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5 replies
3k views
5 replies
Plastic between old facade and additional insulation
Hello everyone,
we are in the process of buying an older house, built in 1912 with a timber frame (joined logs) that has been additionally insulated with a new facade. During the transfer inspection, it was found that there was plastic installed between the old timber facade and the additional insulation. We signed the contract with an open purchase clause for further review of the facade, and yesterday we had a carpenter lift the facade in two places. The plastic was found in these places, but the timber facade underneath was in fresh condition and not at all affected by the incorrectly installed plastic - by today's standards.
The facade looks as follows: Timber frame - construction plastic - 95 mm insulation - wind barrier - tongue and groove panel.
The carpenter suggested that the energy value in the timber frame is likely so poor that the dew point is still correct in the wall. It should be mentioned that the additional insulation was done in 1978 by previous owners.
Should we believe it's fine and go with what the carpenter says? We only did two spot checks on the facade - not in the bathroom where there should be most load.
Other carpenters I've talked to have said that the installation of the plastic is idiotic and that I should be wary of this. Despite this, the facade looks good under the plastic and shows no signs of mold damage.
What to do?
we are in the process of buying an older house, built in 1912 with a timber frame (joined logs) that has been additionally insulated with a new facade. During the transfer inspection, it was found that there was plastic installed between the old timber facade and the additional insulation. We signed the contract with an open purchase clause for further review of the facade, and yesterday we had a carpenter lift the facade in two places. The plastic was found in these places, but the timber facade underneath was in fresh condition and not at all affected by the incorrectly installed plastic - by today's standards.
The facade looks as follows: Timber frame - construction plastic - 95 mm insulation - wind barrier - tongue and groove panel.
The carpenter suggested that the energy value in the timber frame is likely so poor that the dew point is still correct in the wall. It should be mentioned that the additional insulation was done in 1978 by previous owners.
Should we believe it's fine and go with what the carpenter says? We only did two spot checks on the facade - not in the bathroom where there should be most load.
Other carpenters I've talked to have said that the installation of the plastic is idiotic and that I should be wary of this. Despite this, the facade looks good under the plastic and shows no signs of mold damage.
What to do?
The recommendation is that the plastic should be mounted 1/3 into the wall measured from the inside (1/4 according to some). This applies to 1/3 of the insulation value in the wall, not measured in [mm] wall thickness, but in [W/m^2*C] insulation value.
Wood has an insulation value between 0.10-0.16 [W/m*C] (measured perpendicular to the grain direction) and wool insulation ranges between 0.031-0.041 [W/m*C] (use 0.04)
So take the insulation value [W/m*C] for the wall part and divide it by the thickness [m] of the wall part, compare these two values and you should get a ratio that does not exceed 1/3...
Example:
Wool: 0.040 / 0.095 = 0.42
Timber: 0.13 / 0.200 = 0.65
Ratio: 0.42 / 0.65 = 0.65 (Should not exceed 0.33)
Wood has an insulation value between 0.10-0.16 [W/m*C] (measured perpendicular to the grain direction) and wool insulation ranges between 0.031-0.041 [W/m*C] (use 0.04)
So take the insulation value [W/m*C] for the wall part and divide it by the thickness [m] of the wall part, compare these two values and you should get a ratio that does not exceed 1/3...
Example:
Wool: 0.040 / 0.095 = 0.42
Timber: 0.13 / 0.200 = 0.65
Ratio: 0.42 / 0.65 = 0.65 (Should not exceed 0.33)
That the house (by today's standards) has poor insulation can help the situation, since the poor insulation value means the exterior walls have a higher temperature. Which in turn means a lower relative humidity...
Whether there are already damages or not is difficult to say, check outside the bathroom as well, if it's okay there too (+the 2 places you've already checked) then it should be okay everywhere...
To be sure in the future, you can take the opportunity to remove the panel and add additional insulation outside the existing 95mm that already exists. -Then you can achieve the ratio of 0.33 and also reduce energy consumption...
Whether there are already damages or not is difficult to say, check outside the bathroom as well, if it's okay there too (+the 2 places you've already checked) then it should be okay everywhere...
To be sure in the future, you can take the opportunity to remove the panel and add additional insulation outside the existing 95mm that already exists. -Then you can achieve the ratio of 0.33 and also reduce energy consumption...
Thanks for the response, yes I am thinking that additional insulation might be needed, even in the attic where there is 15 cm of sawdust in the old part and 25 cm of mineral wool in an extension. What is recommended for attic insulation? I've been advised not to mix different types of insulation but rather to remove one and add more of the same as the remaining one.
Investigate if you have plastic or another air barrier from inside the house to the cold attic!
(If you have a classic cold attic)
- It's a classic trap to add insulation without an air barrier;
=> You get a colder cold attic & thus moisture/mold problems...
It is always possible to add insulation to an attic, but you should
know what you're doing... otherwise, it can go badly.
* airtight floor structure
* Negative pressure in the house
* Just the right amount of ventilation of the cold attic, not too little & not too much!
(If you have a classic cold attic)
- It's a classic trap to add insulation without an air barrier;
=> You get a colder cold attic & thus moisture/mold problems...
It is always possible to add insulation to an attic, but you should
know what you're doing... otherwise, it can go badly.
* airtight floor structure
* Negative pressure in the house
* Just the right amount of ventilation of the cold attic, not too little & not too much!
Ok, in other words, it's time to consult an expert if it becomes relevant... Thanks for the great response!
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