Heat gain/loss in building? Trying to spec insulation. Building wraps? Spray foam guys are nuts! 1

[fastline12]

Running basic calcs to assess heat loads in a building. However, I have not found anything that directly assesses differences between walls and ceiling? Heat loss is a function of dT and U values but is it that since the warm air will pillow at the ceiling, that air temp is higher causing a higher dT?

In a nutshell, I am working on a wooden frame building, climate controlled, and at an R19 walls and R40 ceiling. However, the ceiling must be done in two layers. The first is easy as it attached directly to the purlins, but the second requires a complicated banding system and I am trying to get around it.

As well, I am considering a building wrap between the framing and sheeting to get a better air seal but I am concerned since we will have an air/vapor barrier on the inside. I think that is generally a no-no?

Talking with several spray foam guys and they all seem to get lost from R values and insist that 2" of spray foam will out perform 12" of glass? I realize air will move through glass but that is why there is an air/vapor barrier on it! But it brings into question if there is any validity to their claims? Performance data?

 

[willard3]

You're the Engineer, what's your opinion?

 

[MintJulep]

You don't get technical information from installers. You get it from product manufacturers. It is easily found in about 15 seconds of Googling.

 

[Dougt115]

Spray foam is R6 per inch while fiberglass is R2.2 per inch or about 3:1 ratio. So about 7 inches of foam will do the job to reach a R40 while almost 20 inches of fiberglass will be needed.

 

[IRstuff]

You can check the R-values here:

https://en.wikipedia.org/wiki/R-value_(insulation)

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[moltenmetal]

R value depends on the type of spray foam. Closed cell spray foams can be R6-7 and do not require a vapour barrier or external air barrier. Open cell foams have lower R value, are cheaper, and do require a vapour barrier. Foams reduce air infiltration, which greatly improves the performance of the home thermally- but the R value requirements are code minimums and you can't get around them.

Double vapour barriers separated by insulation are a no-no, but a building wrap like Typar is NOT a vapour barrier- it is an air and (liquid) water barrier. It goes outside the sheathing, forming the inner barrier of a water drainage plane (behind either siding or brick), not between the sheathing and the framing. And yes, you do still require a building wrap if you use closed cell spray foam.

 

[dbill74]

First, never trust an installer or contractor to give accurate product information. They're going to push something they have on hand or can get very cheap. Verify or get performance data from manufacturers as MintJulup recommends.

Second, if you've never done a heat loss/gain calculation on a building, hire an HVAC engineer to do the calculation for you. The R-value of a wall is affected by more than just the installed insulation, framing material (wood in this case), framing spacing, interior face material, air gaps, exterior facing material for example. Same goes for the roof. Also, you need to take into consideration windows, doors, and skylights. Buildings gain/loose heat through the flooring as well, not just walls and the roof. Is the roof sloped or not, if sloped how much? Other considerations include building location (where it is on earth) and orientation (which way is north).

What is in the structure? How many lights, how many people, what are the people doing, any electronics (TV's, computers, printers, etc.)?

All these things and more need to be considered when calculating a building's heat gain/loss. An HVAC engineer is going to have the knowledge and tools to do this.

 

[fastline12]

UPDATE: I have ran full heat gain/loss calcs on various load conditions and here is where I am.

spray foam direct to the sheet metal surface is a no-no. Sounds great and all but no way to completely lock out moisture as the sheeting itself is NOT sealed! Moisture gets in and has little chance of leaving. Too many testimonials of folks with rust on the metal. Plus repair work becomes next to impossible.

I think a proper air barrier wrap over the framing will be the best route. This will allow "some" air migration and will allow a certain level of vapor, while rejecting liquids that could soak the glass batts.

HOWEVER, I am finding that still, most builders and building OEM's recommending this radiant barrier/bubble wrap stuff for the roof.... I have carefully examined these products in this application and feel they are a waste due to thermal bridging that will occur with direct contact of the radiant foil and the metal sheeting. To get proper radiant blocking performance, you need an air gap. Then you have the issue with the film getting dirty over the years...

So, I am pretty much at using a proper house wrap product?? I am not yet sure what vapor perm to use just yet but I find most feel "house wrap is house wrap"....

Anyone have any input on the radiant barrier thoughts?

