From Thermotech's website: Firstly a double hung window is not as air tight as a casement window. The double hung window has to slide, so its weatherstripping can't be as tight as a casement or it won't slide. That's obvious to most people. What is not so obvious is that sliding windows aren't as air tight as casements because the weatherstripping changes plane. Think of the bottom sash of a double hung. The bottom and side rails are weatherstripped along the edge of the rail. But what about the meeting rail? It's weatherstripped along the outer face. It is difficult, very difficult, to maintain the integrity of the air seal through this change of plane. Despite the fact that double hung windows are inherently draftier than casements, this does not have a major effect on your annual heating bill. The reality is that as long as new windows can maintain their tested air tightness (this topic will be the subject of a future column) very little of the window's heat loss is due to air leakage. Most of the annual heat loss from reasonably tight windows is from conductivity losses through the frame, spacer and glass. This is where double hung windows really fall down, (so to speak). All sliding windows have inherently cold frames compared to hinged windows, like casements. The sash in a casement window is "insulated" on the inside by the hardware cavity. The sash in a double hung window has no such advantage. Another inherent thermal weakness in any sliding window is the meeting rail. Thermally it is the weakest part of any sliding window. The upper sash in a double hung not only loses heat through its outer face, but also through its lower edge. It is cooled on two sides. That's why the bottom edge of the upper sash experiences condensation before the lower sash.

My 1926 house has a mix that is orginial to the house (love wavy glass). The living room, breakfast nook and dining room all have casement. The kitchen, bathroom and bedrooms have double hung (and the other bathroom has frosted glass slats that open and close). So I don't see casements as modern nor do I see a mix as necessarily a problem. That said if the whole rest of your house has double hung, I would probably go with that. In my house there's a kind of logic to the differences--"public" spaces at the front of the house have casement, "private" spaces at the back have double hung. Just one different seems like it might stand out more in a not so great way.

We have Casement and Double-Hung. I am no expert. The following is our experience and research info. Good windows from reputable manufacturers, big or small, will seal well and not have unreasonable infiltration. A company may sell different lines from entry-level to higher-end and each line may have different infiltration rating. You can check the ratings published by the manufacturer to get one that best fits your requirements. In general, casement is supposed to seal better than dbl-hung and slider is supposed to be the hardest to seal tight. Again, good companies knows how to seal them tight, sliders or otherwise. Draft in general has three sources. One is poorly sealed windows, another is poorly installed windows with gaps and seams open to the outside and the other is poorly constructed windows. A properly installed window can seal tight against infiltration but if it transmits heat or cold readily from the exterior to the interior due to poor design or material, then a draft will form as interior air passing the window is rapidly heated or cool. Either case, air movement will happen by the windows. Note that "cheaper" windows often are poorly designed and constructed and will be drafty. We lived with a house full of these when we bought our place and ended up replacing all of them. So in many cases, we do pay for what we get. Modern windows use all sorts of thermal breaks to limit or prevent such contact-transfer of thermal energy. Low-E coating is very effective at keeping heat out in the summer and warmth in during winter months. Double-pane construction with gas-filled space is another method to limit energy transfer. Ours are of such construction and have lived up to expectation. We are currently smack dab in the middle of CA's Summit Fire that started last THursday. Our house has been covered by thick smoke all this time and no hint of smoke in our house yet. Both the dbl-hung and casement are doing their jobs. Good luck with your research!

Thanks. I figured that was the case but wanted to verify. We came down to the wire for ordering, having lost a couple of days to the snow storm in late October. Our contractor ordered 1' transoms to go with our double hung windows, and the glass height is only 5". I wish we could remove the interior trim of the transoms and then replace the vertical dividers just to expose more glass. To my eye, the glass is too small in height when compared to the pane size of the double hung window below it. Live and learn....