Remedial Issues for the Absence of a Vapor Retarder

Archive of Older Topics Archive - Specifications Discussions #5
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The collective experience and technical knowledge of this group is requested to provide remedial suggestions to answer the following question: How does one mitigate moisture migration issues when a vapor retarder is absent or defective?

Scenario:
A +10-year old community swimming pool facility (located in Northern Virginia) has moisture and ventilation issues in a large attic. The roof is of a hip design with asphalt shingles. The roof structure is supported by a wood truss system supplemented by a piggy back truss located on top of the primary (lower) truss with a horizontal OSB diaphragm located about mid-point of the vertical height of the attic space where the two trusses meet. The attic has three vertical draft-stopping partitions creating compartments (two at approximately 4,000 SF and a one at approximately 1,000 SF).

The facility houses an L-shaped swimming pool (approximately 5,000 SF in size) with a coffered ceiling with a 6-mil poly vapor retarder and unfaced, fiberglass batt/blanket insulation (R-19). The vapor retarder is compromised with numerous penetrations and does not have properly taped joints or overlaps.

The remainder of the facility houses locker rooms, fitness areas, dance studio, and several mechanical/pool equipment rooms. These spaces have no vapor retarder above their respective ceilings but do have unfaced fiberglass blanket/batt insulation (R-19) suspended about 3’ to 5’ above the ceilings of these spaces. This plane of insulation is so completely compromised with large gaps and missing insulation that it is useless as a thermal envelope. The net effect of the above scenario is that there are three levels of attic: upper and lower truss spaces separated by the horizontal diaphragm and the horizontal plane of thermal insulation (as defective as it is) located below the trusses.

There are continuous soffit vents that appear to be open and effective (very good) but there are a grossly inadequate supply of ridge vents in the upper portions of the attic (very bad). There is another complicating factor with heat producing mechanical equipment located within the attic space.

The scope of work includes replacement HVAC equipment including a properly designed dehumidification and temperature (water and air) control system for the interior space for the pool.

Design Intent:
Remodel the attic such as to produce a properly designed single attic with a properly functioning passively ventilated attic.

Remedial Actions Scheduled to be Taken:
The following remedial actions are currently proposed:

  1. Installation of additional ridge vents supplemented by multiple off-ridge vents (typical of those used in Florida residential construction) as necessary to achieve a balanced (incoming and exhausted) passive ventilation system.
  2. Removal of at least 50% of the horizontal OSB diaphragm (subject to acceptance of the structural engineer).
  3. Relocation of the unfaced fiberglass insulation from its suspended position down to the top side of the ceiling systems (suspended GWB or suspended acoustical lay-in panels) without the installation of a vapor retarder (just not possible under the existing conditions).

So back to my original question, How does one mitigate moisture migration issues when a vapor retarder is absent or defective? which is directed to item #3 above. Can an excessive application of fiberglass insulation (supplemented by loose insulation), say R-38, retard the flow of moisture to an acceptable level?

My apologies for posting such a long thread question. I tried to make it shorter but there were just too many issues (some not presented).

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Not enough time to respond right now, but first answer is to your last question.

Think U-value, not R. Despite the propoganda, adding insulation past the point of diminishing returns is throwing away money with little to no results.

If you’re putting in a gyp board ceiling anyway, is it feasible to install a closed cell spray polyurethane insulation and use the ceiling as your thermal barrier? That should give you continuous vapor and thermal barriers. The concern then is how to keep your vents open and what affect that will have on your unconditioned space. Does that negate the insulation? Should the vented space be under the shingle roof and not in the attic? Should you condition the attic?

Consider reading up on some of Joe Lstiburek’s articles and his reference information (http://www.buildingscienceconsulting.com/index.html). NoVa is a difficult area to design for because it’s both north and south in terms of design. Housing a swimming pool just makes it that much more difficult.

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I would worry about the structural integrity of the wood roof structure if there is little or no separation from the cholrine in the pool area and the structure above. I am not sure what affect (if any) continued long term exposure the chemicals will have on wood trusses but I know that concrete decks exposed to cholrine will eventually fail if not protected from that exposure.

Regardless of where, or if, you create a vapor barrier you may need to make sure there is not a potential risk to the buildings structural integrity due to the long term chemical exposure. I would start there and that might lead you to the next step that needs to be taken.

