HVAC question...or two

[hunterdan]

Too many post to quote right now but;
The foam plenum sounds like a good and inexpensive idea around the air handler
The sheetrock or sealed house wrap on interior sound like the best options for an interior air barrier

as far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

as far as air barrier If I silicone'd all the tongue and groove and used airtight can lights and exterior wall switches and receptacle's would that be enough?

By the way, I'm not a "Fuller" type guy we have just budgeted for this home down to the last nickel and anything over and above our pre-determined budget will leave us needing a loan. Not only do I not want a loan but I most likely could not get one due to the fact that I'm not employed. My credit is excellent but banks want collateral that I don't want to give or time on the job, tax returns. My current job is working on the home myself where I can to save money while spending it...

 

[rockcity]

Drywall is cheaper than the T&G wood, so you should be able to save a few $ if you decided to use it. It goes up pretty quick and it looks like your interior layout isn't super complicated so it would be fairly easy to install, finish, and paint.

 

[Fabrik8]

as far as air barrier If I silicone'd all the tongue and groove and used airtight can lights and exterior wall switches and receptacle's would that be enough?

Depends on what the inspector says. Probably not though, as the energy code requires an air barrier system and I wouldn't imagine that caulking the T&G joints would qualify as an air barrier system. I remembered seeing that because I was just looking up some insulation requirement stuff in the code before I saw this thread.

 

[kaiser715]

By the way, I'm not a "Fuller" type guy we have just budgeted for this home down to the last nickel and anything over and above our pre-determined budget will leave us needing a loan. Not only do I not want a loan but I most likely could not get one due to the fact that I'm not employed. My credit is excellent but banks want collateral that I don't want to give or time on the job, tax returns. My current job is working on the home myself where I can to save money while spending it...

I know about trying to stick to a budget....in our build, we have crossed more "red lines" than Obama.

Conversations typically went like this:

"We are at the absolute top of our budget. We cannot add one thing extra."
(The next day) "Electrician says it will be another $500 to put in those extra cans you really wanted." "Go ahead, it's only five hundred on top of what we already spent."
Repeat 2 days later.

Things have crept up like that....a few hundred bucks here, a few thousand there.

I added 10% to all the original estimates (land, house, shop combined), I have found that, at least on smaller projects, that's a good reserve, and you usually eat it up. A few things have come in under budget, some have been over (site work mostly). 15% might have been better.

What has surprised me most, and where I was furtherest off, was the budget for little stuff to finish out....decorating items, etc....a piece of furniture here and there, curtains and blinds, rugs, etc....that adds up fast, and right now we are at least 3 or 4 tiimes what I originally guessed.



In the boat and RV world, it's called "two-foot-itis". "We like this model, but the next one up is even better, two feet longer, and it's only 5k more.... Before you know it, you go from looking at 20' TT's to 40' fifth wheels.

 

[hunterdan]

Thanks for the advice; still need to figure out this question below

As far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

 

[Fabrik8]

You're on the right track. What you've identified is the insulation is the source of the problem. If you didn't insulate, there wouldn't be any risk of condensation. But clearly that's not an option either.

You've got the vents, so the air can move. The trick now is making sure that the batts you put in the rafters allow for a sufficient amount of air to move between those vents. If it were me, I'd hang 2x4s off straps below the structural rafters, put the batts between them, and then hang sheetrock from the 2x4s. The reason I say sheetrock is because it's a vapor permeable interior air barrier, t&g wood sheathing is not.

That's the way our house is built, and it's mostly cathedral ceilings. The rafters are furred down to make a deep enough insulation cavity. It's too bad the baffle and insulation installations are not exactly stellar. And we have a hipped roof, and I have no idea how they've handled the vent blocking at the hip junctions because I haven't seen that part... I'm not sure I want to see it, to be honest.

 

[Fabrik8]

So just so I'm clear, why can't he do a sealed attic with a vented roof? He can't do a sealed attic with an un-vented roof because the roof deck is already installed (continuous insulation,.etc), but why not a sealed attic with a vented roof?

Am I totally confused?

 

[Mac5005]

So just so I'm clear, why can't he do a sealed attic with a vented roof? He can't do a sealed attic with an un-vented roof because the roof deck is already installed (continuous insulation,.etc), but why not a sealed attic with a vented roof?

Am I totally confused?

That’s what @shawn was suggesting on the previous page with 2x4s below the rafters, filled with batt insulation, covered with gypsum wall board bc it’s vapor permeable but Air sealed.

That’s about the best case scenario at this point, yet I’m not convinced it still won’t prevent condensation on the underside of the roof sheathing during the winter when roof temps drop below the dew point, especially with the metal roof.