The post build insulation would be fiberglass batts with proper air/vapor barrier seal on the inside. This is a common building product with the barrier directly attached to the glass, though many don't work hard enough IMO to tape seams and get proper air seal causing air migration into the insulated space.

 

[Compositepro]

The radiant barriers do work, and in both directions. An air gap is not strictly required. The foil is less than 0.001" thick.
It reflects heat but also it reduces the heat emitted to the other side.

 

[fastline12]

Do you have data on that? I feel the product is vastly over stated so I would be curious to get some real test data.

As well, how do you account for any improvement when thermal bridging is causing the film to heat up to the same temp as the sheeting? Seems it would just heat and become its own radiator?

 

[Compositepro]

The vendors have data published on the internet. Will 1/4" of it equal 3" of fiberglass? No.

There is no difference between a one inch air gap and a 0.005" air gap as far as the radiant barrier effect is concerned. A one inch air gap will provide better insulation, though not that much if not filled with fiberglass to stop convection.

The absorption and emissivity of materials at a given wavelength are the same. Highly reflective surfaces have low absorptivity and emissivity, so bright aluminum is a poor radiator. I said this in my first response but you did not get it. This a complex subject, so I advise you to study-up. You might try looking up MLI (Multi-Layer Insulation).

 

[fastline12]

Thanks for the reply! I think there are still a few "gaps" in my plans so far. I find getting credible data very taxing... Most "studies" I find are from foam or radiant suppliers in which certain details are not included.

All you have to do is research "radiant barriers" and a pile of sellers pop up with R-2 - R500 values and savings of $1000/mo installing it!

Thanks for reminding me about the emissivity and I will be sure to try and apply the figures but, no where, have I been able to find info regarding metal roofs, only asphalt and we all know that is a huge boost right there.

I am not opposed to using the product, if I can determine the real value in my application. I already need an air seal product anyway.


That is another point and question on fiberglass insulation. Foam sellers want you to know about how air moves through fiberglass! However, what if proper install gets the right air seal required? How much convection are we looking at there? Seems like a good reason to call Owens Corning but I am sure somewhere there is real test data.

 

[moltenmetal]

If you sandwich a layer of aluminum foil between two boards, you will not get any radiant barrier effect in that assembly. The assembly will conduct heat just slightly BETTER than two boards sandwiched together without a piece of aluminum foil between them, because the aluminum foil will reduce the contact resistance between the two pieces, being a better thermal conductor than air. So yes, it does matter whether or not you have an air gap in a radiant barrier assembly, or a gap filled with something like air which is not a strong IR absorber. A vacuum between the radiant reflector and the outer surface is of course even better than an air gap.

Sandwich a piece of aluminum foil between two fibreglass batts and you have a very different assembly. There is substantially more air in a fibreglass batt than there is in a board.

Put a layer of flat aluminized plastic sheet below a piece of corrugated steel roofing and you'll have a thermal bridge at each point where the two touch- but a functioning radiant barrier everywhere they don't touch.

The aluminized bubble wrap stuff provides its own air gap in assemblies where it is used. What is the R value of that 1/4" thick layer of bubblewrap? Certainly nowhere nearly as high as some claim. But as part of an assembly, it definitely can make a difference- again assuming that the assembly is designed properly so there aren't two vapour barriers separated by insulation.

Bulk convective movement of air within a wall cavity insulated with fibreglass batts does occur, and the convective transfer coefficient can be shown to increase as the temperature difference between hot and cold surfaces increases and hence the density difference driving convective movement within the assembly also increases. Denser fibrous materials (i.e. mineral fibre compared with fibreglass) reduce the convective air movement. Open cell foams reduce it even more. Closed cell foams pretty much eliminate it, such that what you observe is more or less just conduction. However, the air sealing (reducing infiltration) is the biggest benefit of the foam insulating systems. But it is very important for the foam not to be asked to do too much- foams are not intended as a weather barrier, and a drainage/ventilation plane in a wall/roof assembly is still required to keep the water out of the wall/ceiling cavity- irrespective of what it is insulated with.

 

[Compositepro]

"If you sandwich a layer of aluminum foil between two boards, you will not get any radiant barrier effect in that assembly. The assembly will conduct heat just slightly BETTER than two boards sandwiched together without a piece of aluminum foil between them, because the aluminum foil will reduce the contact resistance between the two pieces, being a better thermal conductor than air."

