Ventilation Ducts in Wood Floor Trusses

[setexas]

Hello,

I am building my own house in SE Texas and wanted to
ask if anyone has any concerns about the HVAC Ducts I am
installing:

House is a two-story on a slab. I am having my home
insulated with icynene so it will have a very strong
moisture barrier.

The 1st floor ceiling/2nd floor is 18" open wood trusses.
The airhandler will be on the first floor - floor and
the ducts will be distributed thru the open wood
trusses.

The ducts will be made out of 1.5" ductboard on 3 sides
and use the plywood floor for the 4th side. None of
the 4 sides will be next to an UNconditioned space.

Is this OK?

If not, why & what solution do you recommend?

[rimek]

Both supplies and returns will have one side as plywood? Not good for either- what will happen to air when plywood outgasses in heated and cooled airpath? What will happen to plywood when surface is exposed to changes in temp/humidity?
Was this recommended by a pro?
Is this your house?
Why don't you want to use regular 4-sided ductwork?
I am partial to steel, but any closed system would be better than what you are planning.
If you can't afford to do it right, how can you afford to do it over?

[setexas]

supply will only have one side plywood.

WHAT HAPPENS to air when plywood outgasses in heated and cooled airpath? (Only Cooled to 60F & Heated to 85F)

WHAT HAPPENS to plywood when surface is exposed to changes in temp/humidity? (Only Cooled to 60F & Heated to 85F)

[uktra]

All ducts in HVAC need to be hard ducted. Using floor joist cavities in any manner will cause leaks to the outside of the home.

[binford]

Running your duct work inside condition space is great! Best thing you could ever do. You’re not breaking any code concerns. The issue would be warm moist air from the outside getting near the cool air condition air, water will condense and mold will grow on wood, as wood is food for mold. Duct board is ok, but there are advantages to steel, like it can be cleaned later on with out a concern for damage. To bad they don’t make duct board with foil on both sides.

[setexas]

Under normal conditions there will not be any way for warm moist air to come in contact with these ducts. None of the four sides will be in contact with any exterior wall - All four sides will be surrounded by conditioned space.

Without warm moist air coming into contact with these ducts, does anyone see negatives with the air coming in contact with the plywood or 2x4's in the wood trusses?

[41gasman]

Setexas.
You will probibly sart to get some pretty
opinionated responses here.But to
some this is a pretty silly idea.

First, in my neck of the world this
plan will not pass code.I can tell you
the inspectors around here would be
smirking,and breathing funny through
their nose.As they wrote me up with
a really big RED pen in one hand
and their cell phone in the other.
Cause he would just have to get ahold
of the other inspectors for the
"YOU JUST GOT TO SEE THIS"
conference call.

I can appreciate a solid "CAN DO"
attitude.One of coolist parts of
thisforum is that homeowners can ask
questions and learn from people
who have been at this for a long
time.WE all like to help the good
folks out ther in internet land.
What you need to understand is
that based on your question.It
IS fair to say that you have a lot
to learn before you make the choice
to install this system your self!

If you can put your ego in the drawer
for a while.Think about what things
were brought up right off the bat
in response to your question.
You would not come back with
WHAT WOULD THAT HAVE TO DO
WITH IT.You came here to learn.
Not to badger the people who
bring a life time of experience
to this forum for free!!!!!!!!!
So sit back think and type back
that you can see why "X" would
be a problem.But I don't under
stand how 'Y' would be a problem.
Remember you only want to pay
for your system once.Good luck
to you.And I hope we can help
steer you to a good contractor.
41Gasman

[setexas]

41Gasman, I did come here to learn, I am looking for facts & logical reasoning, not one-line statements. I think binford is the only one that read ALL of my initial post.

To help my education, could you answer at least my question number #1?

1. Why will this not pass code? Where is your neck of the world?

2. Exactly what statement did I make that caused you to think that I had a LOT to learn? I asked my original question because even thought it is OK according to IRBC 2003, there may be issues the code people may not have considered.

3. I was NOT asking "what does offgassing got to do with it?" but rather "What is offgasing from plywood, and is it really a concern over the temp range we are talking about?

4. I apologize to everyone that I unintentionally badgered.

[qssfl]

Sounds like you know ALOT about this proposed duct system...maybe even more than some of us here...

From what you have described... it sounds like a wonderful idea to me....

You came here for opinions from people in the hvac trade...

You got some...

Let us know how it all works out...

