Polystyrene boards in attic and vapour issues


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Old 09-09-12, 02:00 AM
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Polystyrene boards in attic and vapour issues

Hi, I have covered the attic floor with 8 cm polystyrene boards (on top of ceiling joists), but now I'm worried about condensation, so I was wondering whether it might be a good idea to drill several 1-inch holes in the polystyrene (maybe 6 for every square meter), as a way to to ventilate and allow moisture to escape into the attic which *is well ventilated. Thanks
 
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Old 09-09-12, 02:37 AM
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Oh, I forgot to say my idea is to drill the 1-inch holes and then fill them with loose rockwool, hopefully allowing the polystyrene sufficient ventilation so the rising moisture can escape.
 
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Old 09-09-12, 03:46 AM
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Welcome to the forums! Fill out your profile and let us know where you are located. So much information relies on that.
Vapor barriers should be against the living or heated/cooled space. What is in the joist bays? Nothing? How thick of XPS did you use and exactly what was the product? Got a product name or where it came from?
 
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Old 09-09-12, 04:14 AM
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I'm in Spain. The polystyrene boards are 8 cm thick. The ceiling underneath is mostly parqquet boards 9 cm wide x 14 mm thick, so not a sealed vapour barrier. There is nothing between the joists, just the insulation boards on top. So now my idea is to drill about 6 1-inch holes per square meter of polystyrene board, and fill the holes with loose rockwool. Can you think of any reasons (scientific or otherwise) why my proposed idea *wouldn't solve the vapour issues? Especially bearing in mind that, otherwise the attic is very well ventilated. I came across this too: Insulation Basics – Vapor Barriers « A New House Thanks.
 
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Old 09-09-12, 05:55 AM
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Drilling holes in those boards is a terrible idea. You will defeat the purpose of the insulation. At 8 cm, approx. 3 1/8", the XPS will form a vapor barrier. If you only have one layer of XPS, you will want to tape the seams with the appropriate tape to prevent air flow.
 
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Old 09-09-12, 06:46 AM
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Not that it matters guys, but polystyrene is EPS... white beadboard. Still a vapor barrier in any case. I agree the seams need to be taped, and maybe the edges even glued down with foam adhesive to try to ensure there is no air leakage around the perimeter. Air leakage is where the moisture problem will occur, as warm humid air meets the cold. If you can stop all the air transfer I can't imagine there would be any problem, especially since the joists are empty, there is plenty of space for humidity to go elsewhere through permeable building materials.

The biggest source of cold air in this space will be the "open end" (the rim joist end) of the joists, where cold air will move down the joist bays unless it is blocked. Also the entire perimeter will be colder than the center due to air leakage from the exterior. So IMO, it would be important to air seal the perimeter of the floor first before laying the polystyrene. Spraying the perimeter with foam would be my first choice.
 
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Old 09-09-12, 06:47 AM
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I wasn't familiar with the coldest weather that Spain gets and the coldest region I checked gets a January average low temperature of around -3 degC which is just below freezing.

Even though a vapor barrier is necessary for insulated spaces that are heated, in some regions they are not always a good idea.
A vapor barrier works by being installed on the warm side of the insulation but if you are in a very warm humid climate and the space is air-conditioned, the vapor barrier should be above the insulation in an attic, not below.

That link you provided is from Australia and it also says;
An Exception:
You don’t usually need a vapor barrier on the ceiling if you have a ventilated roof space as the air flow above the insulation will dry out any moisture in the insulation.
You really need to research your country and region to see what the accepted practice is for your area.

We experience average winter temperatures of -20 degC with extended periods of -30 degC and lower which is where a properly installed vapor barrier is important.
 
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Old 09-09-12, 07:17 AM
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But do you think there is any particular (scientific) reason why drilling the 1-inch holes in the boards and filling with rockwool *wouldn't allow any moisture to escape into the well ventilated attic?
 
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Old 09-09-12, 08:02 AM
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There is a reason you would not want to do that.
It is because when air is encouraged to move through ceiling and into the attic, moisture will condense on its way to the "well ventilated attic".
It will condense around the holes which will become funnels directing the water back to the space.
Perhaps if you put buckets where each hole is you could recycle the water.

If you are looking for justification to drill holes in your insulation it is not something you would get approval for here.
If you want help to remedy your problem we could likely do that.
 
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Old 09-09-12, 08:21 AM
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But then if *all the insulation was rockwool batts, would there not be the same issue with condensation? Also, I don't quite see how it would be a case of hot air meeting cold air and causing condensation *if the holes are well insulated with rockwool. I mean I can understand condensation building up in the holes if there was no rockwool in place.
 
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Old 09-09-12, 09:48 AM
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Hi dubman,
The highest priority is to prevent air flow, which, if well taped as suggested, your new layer of polystyrene boards can accomplish. Drilling the holes, even with rockwool installed will increase the air flow which will increase the moisture in the cavities. I realize rockwool can be very dense, but it would still be installed in a hole and thus never a perfect seal.

With the parquet ceiling I would expect a lot of leakage from the conditioned space to the ceiling cavities. This will not only transfer some moisture, but also heat. If the bottom of your new insulation stays warm, then no condensation. Since the majority of your r-value is above the cavity I don't see a problem with the install. If you were to add insulation to the cavities, then your new barrier would get colder and increase the chance of condensation.

Fabricate a panel where you can lift it out to examine the underside for moisture during the peak of the cold season. Be sure it can be air sealed when put in place. If you detect moisture, then we can discuss what action is appropriate.

Bud
 
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Old 09-09-12, 09:54 AM
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It looks like you are trying to negotiate an answer that you would like to hear but I'll try again!
If you were to insulate an attic with rock wool only, then the moisture infiltration from the living space will dampen all the rock wool.
The correct thing to have is a vapor barrier for moisture to not pass through.

