Spray-Foamed vs. Hybrid Roof Assembly
oldbungalow
| Posted in Energy Efficiency and Durability on
I’ve tried to read through all the vented/unvented attic articles and the BSC pieces. Still trying to understand pros/cons of closed-cell foaming of roof-deck bottom, or something cobbled together like the attached image (also posted to a related ceiling insulation article).
There was an article some time back on “cut and cobble” that mentioned this approach as well (not the article but the comments). The only addition in this diagram is retaining the existing attic ceiling layer to reduce the R level at the upper truss.
If cost were the same is fully sealing and foaming the roof deck better or hybridizing with this approach?
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All other things being equal, including cost and total effective R value, either approach is fine. Other considerations: if you're in wildfire or tornado or hurricane territory the unvented approach can arguably be a bit safer. In deep snow country you'll still want the vented "hybrid" approach to protect against ice dams.
Seems to me your time and money would be better spent undoing the foolish choice someone made of putting the HVAC system in the attic.
Any way you slice it a conditioned attic is a bad idea that is only slightly better that the horrible of putting HVAC in a vented attic.
The conditioned attic has 30-50% more surface area that will be losing heat and must be insulated with very expensive and ungreen foam insulation.
To me the “hybrid attic” is a game of Russian roulette. It seems inevitable the surface temp of foam insulation will sometimes fall below the dew point of the air in the attic and condense water. How much water, how often, what will get wet, will it stay wet lone enough for mold to grow are all unknowable questions.
Walta
"It seems inevitable the surface temp of foam insulation will sometimes fall below the dew point of the air in the attic and condense water"
Highly unlikely. That's the whole point of conditioning the attic-- so the air temp and humidity closely tracks the rest of the conditioned space, where no condensation occurs under normal circumstances.
But you are only partly conditioning the attic with the hybrid plan, which sets up different and uncertain dynamics. It would be helpful to know what climate zone you are in since you have provided insulation numbers that suggest it gets fairly cold. You will be losing some heat just by having the HVAC system on the cooler side.
Offhand I would say it looks OK, but a lot depends on how well the details are handled
The hybrid assemblies don't dictate whether it's partially or fully conditioned. The HVAC system balancing does that, assuming dedicated supply and return are given to the attic space which it should be. The attic is effectively its own zone if insulation is in the floor.
“Highly unlikely. That's the whole point of conditioning the attic-- so the air temp and humidity closely tracks the rest of the conditioned space, where no condensation occurs under normal circumstances.”
I would argue what is drawn in not a “fully conditioned attic” mostly because it is labeled “Hybrid”.
If the plan was to heat and cool the attic so it was more or less the same as the rest of the conditioned spaces, there would be no insulation as shown on the attic floor. The presents of the insulation on the floor means they intend for the attic to be much different temperature than the house.
It seems unlikely that anyone is going to willingly swallow the bitter pill of paying to condition an empty attic.
Walta
I totally agree, but the way to do that is changing the code so builders stop doing it that way, or successfully working with the big builders like Weyerhauser to voluntarily adopt newer practices. Trying to move ducts to the interior or replacing systems with ductless solutions in existing housing is very expensive. When you look at housing stock in this country most of it is pre 1990s, half pre 1950s where attic HVAC is the norm, so moving the bar on EE and climate is something that requires low(er) cost solutions and retrofits and broader adaptability. Not everyone has the means (without significant subsidies) to re-architect homes to absorb ductwork and air handling equipment inside or replace ducted with unducted. Extending or improving the attic envelope seems like the easier way to go (IMHO).
You're fine using cut and cobble rigid foam like your propose, because you will be building a VENTED roof assembly. If you wanted to build an UNvented assembly, then you'd need closed cell spray foam. The issue with cut and cobble is that it inevitably leaks, and then you have moisture getting to the underside of the sheathing which causes problems. If you have a vent channel above the cut and cobble insulation though, the air in that vent channel will carry away any moisture that leaks through and you won't have any problems.