 

[rockcity]

Is the roof on already?

Maybe he could install insulated metal roof panels?

 

[hunterdan]

Metal roof is already on.
From what I can tell I need an air barrier either on the outside, inside or middle of the wall like sealed blue board sheets. I don't see anywhere that calls for inside and outside air barrier. The house wrap is supposed to be an air barrier that is vapor permeable. Why can't I tape all the wrap seams and where wrap folds into door and window frames? Then install self sealing tape 16" OC for siding install and call it good?

* Still looking for an answer to this;as far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

 

[shawn]

Still looking for an answer to this;as far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

Not touching it with a 10ft pole... Equally shitty options.

 

[Fabrik8]

* Still looking for an answer to this;as far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

If you're ultimately concerned about roof condensation, making the roof uninsulated on one part of the house won't solve condensation concerns on the rest of the house (the insulated roof areas not above the attic).

 

[Mac5005]

Metal roof is already on.
From what I can tell I need an air barrier either on the outside, inside or middle of the wall like sealed blue board sheets. I don't see anywhere that calls for inside and outside air barrier. The house wrap is supposed to be an air barrier that is vapor permeable. Why can't I tape all the wrap seams and where wrap folds into door and window frames? Then install self sealing tape 16" OC for siding install and call it good?

* Still looking for an answer to this;as far as air handler If I double insulate ducts and build foam plenum then could I skip insulating the vaulted roof where above attic and do like a member mentioned and insulate bed, bath, closet below attic? Would that eliminate condensation at least on that portion of the homes vaulted ceiling?

If I read this correctly you are mixing up two aspects.

Double insulating the ducts and a plenum around the air handler, is to mitigate heat loss/gain through the hvac system in a vented attic. It also helps to control condensation on the hvac system during the cooling season.

In the linked article, it states you can expect a 5-7% loss/gain through the hvac system in a vented attic with an air sealed duct system and Air tight ceiling/wall between vented attic and living space.

Essentially your “continuous” ceiling insulation plane would extended up and over the duct system and hvac unit, so that in terms of temperatures, it’s in the conditioned space.

That still means Air sealing the walls/ceiling between the conditioned space and the vented attic space.

Think of that aspect as being completely separate from condensation on the underside of the roof sheathing during the time of the year that the sheathing temp drops below the dew point temperature.

***I’m not fluent enough with the code in reference to where an how much insulation is required to make the inspector happy, as well as how the insulated plenum around the hvac unit affects the requirement for access to a piece of equipment.***

I’m just pointing out that is a creative way to have the hvac and duct system in a “conditioned” area while inside a vented attic.

Choosing where, how much, and type of insulation depends on where the difference of temperature is desired to occur and where you want to condensing surface to occur.

Then it’s a matter of vapor permeable Air barrier that allows some drying capacity of the condensation while limiting the Air exchange to reduce the amount of condensation.

 

[hunterdan]

If I read this correctly you are mixing up two aspects.

Double insulating the ducts and a plenum around the air handler, is to mitigate heat loss/gain through the hvac system in a vented attic. It also helps to control condensation on the hvac system during the cooling season.

In the linked article, it states you can expect a 5-7% loss/gain through the hvac system in a vented attic with an air sealed duct system and Air tight ceiling/wall between vented attic and living space.

Essentially your “continuous” ceiling insulation plane would extended up and over the duct system and hvac unit, so that in terms of temperatures, it’s in the conditioned space.

That still means Air sealing the walls/ceiling between the conditioned space and the vented attic space.

Think of that aspect as being completely separate from condensation on the underside of the roof sheathing during the time of the year that the sheathing temp drops below the dew point temperature.

***I’m not fluent enough with the code in reference to where an how much insulation is required to make the inspector happy, as well as how the insulated plenum around the hvac unit affects the requirement for access to a piece of equipment.***

I’m just pointing out that is a creative way to have the hvac and duct system in a “conditioned” area while inside a vented attic.

Choosing where, how much, and type of insulation depends on where the difference of temperature is desired to occur and where you want to condensing surface to occur.

Then it’s a matter of vapor permeable Air barrier that allows some drying capacity of the condensation while limiting the Air exchange to reduce the amount of condensation.

Thanks but I'll need some time to absorb that

 

[shawn]

https://buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf

More things to share later, but haven't had time to put them into a coherent thought. That article covers 99pct of it, though.