That is not correct. Aluminum has resistance to heat flow so adding it in series into the heat flow path does not improve conductivity. Wood also contains a lot of air. Radiation is one of the mechanisms of heat flow through the air spaces in the wood and this becomes part of what is included in the thermal conductivity of wood. The foil barrier also adds two thermal interfaces into the heat path. Keep in mind that in the aluminized bubble wrap the aluminum is so thin that you can see light through it. And we are not discussing a situation where the foil is bonded to the wood. However, even aluminized paint on a surface will absorption and emission of radiant heat.

 

[fastline12]

First, thank you guys for your feedback! VERY much appreciated.

I think I should share the total design and variables for consideration. Keep in mind, I am chasing a "smart and reasonable" design over a rock solid and exponentially more expensive design. Obviously we could throw countless layers of rigid sheeting in there for thermal bridge break, close cell spray, etc, BUT way outside the budget.

Wood frame pole construction with all purlins/gerts on edge, 2x6 walls, 2x8 ceiling. Flush mounted with saddles. Reasons here are improvement of structural performance, easy of installing glass batts, easier to install an air barrier over the frame, and a few others.

Overall design calls for 26ga steel cladding. I know we need a quality and air tight air barrier between the frame and cladding. This barrier will have to shed water but need to spec a perm rating for vapor as it needs to breath a bit.

Glass batts are 5.50" nominal thickness. Still trying to obtain the density but rated at R19 or R21 so I assume different densities on the two. The batts will have an air/vapor retarder with white facing on interior and foil facing that is adhesive bonded to the batts. Each batt will have 3" side tabs to be secured to the studs, then seal tape over each of them. The end points will also have a quality seal.

The cladding will be a painted Kynar 500 spec galvanized sheet.

THE QUESTIONS.....

1. I cannot determine or quantify any real improvement of a radiant barrier used in place of the air barrier in the roof. You can find data comparing apshalt shingles but a metal roof with specialized coating will already reject a lot more radiant than asphalt! You also run into question marks on the vapor perm for this layer as I feel the air seal and vapor specs might be more important than a radiant barrier for this app? As well, there will be the foil layer on the batts already which will help reject radiant.

2. Basic home wraps vs rigid board for air barrier. There is some data floating around about the thermal expansion of foams causing problems in maintaining an air seal at the seams due to failed tapings. This seems to be something home wraps specialize in....staying sealed?? Above all, if I lose air seal on the exterior, my performance will nose dive!

3. Convection currents in otherwise static batts. Thought I have asked ALL the main manufacturers for data on this to properly rate the insulation, I cannot get it! It seems most tests on batts are foam suppliers and don't consider a real seal on both sides. Assuming a quality air seal, how do I account for convective losses? The way I am looking at this is close cell is approx R6/in. installation of 2" is R12. That is already 3x the price of R19 batts so assuming certain convective losses, could we still hold close to an R-12?

4. Thermal bridging. sort of ignoring the long hand way to look at this, I am specifically considering the R values of the framing for consideration to derate the design R value. IE, just taking the U values of the wood and its corresponding thickness to input back into the thermal calcs. I am around an R6-R7 for the framing members, assuming no thermal bridging break.

 

[Compositepro]

Adventures in building science:

https://www.youtube.com/watch?v=rkfAcWpOYAA

This is a long lecture but you may find it interesting. It discusses how even many Professionals get it wrong, particularly with vapor barriers.

 

[fastline12]

Thank you! I made it through most of that and actually hold BSC is rather high regard on quality construction education.

The few things I gathered from that is reaffirming that the majority of the time, a building is at negative pressure due to ventilation. This makes it stand out that controlling vapor in the winter might not be as important as controlling vapor in the summer!

However, when you live in a mixed climate, you have to account for both! Though I feel rather liberated in my requirement for an exterior air barrier/vapor retarder, I think I need to gather more data on the interior specs because I know the materials spec'd is also an air barrier/vapor retarder.

What the vid fails to mention, and seems to be quite a wild card is is WHAT VAPOR PERM RATING DO YOU NEED?? The main issue is you cannot really get much of an air barrier without inherently blocking most of the moisture. In a perfect world, we could seal the air but allow only vapor to pass.

This seems to also reaffirm my position on spray foam directly applied to metal cladding!! You lose your air space in there for drainage. That cladding WILL leak. It was not designed to be water tight. Then it is trapped on the back side.