Making a cavity wall building airtight

paddy236

I am in the process of buying an unfinished house and I need some advice.
It is a 1900sq/ft bungalow.
The work/spec completed so far is:
310mm cavity wall construction (100mm inner and outer leaf). The exterior of the house has been plastered
75mm insulation in the floor
the roof is already on and slated
the windows and external doors are in
it has been plumbed for radiators
there is NO insulation in the house yet.

I plan to convert the attic into bedrooms and a bathroom in a few years.


My main question is:
Is it possible to make this house sufficiently airtight to allow a MHRV to work effictively?

I do not want to externally insulate the house as the windows and doors are already in and it has been plastered. I will therefore have to pump the cavity with insulation.
Is it a good idea to slab the interior of the exterior walls with insulated plasterboard or will it cause problems with mould?

Is there a way of minimising cold bridging between the floor and the walls?

Any advice/help/links would be greatly appreciated.
Paddy

gillad

slabbing the internal walls with in insulated slab is common practice now and makes a big difference to a blockwork house when combined with cavity insulation

paddy236

gillad wrote: »

slabbing the internal walls with in insulated slab is common practice now and makes a big difference to a blockwork house when combined with cavity insulation

That is what I had planned but after reading posts about the dew point being moved inwards I wasn't sure.
What would be a suitable thickness of insulated plasterboard to use?
What cavity insulation should I use with the insulated plasterboard?
How can I minimise cold bridging between the floor and the plasterboard?

Paddy

RKQ

Your designer should specify cavity wall insulation & drylining etc, as part of a complete package including floor & roof insulation etc.

Your BER Assessor will be very interested in these points & will be best suited to advise, having seen your design & site orientation.

There are 3 companies in Ireland that supply airtight membranes & tapes - well known market leaders. I don't think I can say their names now, but they are mentioned in numerous old threads, so have a look or Google "airtight membranes". (Pm for details of all three) At least two of the companies offer one-day Installation courses, which are very useful to do to understand the need for "airtightness" & practice installation.

The correct installation of the insulation board in the cavity has an affect on the level of airtight achievable - so its worth watching these details. T&G joints must be tight.

IMO I think timber frame construction is easier to install airtight membranes. In fact most timber frame kits now come with a factory installed airtight membrane.

No6

If you are pumping your cavity walls and drylining with thermal boards internally give all the external walls a scratch coat of plaster internally before applying the drylining. This seals the blockwork and hughly improves its airtightness. If the ceiling isn't slabbed you can then use a prop airtightness membrane on the ceiling and if you seal all the junctions between window frames and walls, between external walls and internal walls and external walls and ceiling with a prop tape system you should be fine for airtightness.

Get the manufacturer of the thermal board to do a uvalue and dewpoint calculation for their insulating plus what ever you have pumped into the cavity. I wouldnt use more than 50mm insulatuion on the drylining, pumped polystyrene bead is reasonable and slightly better in terms of performance over the white wool types of pumoped insulations.

You should have a minimum of 25mm vertical insulation from the floor slab to finished screed level around all the walls to allow movement in the screed and to reduce thermal bridging.

paddy236

RKQ, thanks. you have pm.
No6, thanks for that detailed info. I will apply a scratch coat before drylining the external walls. The ceiling is not up yet so I will also be able to use the prop airtightness membrane and tape to seal the downstairs.

What would you recommend for sealing the first floor (bungalow with 4 velux on south side)? The roof has not been double latted so I cannot pump the rafters.

Should I also pump the ceiling with polystyrene while i'm doing the walls?

Paddy

paddy236

Just to update
First of all I would like to thank everyone for their help so far. I have learned so much already from this forum.

For my external walls I have chosen a gypsum based scratchcoat, 50mm insulated slab held in place with adhesive and then a skin coat. Airtight tapes will be used around doors, windows, where floor and ceilinG join walls.
For the roof I have decided on 100mm rafterloc(or similar), an airtight membrane, 4"x2" cross battens with rockwool ( or similar) inbetween and then a standard plasterboard slab.

I am having difficulty choosing my cavity insulation. I am trying to decide between ecobead or instafibre.
Is ecobead flammable?
Does ecobead have to be kept 300mm from wires that pass through the cavity, or is it ok to run the wires through a conduit?

Is instafibre outdated?
Does the instafibre sag/settle over time?

Is there anything else I should know about these products?
Should I be looking at any other products? (pm details)

Cheers,
Paddy