 

[hunterdan]

Thanks for the link. Still trying to figure out how one of the designs in the link can fit my situation with ducts and air handler in attic. Here are some new pics of my attic and framing to better show what I'm working with...
rafters are 2x12 (actual 1.5 x 11.25) Looks like I need at least 2" from eve to ridge of air space via baffles. Thinnest R-38 I can find is 10". Maybe I could compress it enough to fit baffles (2") and insulation (10")?

 

[Fabrik8]

There is a design exception to allow R-30 in a cathedral ceiling that doesn't have sufficient space for R-38, but you're limited to 500 sqft. I'm not sure your situation applies. The normal thing to do is to furr down the bottom of the rafters to provide enough depth for the proper insulation.

You won't find much variety in R-38 or really any insulation, as most is meant for a common R-value in a common dimensional lumber depth. Canada has some higher density, but it's apparently hard to find here (from what I hear; I haven't tried).

But, if you want to go with mineral wool, you can use a R-15 3.5" batt, and a R-23 5.5" batt, so you'd be at 9 inches for an R-38 rating.. It's a friction fit, so it holds itself in place and is an absolute joy to work with compared to floppy fiberglass. It's like sticking Lego blocks of felt-like stuff between the studs. But, it's not cheap, and it's not easy to find because relatively few people use it.

Also, don't forget proper rim joist air sealing and insulation, which is important if you're doing a conditioned attic..

 

[hunterdan]

There is a design exception to allow R-30 in a cathedral ceiling that doesn't have sufficient space for R-38, but you're limited to 500 sqft. I'm not sure your situation applies. The normal thing to do is to furr down the bottom of the rafters to provide enough depth for the proper insulation.

Also, don't forget proper rim joist air sealing and insulation, which is important if you're doing a conditioned attic..

Seems like instead of furring out all the rafters I could compress the 10" insulation 3/4- 1" and baffles & insulation would fit in a 11-1/4 " rafter...

 

[Fabrik8]

Seems like instead of furring out all the rafters I could compress the 10" insulation 3/4- 1" and baffles & insulation would fit in a 11-1/4 " rafter...

If you compress the insulation below the nominal rated thickness, it will lose its insulation rating. It works by trapping air, and if you compress it then you lose that air volume. I'm honestly not sure if that will cause an automatic inspection fail, but I would think it would if the rated R-38 insulation is installed in a smaller cavity than it should be. Of course, if the baffles are not visible than it's hard to tell how thick they are, but I'm not a building pro and I'm not an inspector so I'm not touching that.

All the cathedral ceiling rafters in my house are furred down with a 2x2 (2x4 ripped in half, actual 1.5x1.75) to provide that extra insulation room.. I've got 2x10 rafters though, and the house is all R-30 ceilings.


I've been using this on my house for the small flat ceiling sections, with an additional layer of R-15 that I got really cheap (like $10/bag on clearance from a returned special order).

You'll never use fiberglass again after installing that, seriously. The R-30 is about $1.50/sqft in small quantities though....downside. Each of those 30sqft packs is roughly $45.
For the keen-eyed, I got the 24" because the rafter tie spacing is more than 16" in some places, so I have to rip it down to size with my insulation hand saw. That's why there's 24 and 16 inch widths.

 

[Mac5005]

Seems like instead of furring out all the rafters I could compress the 10" insulation 3/4- 1" and baffles & insulation would fit in a 11-1/4 " rafter...

Can’t do that. It loses the rated r value when compressed more than its nominal rated thickness

As far as the attic area, I think you should just build knee walls on either side of the hvac unit, insulate/Sheetrock these walls so that the hvac unit is in a conditioned space. Just make it large enough to meet code for entry and service to equipment. Solves the plenum issue, and keeps the unit from being in a vented attic.

As well I would still at minimum double insulate the ducts and boots. Good time to make sure all holes between living space and attic space are 100% sealed. Light fixtures, receptacles, wire holes, around the duct boots to the wall surfaces etc.

Then it’s just furring strips on the cathedral ceiling to get adequate continuous positive pressure venting, with adequate r value. Still need a vapor permeable air barrier. We have recommended Sheetrock, but you are against that.

You will have to find two products to replace the one. Most likely and acceptable taped and sealed barrier behind your wood paneling.

Somewhat a combination of these two.

Disclaimer, all of the suggestions I made above need to meet the Nc building code, and any applicable local code.

 

[shawn]

Here's a quick rundown of the assumptions I'm working from:

1. Rafters are 2x10" nominal (9.25" actual)
2. Installed roof is metal on an OSB substrate
3. R38 fiberglass batts are intended for the ceiling/roof to comply with the prescriptive requirements in NCRC.
4. There is an air barrier on the exterior of the walls, but no air barrier has been installed to date on the roof or in the soffits.
5. The soffits are/will be vented, the roof has a ridge vent.
6. The roof shape is a simple gable.
7. The air handler has been installed in the ceiling above a portion of the living space.
8. Owens Corning R38 batts are 12" tall installed.
9. The original design intent was to install continuous rafter baffles from the soffit to the ridge, place batt insulation below the baffles, and then install T&G pine boards as the finished interior surface.
10. Rafter baffles vary in height, but 2" tall is a reasonable assumption.