I do agree with the other posters that with a partially conditioned attic space you can't really count on the same performance you'd get with a fully conditioned attic space. I would recommend you try to fully condition your attic space, then remove the insulation between that attic space and the living areas. This way you're moving the building envelope out to the roof in that attic area, so the space inside that attic will be fully conditioned. If you go this route, you need full R value roof insulation though so keep that in mind.
Bill
" then remove the insulation between that attic space and the living areas."
Is the remaining insulation superfluous or does it cause problems, i.e. doesn't code usually include insulation between floors (could be mistaken), like between conditioned basement and 1st floor?
"you need full R value roof insulation "
Seems like you need about 7-8" of foam to hit R48.
Code does not require insulation in walls/ceilings between CONDITIONED spaces. Sometimes insulation is added for sound control, but it's not required. In your specific case, the insulation in the attic floor will act to reduce the temperature in the to-be-conditioned attic space, which would be a problem. You could also solve the problem by putting air vents in the attic space instead of removing the insulation. What you need to do is tie the attic space into the conditioned space so that all of the areas inside the building envelope have approximately the same temperature and humidity levels.
Code is currently R49 for attic insulation. That's about 7.5" of polyiso.
Bill
While I understand this in theory, isn't expanding the conditioned envelope in this way actually adding load?
Say you have a 1,000 sqft house with 9 ft ceilings, 9,000 cubic feet of conditioned space with an unconditioned, vented attic with similar footprint but less volume, maybe 4-5,000 cubic feet.
In scenario 1 you have HVAC equipment and ducts in the unconditioned space, and you're losing potentially 20% of your HVAC energy due to duct losses and cold space.
In scenario 2, you fix this by conditioning the 5,000 cubic feet above and "tie" it into the space below, but that's potentially a 50% increase in load?
In addition, you have an HVAC system that is sized for scenario 1, but adding supply and return in scenario 2 isn't sized in.
I know I'm missing something but feel like I'm not getting it.
I was thinking that by insulating the floor of the attic and isolating it from the conditioned space, it's "semi" conditioned (through duct leakage, whatever) and sheltered from the outside without necessarily requiring additional energy to be conditioned. It may not match the conditioned environment below, but the HVAC will be far happier than in completely vented/unconditioned space. Maybe you drop the 20% attic HVAC loss down to 5-10% (just making up numbers).
I suppose if the attic is useable and easily accessible space the tradeoff makes sense, but if it's just empty storage through an inconvenient hatch or pulldown, seems a shame to condition it.
Energy loss in heating season (it's energy gain in cooling season, but the same process, just in reverse) is primarily related to surface area of the building envelope, not the enclosed volume within it.
What this means is that if you were to "bump out" a building envelope from something like this: __| to something like this: |'''' you end up enclosing more volume, but you don't really change the area of the surface that encloses that volume since the area of the wall and ceiling are about the same, but in slightly different locations. Once you've heated the enclosed air volume to a given temperature, it doesn't take any more energy to keep it there over time -- there is no additional energy loss to make up from the volume itself. What DOES loose energy is the perimeter of the enclosed volume that radiates heat to the outdoors, and air leaks through that perimeter.
What this all comes down to is that adding enclosed volume doesn't add any heating or cooling load to the house. What DOES add heating or cooling load is adding additional WALL/CEILING AREA, since that's what the heat has to go "through" to be lost, and the heating system has to make up for (replace) that lost thermal energy.
Hopefully my explanation makes sense.
Bill
Ah, thanks for the explanation I get it now.
So conditioning an interior closet or something has no penalty. I guess I can follow this but attic volume does come with additional surface area.
Depending on the pitch of the roof the upper surface area of the pitched roof is somewhat larger than the flat below-attic ceiling. And the gable ends would have surface area, i.e. 2 triangles.
So kind of like going from this-
__
| |
to this-
/\
---
| |
The triangular gable ends are maybe 1/2 the surface area of a regular level but additional nevertheless.
