Hello,
I am currently planning an extension to an old, "haphazardly modified" farmhouse and am wondering how to best deal with a thermal bridge. The existing building has a strip foundation cast directly into excavated soil, with unknown depth and reinforcement. This foundation extends up to about the ground floor level from the outside terrain and is topped with a row of concrete hollow blocks before continuing upwards with pumice hollow blocks.
My current plan is to install a U-shaped strip foundation from the outside against the building, avoiding deep excavation around the old structure since the structural risks are unclear. On top of this will be a roughly 10cm (4 inches) concrete slab. Insulation below and perimeter insulation is not planned, as aerated concrete and thick, insulated screed will be used, meaning all insulation will be located above the foundation.
Now I am wondering: would it even make sense to use perimeter insulation on the extension to reduce heat loss through the old strip foundation, which would then be located further inside the building envelope?
I am not an expert on how exactly heat loss behaves when the cold is blocked from the soil beneath the building by perimeter insulation in the upper ground layer but there is still direct contact of concrete with room air and soil further inside.
1. Is the effort for perimeter insulation cost-effective? Without precise building data this is naturally difficult, but any guidance or insight into the typical behavior of ground temperatures under buildings would be helpful.
2. Is it economically feasible to retrofit insulation to the old strip foundation? Partial excavation and then backfilling? I don’t think this is practical since the foundation’s load-bearing capacity depends on direct contact with the soil. Or excavate, install formwork, pre-cast concrete, insulate, then compact again? In my opinion, this is a huge effort with little benefit, as only one side of the strip foundation would then be protected from heat loss, which should not be very high anyway due to the lack of direct contact with the outside soil.
3. A last alternative might be insulating the interior plinth area near the old strip foundation (or the entire wall), but I am not a fan of interior insulation. However, if it turns out that the temperatures beneath the foundation do not drop significantly—because the extension acts as a shield from the outside—and the soil below the building and thus the concrete wall do not get cold enough to cause condensation problems, then this might still be an option. I cannot assess this precisely. If so, what materials would be suitable here, especially those that can handle any potential condensation?
The option of a fully insulated floor slab for the extension is basically off the table for me, as this would be overkill compared to the old building and in my opinion disproportionate to the overall achievable energy efficiency of the house.
I would appreciate any help, additional ideas, or confirmation of my current tendency, which leans towards “leave it as it is, it’s not worth it.”
I am currently planning an extension to an old, "haphazardly modified" farmhouse and am wondering how to best deal with a thermal bridge. The existing building has a strip foundation cast directly into excavated soil, with unknown depth and reinforcement. This foundation extends up to about the ground floor level from the outside terrain and is topped with a row of concrete hollow blocks before continuing upwards with pumice hollow blocks.
My current plan is to install a U-shaped strip foundation from the outside against the building, avoiding deep excavation around the old structure since the structural risks are unclear. On top of this will be a roughly 10cm (4 inches) concrete slab. Insulation below and perimeter insulation is not planned, as aerated concrete and thick, insulated screed will be used, meaning all insulation will be located above the foundation.
Now I am wondering: would it even make sense to use perimeter insulation on the extension to reduce heat loss through the old strip foundation, which would then be located further inside the building envelope?
I am not an expert on how exactly heat loss behaves when the cold is blocked from the soil beneath the building by perimeter insulation in the upper ground layer but there is still direct contact of concrete with room air and soil further inside.
1. Is the effort for perimeter insulation cost-effective? Without precise building data this is naturally difficult, but any guidance or insight into the typical behavior of ground temperatures under buildings would be helpful.
2. Is it economically feasible to retrofit insulation to the old strip foundation? Partial excavation and then backfilling? I don’t think this is practical since the foundation’s load-bearing capacity depends on direct contact with the soil. Or excavate, install formwork, pre-cast concrete, insulate, then compact again? In my opinion, this is a huge effort with little benefit, as only one side of the strip foundation would then be protected from heat loss, which should not be very high anyway due to the lack of direct contact with the outside soil.
3. A last alternative might be insulating the interior plinth area near the old strip foundation (or the entire wall), but I am not a fan of interior insulation. However, if it turns out that the temperatures beneath the foundation do not drop significantly—because the extension acts as a shield from the outside—and the soil below the building and thus the concrete wall do not get cold enough to cause condensation problems, then this might still be an option. I cannot assess this precisely. If so, what materials would be suitable here, especially those that can handle any potential condensation?
The option of a fully insulated floor slab for the extension is basically off the table for me, as this would be overkill compared to the old building and in my opinion disproportionate to the overall achievable energy efficiency of the house.
I would appreciate any help, additional ideas, or confirmation of my current tendency, which leans towards “leave it as it is, it’s not worth it.”
Hi,
your approach already seems quite reasonable in wanting to avoid unnecessary exposure of the old foundation. These post-war strip foundations are often more sensitive to lateral excavation than expected, and perimeter insulation only makes a significant difference if you can fully separate the heat flow to the outside. However, you won’t achieve that because the old concrete base remains inside and conducts heat through. The slight shielding on the outside reduces surface cooling somewhat, but the heat still travels through the inner foundation body toward the ground.
The payback time for perimeter insulation in such mixed situations is usually much lower than manufacturers like to suggest. Have you even roughly calculated how much energy the foundation actually affects on an annual average? The soil beneath your older building typically stabilizes around 8 to 12 degrees Celsius (46 to 54 degrees Fahrenheit), so the foundation does not lose a lot of heat overall. More expensive solutions like extending, formwork, insulating, and backfilling are completely disproportionate for such a low temperature difference.
Theoretically, you could insulate from the inside, but you would need a material that can handle moisture without rotting immediately. There is also the risk of causing localized condensation. I would really recommend checking whether simply adding the extension already sufficiently stabilizes the thermal environment before planning bigger interventions.
Good luck!
your approach already seems quite reasonable in wanting to avoid unnecessary exposure of the old foundation. These post-war strip foundations are often more sensitive to lateral excavation than expected, and perimeter insulation only makes a significant difference if you can fully separate the heat flow to the outside. However, you won’t achieve that because the old concrete base remains inside and conducts heat through. The slight shielding on the outside reduces surface cooling somewhat, but the heat still travels through the inner foundation body toward the ground.
The payback time for perimeter insulation in such mixed situations is usually much lower than manufacturers like to suggest. Have you even roughly calculated how much energy the foundation actually affects on an annual average? The soil beneath your older building typically stabilizes around 8 to 12 degrees Celsius (46 to 54 degrees Fahrenheit), so the foundation does not lose a lot of heat overall. More expensive solutions like extending, formwork, insulating, and backfilling are completely disproportionate for such a low temperature difference.
Theoretically, you could insulate from the inside, but you would need a material that can handle moisture without rotting immediately. There is also the risk of causing localized condensation. I would really recommend checking whether simply adding the extension already sufficiently stabilizes the thermal environment before planning bigger interventions.
Good luck!
Similar topics