I say leave that screen in place. The damper at the hood may help keep critters from coming directly into the house, but you really want to keep them out of the vent turbing and the T-top, as well. When you say "t-vent," I'm picturing something that looks like a roof jack for an attic vent with the opening and screening on the underside of the outer arms to the cross-bar on the tee. Around here, wasps & yellow jackets would be the biggest worry -- their nests could plug up your vent. Nasty to try to clean out. As for attaching the tubing, metal vent tubing comes in sections that plug together -- a slightly crimped end plugs into a slightly expanded end of the next section. The roof connection will be the same -- the top end slides inside the base of the t-vent. (May be a mite tricky getting it all to snap together with the roof unit already in place.) A couple of self-tapping screws through the vertical sides of the tee into the top ring of the tubing would be good if you can do that. The t vent may have screw holes already. Caulk would be okay -- but probably not ordinary vinyl painter's caulk because your local government code people might require a fire-rated caulk as you would with a chimney for a wood stove. It is readily available, just more expensive and more durable. Regardless, you also want to wrap and seal each duct joint with duct tape. (That's actually what duct tape is for.) For this application, you really want to use the aluminized tape instead of the "handyman's secret weapon" --- the common vinyl-cloth duct tape. The vinyl-cloth stuff just won't hold up very well in temperature swings you get in the attic and on the roof. If you have hot summers, that attic and underside of the roof will get very hot. Cloth duct tape softens and degrades in high heat. It also loses its grip in very cold weather. If you live in a place with cold winters, you also probably want to consider adding a second damper up by your t-vent. That mitgates the chilling effect of a cold column of air above your stove vent, particularly on windy days. This post was edited by JWVideo on Tue, Jan 28, 14 at 12:35

That is irresponsible of your builder and is a warning sign about their knowledge, quality and integrity. Reduction from 8" to 6" alone is a problem. It will reduce air flow and so the effectiveness of your hood and it will increase hood noise as well as the amount of electricity used. So increased cost, more noise, less odor, gas combustion by-products and other effluent removal. Flex duct creates two problems; It's a major fire hazard and it increases static pressure (reduces air flow). From a static pressure standpoint 6" flex is roughly the same as 4" rigid in most installations. At a MINIMUM you should have 8" rigid for your entire run if that is what your hood calls for (Many US hoods undersize this). If you have many bends then you should increase that to 10" to keep static pressure down.

I'm not sure I understand the intent of some of the comments. As I stated in the original post we blow hot air out of the upstairs windows while simultaneously opening all the downstairs windows to allow cool air to replace it. This method fills the house with cool air from the bottom up in the early morning when the outside air is coolest. I considered a whole house fan as we installed this in our last house and it worked quite well. Same principle - open all the windows downstairs while the fan blows the hot air out. I'm not sure how this got turned into an issue with not getting enough fresh air into our homes - we live in a mild climate and open windows to bring in fresh air all the time. This house has blown in insulation in the attic that is very light and is easily disturbed so I fear that an unducted whole-house fan would eventually cause the insulation to clog the attic vents. I suppose a ducted whole-house fan would work and I understand the reasoning behind separating functions. Since it seldom gets really hot we typically only do this for a small percentage of the days during the summer and it seemed to only have to run for about an hour so I thought it might be a good case for dual purpose. I just thought there might be some clever solution that could accomplish this without a lot of effort and expense. Thanks for the input so far.

Yes, I meant 8-inch will be better, in general. Grease collection at various points is a complex matter. For example, one reason simmering bacon, in spite of its low upward plume velocity, needs a higher than minimum air flow is that the baffle centrifugal extraction process needs enough velocity to work. In general, however, hotter cooking generates faster plumes and needs higher hood intake air velocities. If the duct is warm enough and the flow rate not too low, the grease collection via condensation will be low. If the velocity in the duct is not too high, the grease collection via impingement will be low. Most home cooking levels are such that what does collect in the duct rancidifies (hardens to a varnish) and doesn't become a nuisance or fire hazard. Commercial ducts, however, need yearly cleaning and inspection. If performance is your priority, then review of hood threads here will provide insights into the requirements needed to achieve good capture and containment.