Good luck :)

[Gunnery Sergeant Hartman]

Originally posted by setexas
41Gasman, I did come here to learn, I am looking for facts & logical reasoning, not one-line statements. I think binford is the only one that read ALL of my initial post.

To help my education, could you answer at least my question number #1?

1. Why will this not pass code? Where is your neck of the world?

2. Exactly what statement did I make that caused you to think that I had a LOT to learn? I asked my original question because even thought it is OK according to IRBC 2003, there may be issues the code people may not have considered.

3. I was NOT asking "what does offgassing got to do with it?" but rather "What is offgasing from plywood, and is it really a concern over the temp range we are talking about?

4. I apologize to everyone that I unintentionally badgered.

1. Because it is in the National Code.

2. How are you sizing this ductwork? (what method)
It sounds like you are a DIY'er...

[se_texas]

thank you.

under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

Does the National code not allow this?

[tinner73]

if you are using the second story floor as the top of the duct you are gonna have a hot/cold spot on the floor material. i have never heard of an idea such as yours. how are you sizing this? did you contact the building department in your area? they are the ones that have to be satisfied for occupancy. that would be my first call.

[Gunnery Sergeant Hartman]

My last thought is:

If you are builing your own house, why do it half assed?

Have your ductwork sized properly. Also use sheetmetal. (all 4 sides)

[supremehvac]

It is hard to say if what you are doing is ok. We need to hear more details about how you intend to construct this duct system. Do you intend on just enclosing the three sides of the 18" trusses with ductboard, cut out your openings for the registers, and direct all the air back to the furnace. If this is the case, and it is all that you have specified so far, then no it won't work. Because the air ducts need to be sized properly to get the correct air volume and velocity to each room. This is calculated by heat loss and heat gain factors. I have worked on condos that didn't use an enclosed duct system and even on a small scale the performance of the system was very poor. Of course now that it's done to change it to a ducted system would be very expensive. So the tennants have to deal with low levels of comfort, mostly in the A/C season. If you do not size the ducts or enclosurer the system will not even come close to working right. So now that you know the ducts need to be sized, is it worth your time to mess around cutting and securing ductboard to the plywood? I have installed many systems and I just don't see how what you intend on doing could work and/or be worth the time.

[se_texas]

thank you.

under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

Does the National code not allow this?

[supremehvac]

Your welcome.

To answer your question, yes, use of under-floor space as supply plenum is ok, as long as you follow code when making the installation.

[tinner73]

take pics and post.

[se_texas]

thank you.

under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

Does the National code not allow this?

[se_texas]

sorry for the triple post. That's what I get for using my cell phone to check this website.

Thanks for everyones advice, lots a work todo.

[Gunnery Sergeant Hartman]

If your A/C doesn't maintain a RH below 50%, expect growth problem immediatly in the near future.

[Toolpusher]

Be sure to install a drop ceiling on the 1st level.
This way when your system falls apart, you can go beck in with metal and install a reducing trunk line system.

1 question, Why would you consider this ?
The cost of the 4th side is less then nothing compared to the cost of having the duct come apart.
Which if it does you not only just lost air to the area that came apart, you lost it to the whole system.
And not knowiing where it seperated, your looking at removing the ceiling.
Besides, with duct board you'll loose 4" of valiable duct space. Well 3" your way.

[qssfl]

The idea of using the floor joists is similar to the way they duct out mobile homes.... With no reducing trunk line... there's lot's of air at the begining of the duct and hardly any at the end...

Doing nothing to account for static pressure loss=big mistake...

It's your house...If you think there's lots todo now... wait till later when you see how well it works....

Lots more todo later to rectify the problem....

Like I said.... "Good Luck"......you the man :)

[binford]

I read right through supply air. Look up UMC 602.1
stud/partition cavities not OK for supply air.
NCBolton is correct you would get mold if above 50% R.H. The supply air is very cold and water condenses, this help to lower the R.H. of your home. Fire code also comes in play as flame spread of your ducts also need to conform.
If it were my house I would use sheet metal, but duct board seems to be taking over. I would not want fiberglass duct board coming apart and blowing around in my house. Duct cleaning companies here offer to spray the inside of the duct board to protect it. Also if you add a UV light that could break down the duct board. The Sheet metal guy I use is very knowledgeable with air flow. He makes what I ask for and offers ideas too. I only have a small brake so it easier to get what I need made.

[se_texas]

thanks binford,

I misunderstood "supply" air for combustion air.