Some times transparency need to be assigned manually in Sweet home 3D
Try these steps,
1. Make/design your model at any 3d-modeling software
2. Apply transparency texture(color=black&white) to it( use *.png only to work with SweetHome3D).
3. Align UVW mapping.
4. Export as .obj(wavefrot OBJ) or .3ds (check Transparency texture must be added in the same folder ).
5. Import exported obj/3ds file in sketch-up/any other 3d model viewing application to check transparency map working or not.If transparency does not load you failed to create model correctly(Need re creation).
6. If transparency working correctly then Import 3d model as furniture in Sweet-home-3D.
7. Select imported furniture, go to Furniture -> Modify in main menu
8. Select materials radio button in color& texture, then click Modify
9. Select model material(your transparent object material) then select Texture radio button and click browse button.
10. In the new window click Import button & browse, open your texture map and click OK for all windows.
*10. If object don't have a texture file, fully transparent like glass
then select color radio button and select white color in modify Furniture material window.
11. Now render your image.
TIP.
If you want to hide(invisible in render) a particular part of an object then use my texture.
(This file consist zero color pixel).
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