Ideally the inner leaf to ceiling should be airtight. If the inner leaf is very porous then it should be made airtight over its entire surface. That would be part of a major retrofit unless the walls are already wet plastered rather than boarded. Then just the inter storey surface needs treatment.
There shouldn’t be plumes of high relative humidity air coming up through the cavity, particularly as it is insulated. If you believe that there are, then the top of the cavity needs serious airtight capping, but that is a symptomatic treatment, not a cure. The source of the humid air needs to be found and eliminated.
A common source of abundant humid air in a cavity is unlined airbricks. Check that all airbricks are lined and the lining/wall interfaces are airtight. Another source is where penetrations have been made for pipes, wires and ducts. All such penetrations need to be airtight to both leaves of the wall, inside and out.
I have started work on our bathroom cavity, what do you think? I am going to clear out the mess first as there is a lot of waste left behind. I think I need to make the plasterboard side airtight. There are holes for pipes for example. Most of the brick wall is internal.
The bathroom needs to be air tight to the roof space and to my mind vapour proof too, although adequate roof space ventilation should compensate for small amounts of vapour getting into the roof.
Ventilation of the roof is a whole topic in its own right.
I have opened up the area in one of the bedrooms. I can see why the ground floor ceiling and 1st floor is cold. Air is coming through the old fibreglass and under the floor, also there is an air gap between the stud wall and chipboard.
I am thinking fill the space between the joists with 200mm rockwool and tuck in under the floor then cover over with something to make the space air tight. Then fill with more rockwool.
I see that in places efforts have been made to remedy insulation, possibly at the time of installation. Most probably someone who didn’t have our understanding was just doing their job. What you have photographed is all too normal.
Make sure that any insulation over the airtight layer is as least double the thickness of what is below, otherwise you risk the membrane getting cold and condensation forming on the inside, soaking into insulation and timber. A vapour permeable barrier over the insulation will minimise wind washing and clogging of air pores in the insulation.
I have opted to us air tight membrane on the ground floor ceiling then wrap over the joists. Then bonding and taping to the blocks for the internal wall. I have also taped to the inner stud wall. The membrane is now a pocket to fill with insulation and then overlay. A breather membrane has also been fixed to the roof joists to stop wind washing and dirt degrading the insulation.
I have a question about the master bedroom. We have opened it up today and found the inner wall is incomplete at one end. I can’t fasten the airtight membrane securely at this area. Any suggestions?
As others can confirm, home retrofit never quite goes as expected. Where there are upstanding blocks I would smooth off/fill in with mortar. Although not strictly necessary it will give a less abrasive surface to rub the corners and edges of the airtight membrane over its lifetime.
The options seem to be to fit new blocks to the top of the wall, fit timber or find some PIR off cuts to cut and fit as an end wall to the insulation. The use of the PIR off cuts is probably the option with lowest carbon footprint (as the material would otherwise be scrapped) and the easiest to work with. There is however the additional option of moving the edge of the insulation to overlap the cavity, improving the thermal bridge between roof and wall insulation.
I have often asked for PIR offcuts from a skip and never been refused.
I would say that insulating our two bedrooms roof space has had a positive effect. We haven’t had to change to winter duvets and the amount of gas used for november is 95kwh less than November 2022. I looked at temperature data from the met office and November 2022 was 1.8c above average, November 2023 was -0.1 below average. Our usage for this November was 1504 kwh so is this a high figure for a 3 bedroom detached house?
Passivhaus specifies max. 15kWh/m² year.
EnerPHit specifies max. 20kWh/m² year.
Low Energy Building … 30kWh/m² year.
In 2022 I used 53kWh/m² year.
In November I used 952.33kWh, or 6.5kWh/m² .
I can’t compare November this year as I am no longer on gas.
Regarding the summer use, I have cooked on induction for 17 years and my solar thermal has met my spring to autumn hot water requirements for 20 years.
Is there a way of correlating the gas usage to a heat pump size? I don’t think we could switch over at the moment as I think the house is losing to much heat. I assume there is a way of doing this in a detailed way or I suppose get a survey done?
The rule of thumb calculation comes in about 3kw using a years worth of heating gas consumption. I think that is low, we don’t run our heating 24/7, its off during the day while we are at work and over night. Maybe I need to run the heating on a constant setting for a few days or do something else.
Before switching to a heat pump I ran my gas heating at 45 °C for a year, well, in the winter anyway, so I was confident that a heat pump would work. I had to get the heating to come on up to an hour earlier than previously to give a longer warmup time but it did save gas.
For the last few days my 6kW heat pump has been running at 28 °C to 32 °C but I am now home most of the time (retired, house husband) and the heating is on whenever it gets the urge throughout the day.
I suggest starting a new thread if you wish to discuss heat pumps further.
