House purchase - How to deal with cold exterior walls?

Hello everyone,

we recently purchased a house, and in a few weeks the renovation and refurbishment work will begin. The house was built in 1957, the exterior walls are made of 24cm pumice stone (all floor slabs are made of reinforced concrete, basement walls and foundation are made of rammed concrete).

Now, the previous owner has provisionally insulated some exterior walls from the inside, partly very simply with plastered polystyrene panels, partly with professionally installed wooden cladding with a vapor barrier between the wood and the wall (which we will probably initially keep as is). We are aware that we have to expect mold behind the polystyrene panels - but that is another issue. ;)

We are currently wondering how to deal with these apparently very cold exterior walls in winter. From our point of view, there are basically three options:

In any case, we will remove the styrofoam crap from the walls, remove all mold (possibly also from the plaster)

1. and simply renovate, i.e., do no insulation measures. We will wait through the first winter and then see how the temperatures and the livability in the rooms are. In the worst case, we will just have to heat a lot. But then the walls can breathe and not get moldy again (assuming proper ventilation ;) )

2. and insulate from the inside at the places where the previous owner has already insulated, with applied walls and vapor barrier (there are apparently prefabricated walls made of expanded clay, but they are very expensive and complicated to install – plus they apparently protrude about 8cm into the room). But that takes away the walls’ ability to breathe, right?

3. and insulate the entire house from the outside. This would probably be the "nicest" solution, but unfortunately is definitely out of the question for us because of the budget!

Do you see any other possibilities?

Sorry if this is probably a frequently asked fundamental question, but we are currently desperate because we don't want to do anything wrong. Would an energy expert be the right contact for an independent on-site consultation?

Many thanks for all tips and opinions.

Best regards
NFA

For the energetic renovation of a building, a renovation concept in the context of the entire building is necessary, which also integrates the heating/the heating system. Intended partial insulation of the building must be critically examined, as there is a risk of increased condensation/mold formation at particularly cold spots (thermal bridges). Already implemented insulation measures—especially interior insulation—must be checked for building physics consistency, particularly if they were carried out as own work and involve vapor retarders/vapor barriers. The latter always have defects and thus leakage points.

If you want to insulate in sections, this is possible, but only after a previously created overall concept with clearly defined interfaces. Regarding your insulation variants: 1.) Feasible, but not economical, as you may turn your house back into a construction site after 1-2 years. Walls do not breathe, by the way, but merely sorptively absorb moisture to a limited extent and also release it back into the room air. Nothing passes through the wall except heat radiation. Mold arises from insufficient or incorrect ventilation, inadequate building insulation, and insufficient room tempering (... the cold bedroom ...) and is mostly user-related. 2.) Not recommended, as it is complex in building physics. Interior insulation must be cavity-free, diffusion-open, and capillary-active. Appropriate materials are available today but must be installed professionally; expanded clay does not count. 3.) The best solution. Insulation is always applied on the cold side. Interior insulation is a last resort when it is not possible to do otherwise due to the building construction or if monument protection prohibits facade insulation.

I recommend direct cooperation with a combined building energy consultant/architect who will develop an individual renovation concept for you, taking your budget and your family situation (move-in, own work, etc.) into account. He can also advise on potentially applicable funding programs from KfW Bank or BAFA.

What I strongly advise against: 1.) spontaneous action without a concept, 2.) action without professional planning, 3.) action based on uncoordinated contractor offers, without independent planning.

You can find regional building energy consultants for a first on-site consultation via the dena list of [energie-Effizienz-experten].