall-window etc and penetrations such as steels going from inside to outside. They can all be eliminated at the design stage but with much more difficulty on en existing structure.

As well as part J, part F deals with ventilation requirements including work on existing houses. I would not be without MVHR now, and I suspect many others here would also recommend fitting a system if possible. Failing that, Part F sets out what ventilation openings are required.

AIR TIGHT vs air tight - where does all the air come from in a standard house?

RobL — Wed, 06 Jan 2021 17:44:06 +0000

We have a '63 house, solid floor, wet plaster walls - so it's maybe similar. Leaky bits were:
All sockets / light switches on external walls, as the wires were routed via the cavity wall gap.
All ceiling roses on the top floor.
Around windows
Under internal windowsill
Wall - ground floor
Between floors there is no wet plaster. Our inner leaf is breezeblock, which is not airtight!
Any plumbing pipe through an external wall if not visible will not be airtight
Any upvc / wood cladding on the exteriour (we had) will cover up very dodgy brick/blockwork with big gaps
Door/windowseals /lofthatch

AIR TIGHT vs air tight - where does all the air come from in a standard house?

Dominic Cooney — Thu, 07 Jan 2021 12:04:37 +0000

Posted By: greenfingerWould there still be enough for the 5kW wood stove without an air brick?

Fit a direct air supply to the stove. I did this for the stove in the previous house, and for one of the stoves in this house. Air brick = uncontrolled draughts into and out of your heated envelope. Looking forward to fitting a direct air supply for the stove in the room that I am sat in now (possibly with a new stove as well if funds allow) and sealing up the airbrick.

I don't know why anyone wouldn't do it. My in-laws had a fireplace remodelled and stove fitted to replace an old open fire that had an external air supply fed from below. The air supply was closed off and a new airbrick put in the external wall of the house. What a wasted opportunity!

AIR TIGHT vs air tight - where does all the air come from in a standard house?

gravelld — Thu, 07 Jan 2021 20:14:47 +0000

It really depends on both the practices at the time the house was built and how much the house has been frigged about with since.

My house was built in the 50s, wet plastered. In those areas of the house, it performs far better than the modern new build requirements (which are poor, but anyway...). However, it was extended in the early 2000s. These parts are a train wreck. That's both because of building practices (dot and dab, "a house needs to breathe (sic)" etc) and also fashion (downlighters, non-room sealed stoves and more).

The only way you know a house is air tight is if *that house* has been measured by a *reputable independent* organisation/individual. That removes >95% of new builds, by the way.

The uncomfortable truth is that each house has to be measured individually and action taken from there. You can't really make judgements on types of housing.

AIR TIGHT vs air tight - where does all the air come from in a standard house?

tony — Thu, 07 Jan 2021 20:28:17 +0000

95% new builds gone

AIR TIGHT vs air tight - where does all the air come from in a standard house?

LF — Thu, 07 Jan 2021 20:29:58 +0000

Posted By: gravelldIt really depends on both the practices at the time the house was built and how much the house has been frigged about with since.

My house was built in the 50s, wet plastered. In those areas of the house, it performs far better than the modern new build requirements (which are poor, but anyway...). However, it was extended in the early 2000s. These parts are a train wreck. That's both because of building practices (dot and dab, "a house needs to breathe (sic)" etc) and also fashion (downlighters, non-room sealed stoves and more).

The only way you know a house is air tight is if *that house* has been measured by a *reputable independent* organisation/individual. That removes >95% of new builds, by the way.

The uncomfortable truth is that each house has to be measured individually and action taken from there. You can't really make judgements on types of housing.

This is my house. Slowly sorting the earlys extension 2000s stuff and badly fitted double galazing.
Double height Bay window was a drafty affair with tiles and no insulation. Now sorted.
2000s work was supposed to be higher standard but I have had to sort out all cold bridging/gales on windows.
Plasterboard is ongoing - foaming the plasterboard tent - where cold air sinks from loft.
Floor downstairs is better/warmer as well insulated under concrete.