"moisture getting to the underside of the sheathing"
For closed cell on the underside, some articles mentioned the possibility solar-driven vapor drive from roof to underside, or is this not possible? i.e. can moisture above the roof deck get driven in between the bottom of the deck and the closed cell, causing rot? What about nail holes?
You shouldn't get much moisture from the roof side unless you have a leak, and the nail holes basically "self seal" as they punch through the roofing materials. I wouldn't say moisture drive from above is "not possible", but it's not a big moisture source here. It's true that you create a bit of a moisture trap this way, but it's done all the time in this paricular situation without much issue.
Bill
makes sense! thank you.
So, what happens to underside-foamed roofs during shingle replacement? Presumably nails are removed/loosened. I guess if this were a problem one would hear about it given all the foamed attics out there.
By vapor drive from above I was thinking after a heavy rain on a humid/sunny day.
I have a 2300 sf 1906 build in upstate NY. Finished off half of the attic and opened a loft into it. Newer roof and decking, 2x6 rafters. I sprayed closed cell about 3 years ago to the entire roof deck and gable walls. Floors 1-3 are not about 2800 sf conditioned area. It’s 10 out today, feels like -5. The unconditioned side of the attic with a 12:12 pitch has about a 13’ peak that’s separated by a 2x6 R19 wall with an exterior door installed and R19 batts in the attic floor. The cold side is about 15 degrees cooler than the rest of the house but still a 50 degree difference from outside right now. I can see the roof framing layout when it warms up outside from thermal bridging, but am very pleased with the decision. Structural rigidity on a tall house that occasionally creaks with high wind was a plus. Not sure what info you’re looking for though. I maintain the RH around 35-40% year round, and plan installing a layer of rigid to the roof deck in the future with an ERV installed in this area. Ice dams are a thing of the past finally. Eventually we will be finishing this portion off as well and likely introducing a mini split to serve the attic. In my experience, r values are less applicable than u factors as it is a generalized concept. Wet r30 or high winds/leakage aren’t r30 whereas 4” of closed cell (soy based in my case for the green comment earlier) is still 4” and impermeable
Just to clarify, are you describing a fully foamed attic but only partially conditioned, with the unconditioned section insulated on the bottom as well, between attic and interior of home?
I think that is closer to what I was proposing above, essentially an enclosed attic space that is insulated and isolated from the elements but not directly conditioned. What is the R-value of the foamed attic roof?
KT - I believe this was aimed towards me…fully foamed attic, yes. Walls are ~3”, deck is 4-5”. Closed cell soy based product so r-values are slightly less. Floors within the unconditioned space are R-19 friction fit from 2nd floor before new ceilings went in. Unvented roof assembly. Sealed ridge vent and encapsulated eaves from fascia to top plate. There is a 2x6 wall with an exterior door that divides the loft from the unfinished side. Installing an ERV on unfinished side soon which is how I found this forum.
“What this means is that if you were to "bump out" a building envelope from something like this: __| to something like this: |'''' you end up enclosing more volume, but you don't really change the area of the surface that encloses that volume since the area of the wall and ceiling are about the same, but in slightly different locations.”
Bill let’s do the math for 1000 squ 20x50 roof with a 12-12 pitch.
The ceiling has an area 1000 sqf the face of the roof has an area of 1414 sqf + 200 sqf of gables = 1614 total or 60% more area leaking energy. 60% more area you must buy insulation to cover with the most expensive and ungreen insulation on the market today.
Both roof sit on 8 foot walls with 1120 sqf of area. One house has an area above grade of 2120 Vs 2614 or 20% more over all. It is pretty cheap to cover a flat ceiling with R60 but almost know body spends the ton on money it would cost to do R60 at the roof line. If you are losing only 20% of the AC in the attic conditioning the attic could cost you money.
I picked 12-12 to make the math easy and make my point a flatter hipped roof would have lower numbers.
Walta