External AND internal insulation on same walls

Beatz

Looking for some thoughts/advice on a strange situation where external AND internal insulation may be used on the same walls.

We are renovating an old house which has existing dry lining. I’ve no idea how long its there but its very old and the insulation is poor (about 40mm of polystyrene). As part of an Energy upgrade we are adding EWI.

Since removing the dry lining would impact a lot of the existing interior (wall finish/floors/sills/skirting/painting etc etc) and because our budget is blown, the insulation guy and others are saying there is no problem having both.

My concern is that with EWI you are mean to be using the thermal mass of the block. However, if the existing dry lining is left in place, then it slows the heat going into the block. That is probably ok (since it will get there eventually) however it also slows it coming back into the room.

Would we be creating a situation where the heat is trapped or are would we effectively be reducing the effectiveness of the EWI by having a barrier for the heat to come back into the room ?

In an ideal scenario would be we’d strip the whole lot out, but for budget reasons we’d prefer to leave unless it is compromising the EWI.

Any thoughts/advice !?

Calahonda52

Please describe, in as much detail as you can, the construction of the exterior walls from outside in:
eg:
render
wall
inside wall finish
dry ling batten
40mm ploy styrene
plaster board
gypsum skim

also how is the new wiring/plumbing going to be catered for?

regardless of the answers, it is absolutely imperative that the ewi goes down below ground level and up through the soffit board to meet, in an airtight way, the ceiling insulation.

In addition, if replacing the windows, either hang them on the outside before the ewi goes on, or else allow for a decent amount of ewi on the external reveals: standard window frame sections are too narrow.

stiofan85

Beatz wrote: »

Would we be creating a situation where the heat is trapped or are would we effectively be reducing the effectiveness of the EWI by having a barrier for the heat to come back into the room ?

I know someone with this situation and I had your exact thoughts, but he said his house is wonderfully warm. Would love to know the truth as we drylined and it is pretty effective but always have the option of external.

Beatz

Hi – Given I’m not versed in construction methods my best answers would be:

The exterior wall is a 9” solid block construction (from late 1940’s). Currently there is a poor/dated dry lining system in place, which appears to be about 40mm of polystyrene which I think is between battens attached to the interior wall. A plasterboard finish is then the inside wall finish.

As mentioned, I’m concerned that we may effectively be reducing the effectiveness of the EWI form 100mm to 60mm in a roundabout way (if there is 40mm to pass through to get back into the room would we have lost heat through the EWI by the time the heat makes it to the room ?).

We are not proposing to rewire as it was done about 15 years ago (not perfect but doesn’t seem to justify costs). Plumbing will remain with replacement of rads etc.

The EWI will go about a meter below floor level but not beneath ground level. As there is a suspended floor (which will be insulated) the floor level is much higher than ground level. I guess ideally you’d bring the EWI to the foundation but the cost/disruption of this would be huge relative to the heat loss ? The soffit will be taken off and replaced so the EWI will meet the roof insulation (and floor insulation as mentioned). The existing windows are good and not being moved out. 25mm of EWI will be put into the reveals (again a question of cost v’s impact).

In terms of the internal v external heat loss question, I’m trying to gauge whether I’m over thinking this or not though !!

GreeBo

If the internal insulation is doing its job then it should by definition be preventing the external insulation from doing its job.

I'm sure your rooms will be warm but you will have no thermal mass so only the air will be warm, open a door/window and your house will be cold again.

Also any heat energy that slowly makes its way to the blocks wont be able to get back to the rooms to keep them warm and you may get condensation/mold issues between both layers.

BryanF

GreeBo wrote: »

If the internal insulation is doing its job then it should by definition be preventing the external insulation from doing its job.

I'm sure your rooms will be warm but you will have no thermal mass so only the air will be warm, open a door/window and your house will be cold again.

Also any heat energy that slowly makes its way to the blocks wont be able to get back to the rooms to keep them warm and you may get condensation/mold issues between both layers.

Op already said the drylining is 40mm eps. So everything else above is not applicable and in the context in just scare mongering

GreeBo

BryanF wrote: »

Op already said the drylining is 40mm eps. So everything else above is not applicable and in the context in just scare mongering

So you don't think the internal insulation is preventing or at best slowing heat getting to and from the blockwork?

jmBuildExt

GreeBo wrote: »

So you don't think the internal insulation is preventing or at best slowing heat getting to and from the blockwork?

Purpose of insulation is to stop heat escaping from the room.
"Allowing the room to heat up via heat stored in the block work" is not an advantage of external insulation.

OP:
The only problems you should have here are the usual problems associated with internal insulation:
i.e. moisture in the air condensing inside the house/behind the internal plasterboard and causing mold. So someone needs to do a dew point calculation. The external insulation will more than likely have a baring on this.

GreeBo

jmBuildExt wrote: »

Purpose of insulation is to stop heat escaping from the room.
"Allowing the room to heat up via heat stored in the block work" is not an advantage of external insulation.

You don't think that external insulation allows the structure of the house (block work) to act as a heat store and that heat will transfer back into the rooms?

Where do you think this heat goes if not back into the rooms (assuming they are colder)?

jmBuildExt wrote: »

OP:
The only problems you should have here are the usual problems associated with internal insulation:
i.e. moisture in the air condensing inside the house/behind the internal plasterboard and causing mold. So someone needs to do a dew point calculation. The external insulation will more than likely have a baring on this.

Isn't that what I said?

listermint

If you are not going below the ground level with the EWI, you are already defeating it. As your blockwork will be attached to any external paths surrounding the house, meaning a nice conductive layer of external concrete that is attached to the blockwork allowing that to travel up the blockwork beyond the EWI.

Its just food for thought. Retrofits are super tough (going through it myself) but if you strive to hit all the highlights then you will end up with a good results.

Obviously alot is cost prohibitive espectially ground works which many installers dont want to do, quick and easy jobs is handier money.

RTT

I am also interested in hearing some advice on this as I am considering getting external wall insulation.
I currently have 50mm, foil backed, insulated plasterboard slabs. They are fixed with mushroom fittings to the internal walls and skimmed over. Would ewi be a good idea in addition to the existing internal insulation?