By stuffing these holes you want to make with anything you will slow down the transfer of moisture causing what can't move through these holes to travel through the polystyrene.
Humidity transfer has a great deal of force behind it and one way or another will wind up in your attic.
That is why it takes an impermeable barrier, thew best one being plastic, to stop its flow.
IOW if you restrict those holes you will force moist air to travel to the attic through all the leaks in the entire ceiling.

You need to trap the moisture and prevent it from passing from the warm to the cold side of your insulation with a vapor barrier.......end of story.
 
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Old 09-09-12, 10:23 AM
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It's a little confusing about the vapour barrier. I understand that in some cases of well ventilated attics, a vapour barrier isn't always necessary, as explained here: Insulation Basics – Vapor Barriers « A New House And I probably have lots of air passing through the ceiling parquet, which I have no idea how to seal fully. I mean, if I install a vapour barrier on top of the joists and under the polystyrene, would condensation not build up in that empty space between the joists? On the other hand I think having a plaster ceiling would in itself be a good air seal, but it's not my case... *But, if taping /sealing the polystyrene boards is a solution, that would be great! Maybe some kind of high quality silicone or air-tight gap filler would work? By the way, what exactly is caulk - not a common word here). Also, I assume I would have to use some similar product for the electric cables passing through the Polystyrene, so air doesn't get through there?
 
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Old 09-09-12, 11:11 AM
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Caulk is short for caulking and is a sealant like what you would put around windows to seal them from water getting behind them.

A vapor barrier in an attic is easy to understand if you are in a climate with sub-zero freezing temperatures.
If you were to install sheet insulation and drill holes in it in our climate you would see frost volcanoes forming around those holes where moisture would enter the attic and freeze.
In your climate it might tend to cause condensation to form and cause the structure to rot.

If you were to install a vapor barrier in the attic between the ceiling and insulation, the insulation against the vapor barrier would keep this area at the same temperature as inside the house, preventing condensation from forming.
In other words, sealing the moisture has to be on the warm side which is nearest the ceiling.
You can not seal it from the top side.

In other words, if you ant to do it properly you must remove the sheet insulation, place a plastic vapor barrier against the ceiling surface, seal wiring penetrations with "caulk' then place the insulation against the plastic vapor barrier.
 
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Old 09-09-12, 11:25 AM
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Ok, lets say I lift the polystyrene boards, then I have the joists, and below that the parquet (a small part of the ceiling has plaster instead...). So how exactly can I seal that area between joists and on top of parquet ceiling? Do you mean something like plastic sheets on top of the parquet, between the joists, or also going up and around each joist? Or could I just place the plastic sheets in strips between the joists, and caulk them to the sides of the joists? So you don't think there would be any point, in this case, of taping /sealing around the top of the polystyrene boards? I thought Bud was recommending taping /sealing on top of the insulation boards? In this case, the vapour barrier would be the insulation itself? Another thought: would there be any possible advantage in adding a bit of extra rockwool insulation first, between the joists (to keep more heat below the ceiling), then laying the vapour barrier across the top of the joists, fully sealed, and then the polystyrene boards on top of that? Thanks
 

Last edited by dubman; 09-09-12 at 12:58 PM.
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Old 09-09-12, 01:01 PM
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Are you laying this EPS on top of the joists? What is in the joist bay cavity??? Nothing?? Why be fixated on EPS when rock wool in the joist bay cavity on top of a 6 mil vapor barrier would give excellent insulative qualites. If it is priced where you are like it is priced here, the boards are way too expensive compared to rock wool.
 
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Old 09-09-12, 01:10 PM
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Unfortunately, I already have the extruded polystyrene boards installed on top of the joists (nothing between joists), and now I think the best option would be to install the vapour barrier under the boards.....I was just wondering if there may be any advantage in first laying some rockwoll between the joists and the vapour barrier on top of the joists, under the polystyrene boards? Thereby keeping more heat below ceiling level.
 
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Old 09-09-12, 05:36 PM
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The polystyrene is a vapor barrier. I would have installed one or the other (rock wool or poly), but not both in the order you have them.
 
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Old 09-10-12, 12:16 AM
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Ok, but I gather that, to avoid condensation, I must install a separate vapour barrier *under the polystyrene, eventhough the polystyrene itself is a vapour barrier. Or would it be sufficient to seal /tape or caulk the polystyrene from the top, where gaps exist - especially along walls etc, without removing it to place separate barrier under it?
 
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Old 09-10-12, 03:25 AM
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I can't tell you to do that because it is an incorrect method of insulation to begin with. If you do it, then consider the failure that is imminent. You have purchased a product you can't use properly to do a job it wasn't meant to do and you want our blessings. I don't think it will work.
 
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Old 09-10-12, 05:45 AM
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One vapor barrier = good
Two vapor barriers = bad

Stick with the polystyrene, tape the joints and seal the edges and be done with it. Don't add plastic and don't punch any holes in it.
 
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Old 09-10-12, 05:58 AM
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Well, that would now be my preferred solution (sealing / taping all gaps in the polystyrene). Since, as it seems to me, the polystyrene is both a vapour barrier and an insulation. Thanks for all the ideas. I learned a lot.
 
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Old 09-10-12, 06:40 AM
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"as it seems to me, the polystyrene is both a vapour barrier and an insulation"

Yes, that's exactly correct
 
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Old 09-11-12, 08:34 PM
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I'm in Spain.
Thank you. Please
Originally Posted by chandler
Fill out your profile and let us know where you are located. So much information relies on that.
Thank you.
 
 

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