Do not use flex duct, It clogs It's Dangerous and will choke your dryer!
Start your system out right, if you can't reach the vent to attach metal pipe use a bracket it's the best dryer vent solution available!
1) Locks dryer in place so the metal duct you use will not move around and fail! You will secure the bracket to the floor or the top of your washer or pedestal. I use a bracket almost always just because of the fact it locks everything in place.
2) Makes servicing easier and eliminates transition duct.
Let's talk a little about transition duct, so we can better understand its role in the operation of your dryer. It seems that the only reason it even exists was to create a way for a dryer to be connected easily and therefore not requiring better planning. Its functionality in a system that requires good airflow for efficient operation with wet lent being a major player seems irresponsible, a lack of a better method could be its only rational. There are codes for its use and that code states a four-foot maximum length may be used to connect the dryer to a code complaint system [hard metal duct]. So it is also misleading being its sold in much longer lengths, that's probably why I see installations with ten or more feet not uncommon.
Considering this information you can see where a challenge might be. It's trying to correct a mentality, that this easier method works just fine! Well in most cases it doesn't! With clothes dryers, it's apparent that there is a large risk of fire and even bigger loss of energy. This is where I would ask of professionals to help aid me in correcting this wrong. Adding credibility by either sharing your experience are your understanding of what I'm proposing for this new method. I propose that transition duct is eliminated and that proper planning be made and not compromised and when conflicts occur they be addressed, planned out, and corrected. It's all too apparent that an anything goes attitude has been the norm in this industry.... leaving many new homeowners with poorly designed systems all too often going unrealized do to the fact that you can't distinguish its inefficiency in the energy bill and the system seem's to work. I would suggest that the powers both in fire protection and consumer product testing look to correct this soon.
The reason I believe most installations are improperly installed fall into three category's
1) Poor Planning in the construction period
2) Lack of a viable method to achieve proper connection
3) Matrix and bottom line
A quick overview of my career. I feel that it needs to be stated, I started out with a four-year enlistment in the US Navy in an engineering and propulsion occupation, then honorably discharge. After that, I started a thirty-year career in electrical contracting achieving my master's license in 1991 and running an electrical contracting business since
In the earlier years of my contracting I learned of the importance of correctly installing the vent system, we're as most of my competitive companies would shun off the job, I took a different approach. I mastered some Techniques, wouldn't compromise, was fair with cost, came up with solutions and made a profit doing them. In all fairness, I would have to admit though most of my clients and projects or in the higher end market, leaving it easier to sell these measures of safety and efficiency.
I would like to share my experience with transition duct. There are several types some better than others. Some even have the UL listing but all fall short of the safe functional properties of hard metal duct. Their ribbed construction makes it impossible to not collect lint and therefore reducing airflow. Their only redeeming quality is as stated above, their so easy to install! and in every installation that I would use them, they would fail. Today I very rarely use them and all my systems work without ever needing to be cleaned
When a new code is passed we scramble, we complain, then we adapt and master techniques to facilitate that code, usually theirs an advancement in safety so we all follow. Cost eventually come down and soon it becomes the way. My point is we shouldn't have to sell this measure of "safety" it should be code.