Just based on those 10 things, the math doesn't work. You need the equivalent of a 2x6" framed under the existing rafters just to get enough cavity space for the baffles (2") and insulation (12"). You could get the same height with a 2x4" hung from strapping, but that's neither here nor there.

You could substitute the baffles for 3" of XPS in lieu of the 2" baffles in an unvented roof assembly per Joseph Lsitburek's calculations in the PDF I posted earlier, but it appears you're too late in the construction phase to make this change (roof is already installed).

If you can get baffles that are at least 2" deep and satisfy the free air requirements in your roof venting calculations (bottom of p69 in the link), then you could install baffles continuously in each rafter space, then batts below (between 2x4s), and GWB. Field-constructed baffles constructed out of XPS (as shown in the PDF) are attractive in lieu of prefab baffles, since they'll allow you to customize the free air space, and might work as an exterior air barrier. You'll need an air barrier in the soffit regardless to prevent wind washing of the insulation. XPS would work well for that, sealed in place with expanding foam.

Clear as mud?

 

[shawn]

See how the blue board (probably XPS) separates the batt insulation from the air moving through the soffit? That's necessary to keep wind washing from the batts. If you put the angled piece (baffle) in place without the vertical piece, the end of the batts are exposed to the air blowing into the soffit, the cold air moves through the batts, and pulls heat out.

 

[hunterdan]

Here's a quick rundown of the assumptions I'm working from:

1. Rafters are 2x10" nominal (9.25" actual) 2 x 12 (actual 11.25")
2. Installed roof is metal on an OSB substrate yes with that fancy roof wrap
3. R38 fiberglass batts are intended for the ceiling/roof to comply with the prescriptive requirements in NCRC.
4. There is an air barrier on the exterior of the walls, but no air barrier has been installed to date on the roof or in the soffits. roof has been wrapped but not soffit. I see what you mean anout batt ends and the blue board in the diagram. I will either do taht or use what's in my digram transitioning to a 2" baffle.
5. The soffits are/will be vented, the roof has a ridge vent. They are both vented
6. The roof shape is a simple gable.
7. The air handler has been installed in the ceiling above a portion of the living space.
8. Owens Corning R38 batts are 12" tall installed. I found some that is 10-10.25 R-38
9. The original design intent was to install continuous rafter baffles from the soffit to the ridge, place batt insulation below the baffles, and then install T&G pine boards as the finished interior surface. thinking now about doing sheetrock ceilings with tight corrugated sheet metal over it like in steak hoses
10. Rafter baffles vary in height, but 2" tall is a reasonable assumption.

Just based on those 10 things, the math doesn't work. You need the equivalent of a 2x6" framed under the existing rafters just to get enough cavity space for the baffles (2") and insulation (12"). You could get the same height with a 2x4" hung from strapping, but that's neither here nor there.

You could substitute the baffles for 3" of XPS in lieu of the 2" baffles in an unvented roof assembly per Joseph Lsitburek's calculations in the PDF I posted earlier, but it appears you're too late in the construction phase to make this change (roof is already installed).

If you can get baffles that are at least 2" deep and satisfy the free air requirements in your roof venting calculations (bottom of p69 in the link), then you could install baffles continuously in each rafter space, then batts below (between 2x4s), and GWB. Field-constructed baffles constructed out of XPS (as shown in the PDF) are attractive in lieu of prefab baffles, since they'll allow you to customize the free air space, and might work as an exterior air barrier. You'll need an air barrier in the soffit regardless to prevent wind washing of the insulation. XPS would work well for that, sealed in place with expanding foam.

Clear as mud?

Also talked to the inspector today about a number of things. I asked about air barrier and he said my wrap on walls and roof will suffice.Also asked about T&G on walls and he said my idea of caulking all the joints between runs and around wall perimeters would be ok especially if I was going to laquer the walls.

looks like nailing a 2x4 directly to the 2x12 rafter would give me 12-3/4". baffles (2") + insulation (10.25") equals 12.25"

 

[kaiser715]

Remember that the inspector is an expert on the code book, not energy efficiency, moisture, building science, etc. "To code" does not necessarily mean "the best way".

 

[shawn]

Remember that the inspector is an expert on the code book

If only....