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

[Oxbow16]

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

[markc]

Airflow is good, draughts are bad.

The obvious ones are windows, doors and chimneys, but there are a lot of draughts under skirting boards of suspended floor houses.

But sealing a building will just lead to damp if it is not adequately ventilated

[Ian]

I did a new-build holiday home 4 years ago. It is timber frame construction, single-storey and naturally ventilated built to minimum building regs. Using no air-tightness membrane or tapes and no special design or workmanship measures used to increase air-tightness it achieved an air-test score of 3.7 m3 /h/m2). On this particular house this equates to 4 ACH @ 50pa.

There's no draughts and it is very cheap to keep it warm & comfortable.

Edited January 6, 2021 by Ian

[Temp]

In some old houses they have little to prevent damp getting in. Solid walls, floors without DPM etc. These can sometimes rely on draughts to remove water and keep them dry. Seal them up and you increase humidity and risk condensation and damp issues. Don't seal them and you have draughts and expensive heating bills. The only way I can see to solve this is an MHRV system but to be effective you need to do a good job of the sealing.

[Thedreamer]

34 minutes ago, Oxbow16 said:

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

 

 

Hi Oxbow 16,

 

I am heating my house with just a Charnwood C4 maximum output is 4.8kW. I am in strange position of having a suspended timber floor in a new self build which is well insulated but not to passive standard and take the air from the void below the floor. Is your floor a slab?

 

My wife did the caulking it makes a big difference for us it was a final layer of airtightness.

 

Also do you have trickle vents in your windows?

 

It all add up but older properties need some airflow to prevent condensation both in the internal and external spaces, so each bit needs to be considered. 

 

 

 

 

[Oxbow16]

Thanks for the great quick replies.  

 

I'll respond more later today, but for now just want to add that we have solid floors; seemingly all concrete and at least some with a DPM beneath.  And no trickle vents on windows, although some need replacing so that will be a decision to make for those.  

 

Cheers

Edited January 6, 2021 by Oxbow16

[Declan52]

Once you start to seal up gaps then the stale damp air won't get removed. The more you seal up the more of this air won't be escaping so will start to cause issues with condensation and mould. When it gets to this point then you need to think about removing this air mechanically. Humidity controlled fans or mhrv is the easiest way.

[SteamyTea]

Air tightness can be looked at in isolation.

Where your insulation is, and what type it is, also has to be taken into account, as does humidity control.

 

So it is not a simple case of just filling the gaps on the inside, it is why houses have breathable, but wind tight wrappings and vapour control membranes.

[ProDave]

Actually it's not as simple as you might think.

 

"modern" but not air tight houses, are probably the worst, because of the amount of extract fans, and things you are mandated to have so you have lots of large holes in the walls with almost uncontrolled ventilation.  Oh and you have trickle vents in all the windows as well, and if you have a stove you have a chimney, and a duct close to the stove to bring air into the room (and you are not allowed to shut it off)

 

Contrast that to my previous 1930's house. Yes it had a chimney, but no specific duct to let air on for the fire.  It had no kitchen or bathroom fans, if you wanted ventilation you opened a window.  No trickle vents in the windows, again you want ventilation, you open a window.  It was surprisingly air tight, to the point if you lit the fire in the living room, the air that went up the chimney was mostly replaced by air being drawn down the bedroom chimney, down the stairs and a draught under the door that almost whistled.  It was so bad that on a still day with no wind to take the smoke away from the chimney, it would actually draw smoke down the bedroom chimney into the bedroom.

[Rich123]

Hi , in an old property the air infiltration rate will be high as the structure is not designed to be air tight.

In 2008 air testing of buildings was introduced and most new builds struggled to achieve the desired air test of 10 m3 per m2 per hour at 50pascals pressure.

in recent years most buildings are achieving around 2 , this is by the method of construction , particularly timber frame which is more air tight .

Passivhaus is less than 1 .

I would expect your property to be around 25 .

New builds have balanced heat recovery ventilation , therefore external infiltration isn’t required to reduce humidity.

It would be close to impossible to get an old house to meet the new build standard without a radical refurb, lining all of the walls and roof and over cladding with insulation. Plus a whole house mechanical vent system.

 

 

 

[mathraki]

But is the fact that it is v. difficult to get an old house (1906 mid terrace, 3 storey in my case ) to the new-build air tightness standards, still reason enough for NOT going MHRV .....when at least, many of the 'big leaks' have been addressed. Won't  MHRV still ventilate the spaces whilst removing moisture rich and smelly air and recover 'some' of the heat in so doing ? 

 

As I seal up and fix my new Victorian house (room by room), I am reducing the air ingress into the living areas via floor, skirting, sockets, windows, fittings and services. I am conscious I need to provide ventilation and despite acknowledging I may never get all leaks addressed, MHRV is still my preferred  option.

