How do I give a div a responsive height?

Question

I'm starting to learn responsive design, but there's one thing I can't seem to figure out, so far...

I'd like the div element (contactBg) with text inside of it to come down to the bottom of the image to the left. Obviously I can do that by defining the height in px, but then that height is maintained when the rest of the window resizes instead of being fluid.

How can I do this?

Here's the mark up:

<div class="row-fluid">
        <div class="span10 offset1">
            <div class="container-fluid">
                <div class="row-fluid">
                    <div class="span5">
                        <img src="images/logo.jpg" alt="logo" />
                        <img src="images/store.jpg" alt="store" />
                    </div><!-- /span5 -->
                    <div class="span7">
                        <img src="images/portal.jpg" alt="portal">
                        <div class="contactBg">
                            Administration: Emma Jane Julien<br />
                            <a href="mailto:[email protected]">[email protected]</a>
                        </div><!-- /contactBg -->
                    </div><!-- /span7 -->
                </div><!-- /row-fluid -->
            </div><!-- /container-fluid -->
        </div><!-- /span10 offset1 -->
    </div> <!-- /row-fluid -->

And here's the CSS:

   body {
        background: url(../images/3D2A1698A177AF9B71_218.png) repeat;
    }
    
    img {
        max-width:100%;
        padding-bottom:1%;
    }
    
     .container-fluid {
        background:#fff;
        padding: 1%;
    }
     
      .row-fluid {
        width: 100%;
        *zoom: 1;
 }
      .row-fluid [class*="span"] {
        display: block;
        float: left;
        width: 100%;
        min-height: 30px;
        margin-left: 1%;
        *margin-left: 2.709239449864817%;
        -webkit-box-sizing: border-box;
           -moz-box-sizing: border-box;
                box-sizing: border-box;
}

      .row-fluid .span10 {
        width: 82.87292817679558%;
        *width: 82.81973668743387%;
      }
 
      .row-fluid .span7 {
        width: 58.06353591160195%;
        *width: 57.12912895262725%;
      }

      .row-fluid .span5 {
        width: 40.93646408839753%;
        *width: 40.00205712942283%;
      }
      .row-fluid .span4 {
        width: 31.491712707182323%;
        *width: 31.43852121782062%;
      }

    .contactBg {
        background: #282624;
        padding: 3%;
        max-width:100%;
        height:auto;
    }

Answer 1

I know this is a little late to the party but you could use viewport units

From caniuse.com:

Viewport units: vw, vh, vmin, vmax - CR Length units representing 1% of the viewport size for viewport width (vw), height (vh), the smaller of the two (vmin), or the larger of the two (vmax).

Support: http://caniuse.com/#feat=viewport-units

div {
/* 25% of viewport */
  height: 25vh;
  width: 15rem;
  background-color: #222;
  color: #eee;
  font-family: monospace;
  padding: 2rem;
}

<div>responsive height</div>