Can you help with just one German word? It is "Trennvlies". It is used
in connection with construction, but I do not know to what part of the
building it refers. I've searched everywhere!

I speak German, and Trennvlies is the German word for an unwoven
fibrous material (fleece) used to separate or keep keep something one
layer mixing or growing into the lower level  (Trennen)  It is used
for example in gardening and also in consturction (your "Bauen"),
where it may be laid und the roof tiles, or laid around the
underground portion of the walls to protect the asphalt insulation
from damage.
That probably suggests to you what is meant in the context you have. 
I hope so, but if not, please feel free to come back with the German
text in which it is mentioned.

Your comment is indeed most helpful. The trouble is, I only have the
word as a caption, on its own, and I do not know which part of the
construction it is used for.
I am wondering if "separation fleece" would be suitable, or
"separating fleece". What do you think?
By the way, could you please tell "the powers that be" that I have
changed my credit card.I am not sure how I inform Google Answers of
this, and I ceertainly do not want it to look as if I had refused
payment.
Many thanks!

Well, not a precise answer to your question ...

But if you choose "Trennvlies" and "separate" as search terms you can
see that both "separating layer" and "separating mat" are being used
as translations:

://www.google.co.uk/search?q=trennvlies+separating

"Separating layer":
http://www.detail.de/Db/DbFiles/archiv/3421/ansichts.pdf (page 3 of 3)

"Separating mat":
http://www.detail.de/Db/DbFiles/archiv/1783/ansichts.pdf (page 2 of 4;
access restricted but searchable pdf-page).


Regarding your change of credit card I suppose the best would be to
send an e-mail to answers-editors@google.com

Thank you very much. The answer is now perfect!
Regards,
Leander1