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In addition to the thoughtful comments above, I would suggest that you contribute on getting the best HVAC system that you can afford to increase the ventilation in the attic area and reduce the warm humid air that may collect in the upper ceiling areas of the main occupied space. A good MEP consultant should be able to propose a “stratified” solution that puts more conditioned air in the occupied areas and concentrates on ventilating the upper areas. This also addresses the concerns of chlorine.

I would suggest that you avoid GWP products (I would even avoid the exterior soffit material) in favor of glass fiber panels or ceramic mineral panels with a perforated face (for acoustical and permeability). A much better solution would be portland cement plaster (probably out of the question) or a non-ferrous metallic ceiling system.

There will probably always be a positive vapor pressure toward the outside of the building (even in the summer), with a tendency of condensation to form on the surface of the roof deck in the worst case scenarios. I think the thermal envelope should remain at the ceiling. A well venitlated attic should address the worst conditions in the attic. I am not sure that I would focus on a vapor retarder, but if you want to look at it, I would strongly suggest a WUFI analysis to determine the proper location for it (my hunch is occupied-space-side of batt insulation above the ceiling, but I would not stake my first-born on it).

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I’m not on-board with the acoustical ceiling products in this application. Even the aluminum panels and suspension systems can have corrosion issues and air from pools can be very corrosive, even compared to exterior soffit conditions. Probably depends on the product.

Air movement and ventilation are certainly important. Keep in mind that running ducts in an unconditioned attic will probably result in condensation on the ductwork, not necessarily your best solution.

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Can you take out the ceiling over the pool to reinstall that vapor barrier properly?

One possibility is to put a ceiling, with vapor barrier and insulation above it, at the bottom of the trusses over the entire building, then run ductwork below this ceiling so it’s in a conditioned space. Suspended ceilings could be hung in the rooms that don’t need a high ceiling.

A conventional HVAC system, in which dehumidification is just a side effect of cooling, will allow wide swings in humidity levels. The HVAC system should be designed primarily as a dehumidification system with supplemental temperature control.

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I don’t have very much experience with pools, but my initial thought on the insulation issue would be to look at a SPF (Spray Polyurethane Foam). There are Products out there (and we use them A LOT in our typical envelopes in the Great White North eh?) that combine air barrier/vapour retarder into the foam insulation when they cure, with tested perm values that qualify them as air barrier & vapour retarder.

Another thing that jumped out at me when I read the post is the ventilation (which a couple of people already commented on in more detail); but my original thought was with the humidity level of a pool and HVAC equipment-generated heat in the attic space, I would think that more ventilation than relying on passive soffit and ridge vents would be needed to address that issue and ensure you don’t get mould growth. Depending on the replacement equipment specified, my gut would say you would want some wall louvres if possible and maybe even some power dampers to ensure proper evacuation of hot, moisture-laden air out of the attic space.

Please keep us up-to-date on what is decided for the final design solution. I think this could personally be a real learning experience to tuck away for future reference in an uncertain situation.

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A few observations and comments:

*OSB is a vapor retarder. So you have plenty of “vapor retarder” already present within the structure - maybe not continuous, but certainly a part of the mix that cannot be ignored.

*A vapor retarder that has holes in 1% of its surface area is 99% effective!

*An air barrier that has holes in 1% of its surface area is INEFFECTIVE. And will allow huge amounts of moisture laden air movement - in either direction (depending on the pressure difference)

*The biggest issue with regard to condensation potential is AIR TRANSPORT of moisture laden air, NOT, I repeat: NOT vapor diffusion. Concentrate on PLUGGING HOLES, not on fussing over whether or not you have little holes in the polyethylene.

*An effective vapor diffusion retarder can be as easy as applying 3 coats of latex paint over gypsum board. How many perms you need is a building science question - and depends on relative humidity and temperature differential.

Best advice is to perform a blower door test now and at the conclusion of the remodel to find the holes that will cause condensation. These tests are inexpensive, at about $300 a pop, and tell you what you need to know about the imperfections in the separation between the hot, humid environment of the natatorium and the rest of the building.

If there is no way to cost effectively create an air barrier separating the spaces - another way to address this is to use pressurization with RH and Temperature sensors - fans go on when conditions merit (when condensation potential exists). Lstiburek has done this many times with great success - when no other “expert” could solve the pre-existing problem(s). I have seen case studies in which Lstiburek used this to pressurize interstitial spaces (like between roof rafters, for example - and another example where he used pressurization in a brick veneer cavity wall). Fascinating stuff.

Plug the holes, plug the holes, plug the holes!