I am not familiar with that brand but i have in a few builder grade sliders before and have not had problems with air infiltration.
Adding more weatherstripping to a new windows would be considered wrong and highly unprofessional.
I would be suspect of the installation. My guess would be that did not foam around the windows and underneath the sill. Sliders are some what forgiving in installed out of sqaure or plumb because of how they seal, but could be the problem if they really botched it up.

The Double Hungs shouldn't be leaking that badly by any stretch.

Sliders can be a bit leakier by comparison and sometimes there are missing weatherstripping corners that are either field applied or factory applied.

The design and interface at the rail and sill of a slider is not going to be as tight as the double hung.

A well designed window usually have some features to prevent air infiltration;
Mortised sill wall, sill interlock, welded sloped sill.

Obviously a high quality window made with tight manufacturing tolerances.
I know Okna, HiMark, Soft Lite, and Sunrise are all very air tight.
Thermal Industries use very thin outer walls and thats always been a concern of mine. This sounds mostly installtion related though.

I'm posting a photo of one of the sliders. The arrow shows where the air is entering. I watched the installers and they thoroughly foamed completely around the windows including under the sill and they carefully leveled all windows. This air leak appears to be a flaw in the window itself because of where it's located, but I could be wrong...not knowing the ins and outs of windows.

This post was edited by lakefamily on Fri, Apr 26, 13 at 14:50

That area is inherently leaky in sliders in general. That said, the TI product does not have great air infiltration ratings in comparison with the top performing lines, so that should be kept in mind.
Adding weatherstripping tabs at those corners or addressing where weatherstripping may have been missed or cut short is not unprofessional at all, and is about the only way that you will be able to improve the situation assuming that the unit is installed square. Many manufacturers will even send out "service" kits with exactly that to address those issues.

+1 to HomeSealed feedback.

That is where sliders huff for certain.

Look for weatherstripping taps that can be placed on the bottom of the sliding section and where the interlock stops.

windowsonwashington...is that something that the salesman and the technician from the company should be familiar with? They're coming to check out the windows in the next week or so.

Thanks for the input everyone...

It is, and it is more than likely that is what they will bring out with them.

Probably not the first time they have heard the same problem, almost an inherent issue with sliders.

Thanks for the suggestions on fixing the sliders. What kind of fix is suggested for the leaky double hung? Significant air was felt from along the bottom where the window fits into the rail...the corners in particular. Thanks again.

Sorry to say, but you have a fever... and the only prescription is better windows!

Here is a link that might be useful: Prescription

For air leakge at the sill on a DH, you'll want to check to make the window is installed square so the weatherstripping is making good contact, and also that the weatherstripping is not missing or cut short.

+1

Check the corners of the interlock as well. Sometimes they are cut short and tabs can be dropped in those locations as well as chimney draft blocks.