Houses are using an armor map. And the armor value are so low so that AP goes throu it (for most of the guns anyway). HE on the other hand explodes on first contact. That is how i have understood the mechanics behind this. What you can try to do is to change values on the armor map to a quite high value (or lower it for bushes etc) and see if there is a difference. You can find them here: \data\k42\loc_rus\armor_maps\objs.
When you open it you can see that for example that the house called saray0 have 55mm armor. And if you also open the object.engcfg files wich can be found here: \data\k42\loc_rus\levels\LEVELS\obj_cfg\objects (or here: \data\k42\loc_rus\levels\LEVELS\obj_cfg\flora for bushes/trees) you can see what armor map all objects are using.
And you also have one more parameter to take a look at: material = wood;
I have no idea what difference this parameter does. Maybe the sound when you hit it? Maybe different strenghts? (you probably find info about them in the common_res) But i have never tested to change it so i have no idea. But this is atleast a good start to experiment if you can do it better somehow
EDIT: just checked the armor map for bushes, shows that it have 77mm armor
So maybe lower it would solve some trouble, or give new ones, i dont know.