 

Further I am only planning on doing MHRV in the middle and top floors, with the MHRV unit in the COLD, eaves ventilated attic:

Top: 3 bedrooms and 1 x WC/Shower Room.

Middle: 1 bedrooms,1 x WC/Shower Room and 1 x Office.

 

Ground: Entrance Hall & Sitting room (out of scope because too far from Attic and too diff. for ducting)

The Kitchen, utility and another WC will all be new-build to rear from the entrance hall ....down 2 steps, you know the type.

 

So whilst my set-up is far from standard, ideal or probably recommended ...MHRV will allow me vents wet rooms with minimum holes punched in the walls whilst refreshing the air in the bedrooms and office without the need for the windows to be opened (often).

 

Cross your fingers for me.

[Bitpipe]

Having observed the build process of our passive standard timber frame, the most obvious difference was the airtightness provision between floors - i.e. there is a 4m airtight membrane that goes up the inside of the ground floor wall, out and over the exterior floor deck and back inside the wall of the upper floor. There is an internal airtightness membrane over the interior walls and the underside of the roof.

 

Everything is taped together and the airtightness detail is continued onto windows and doors (which themselves are airtight. Final detail is that any external penetrations are sealed and the follow on trades take care not to penetrate the airtightness layer (this is mostly achieved by creating a service void).

 

Modern houses are often described as 'plasterboard tents' as the structural wall is very leaky onto which is dot and dab plasterboard which is skim coated. Draughts are therefore concentrated at the joins of the plasterboard - typically ceilings and floors and these will therefore be colder than the other areas. The warm moist air in the home will condense out on these colder areas leading to damp.

 

Older houses tend to have fully plastered walls onto the internal brick so are somewhat more airtight.   

[mathraki]

"The warm moist air in the home will condense out on these colder areas leading to damp"

 

Potentially, if the conditions are present for condensation to occur. If the house fabric is kept at a temperature above the dew point then the water vapour will not (cannot) condense.

If however the localised effect of a draught i.e. cooling the wall in the gap-area between two boards, could potentially cool to the gap-area to a temperature below the dew point and water could condense in these areas.

 

Sealing the gap, avoiding the 'cool' air getting in is a good remedy; the correct insulation is a better solution again and furthermore the dilution and extraction of the saturated air using MHRV or similar is also a mitigation.

 

Pedantic perhaps but i wanted to clarify that not sealing gaps does not automatically lead to damp and in many cases the additional air flow is exactly what prevents it.

[larry]

On 18/01/2021 at 14:04, mathraki said:

But is the fact that it is v. difficult to get an old house (1906 mid terrace, 3 storey in my case ) to the new-build air tightness standards, still reason enough for NOT going MHRV .....when at least, many of the 'big leaks' have been addressed. Won't  MHRV still ventilate the spaces whilst removing moisture rich and smelly air and recover 'some' of the heat in so doing ? 

 

As I seal up and fix my new Victorian house (room by room), I am reducing the air ingress into the living areas via floor, skirting, sockets, windows, fittings and services. I am conscious I need to provide ventilation and despite acknowledging I may never get all leaks addressed, MHRV is still my preferred  option.

 

Further I am only planning on doing MHRV in the middle and top floors, with the MHRV unit in the COLD, eaves ventilated attic:

Top: 3 bedrooms and 1 x WC/Shower Room.

Middle: 1 bedrooms,1 x WC/Shower Room and 1 x Office.

 

Ground: Entrance Hall & Sitting room (out of scope because too far from Attic and too diff. for ducting)

The Kitchen, utility and another WC will all be new-build to rear from the entrance hall ....down 2 steps, you know the type.

 

So whilst my set-up is far from standard, ideal or probably recommended ...MHRV will allow me vents wet rooms with minimum holes punched in the walls whilst refreshing the air in the bedrooms and office without the need for the windows to be opened (often).

 

Cross your fingers for me.

 

I'm putting in an MHVR into a 1920s build house, mostly solid walls but with some newer extension. My internal brain model is that it should allow for ventilation without the associated loss of heat of opening a window, using an extractor etc etc. 

We will progressively be working through and insulating each room as we go (and at least we have started this with wood fibre board IWI, Pavatherm recommended by Mike Wye) and sealing as we go so I expect the airtightness to improve, but of course realistically we are never to be anything like that of a modern house. However I expect the benefit from the MVHR will increase as we progress with this job. 

(We're also ripping out a lot of the upstairs so I have one chance to have simultaneous access to walls, floors and ceilings for fitting the unit and ducting, so it felt like the best time to do it). 

Ask me in 5 years how it went (!!), but from other forum posts I've read it seems people have been pleasantly surprised at least from the fresh air perspective of a MVHR in an older house.