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

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.

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.

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!

95% new builds gone