we are getting ready for such a project. in 2008 replaced half the house w Marvin Infinity . Used casements and sliders both.[different areas and usage on the house]. Truthfully, I really like the sliders and when closed up for the winter had no draft and they were quiet. You can nudge them open just an inch or two. Casement panel pivots the entire panel out when you crank even a bit...the casement window was chosen for the looks in the configuration of the spots I placed it …..but the cupping of the air in through the casement and the panel pivoting out , VS. just a "crack" open of a slider is a factor. Do not feel that a better or best slider, say in fiberglass or wood w fiberglass exterior will be a sub par window. An issue is getting the correct function/ sizing/opening features for the spot you are putting it. Fiberglass can be made into any size but is generally going to be better than vinyl . Marvin just redid their window series and added" Marvin Modern"....Now I have the second half of house and looking again but may use Marvin Integrity, which is fiberglass but I won't be doing gelstain on all windows like I did with Infinity. I think Marvin is a good window...we like the sliders. I just spent a few hours on the FENESTRATION website this week.....if you take two windows..say Casement vs Slider...the differences are not that great.....but the material may be a factor: fiberglass interior and exterior in a very good window may be better than some alum clad windows it appears.... [any aluminum or fiberglass exterior can be painted later-vinyl can not] the BIG difference comes w Argon and Low E ...should be standard [and look for the lowest U rating in your product search] Krypton, triple panes of glass, laminated glass, sound glass are all extra things depending on what needs you have. Good luck.

There are fundamental differences in the closure interfaces that make the average casement more airtight than the average slider. It is just a function of design. Given that fact and assuming similar glass packages, the casement WILL be more efficient. Enough so to justify the delta mostly depends on the application and what you are going for.

It, however, is not poppycock that sales people are "selling" you that casements are more efficient (on the average).

Every layer over the window also adds an R value.....many people are Not..but I AM a window treatment person.....I have layered window treatments...blinds or shades with fabric panels. We live in a slightly urban place....some thinner layers for privacy, then thicker layers to pull for blackout at nite or when complete block is desired for other reasons. this can be part of your scheme as well.....I like the gentle nudge of a slider . When the conditions outside are such that "complete closure" is required....I usually have all or some of the layers of window treatments working . Since the 3 sliders[along with a couple casements as well] were installed....our heating bills have declined. think of old traditional places in the world......in Britain with many cold windy days and SINGLE pain windows with leaks galore.....what do they do?.....custom made full length THICK LINED DRAPES.....pooling on the floor, no less.....Yes a lot of that was to keep out drafts. Get the window you want but don't expect much from cheap windows of any sort.

Shannon, skeptical - absolutely. Contemptuous - absolutely not! Oh and I love Consumer Reports and have had a subscription on and off for years but that is not to say that sometimes they don't leave out parts of testing that, while might not be important to the majority, may be interesting to others.

Everyone else so far - thanks you have had some constructive thoughts.

It is obvious the casements will seal better and so for the same glass area, glass type and covering, and vinyl frame area casements will likely perform better. What I am trying to do is get a feel for how much better?

I want to be able to decide is: if I keep my house for another 10 years, how much will I save in heating bills by using casements vs sliders? (I don't care about cooling - this place is never too hot for me.)

If someone can say it will likely be around 10% or 30% that gives me something I can relate back to my heating costs and then decide if the extra $200 or so I will spend for windows is worth it.

I will have to take a closer look at the U-factor and figure it out from there.

Ha, this site is quite useful:

https://www.efficientwindows.org

Wait, you asked about "vertical sliders". That is typically a DOUBLE HUNG.

You can look at the air infiltration numbers from the manufacturers to see which seals tightest. Sometimes hung windows out perform casements in laboratory settings.

Sorry millworkman, I meant horizontal. Left to right or visa versa. Though anything that applies to vertical would likely mostly apply to horizontal too.