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Old 10-23-2009, 10:09 PM   #1
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insulating interior walls


I have a question about insulating an interior wall.

I have 2 reasons for wanting to do so. This wall is in an ongoing renovation area in my home. One room will be heated throughout the winter, while the other will be heated only on an "as needed" basis. Secondly, when both rooms are finished and both are heated all the time, there will be a temperature differential between the 2 rooms; one thermostat will be set to normal room temperature, and the other will about 15 degrees farenheit cooler.

I'm wondering whether I can just stuff some fiberglass insulation in the wall and cover it with drywall, or whether one side, or both, require some sort of vapour barrier. I can't quite wrap my head around this issue.

I was considering putting up a layer of Typar, on the cold room side, until I get the room finished. I figured that when both rooms are finished, and the temperature differential is only around 15 degrees, that moisture/mold would not be an issue.

Am I entirely out to lunch here?

Thank you, in advance, for any advice you can provide.

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Old 10-23-2009, 11:00 PM   #2
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You don't need a vapor barrier on an interior wall. However, in this case it can't hurt. You always install the vapor barrier to the warm side. No need for typar or anything like that at all. Be sure to not cram the insulation in the space....The loft of the insulation is what gives it its insulating value...More isn't better.

One thing to be really wary of is if there are any plumbing pipes in the floor or wall of the unheated room if the walls are exterior and the floor is over unheated space. That could cause problems when it gets cold.

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Old 10-23-2009, 11:41 PM   #3
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Thank you for your response.

The problem is..........what is now "the warm side" will, in the future be the side that's about 10-15 degrees F cooler than maintained room temperature.

My question now is..........If I put up a vapour barrier on the "now warm side" should I remove it when that side becomes the slightly cooler side?.........Or can I vapour barrier both sides? Something about that worries me; about creating a little plastic moisture capsule inside the wall.

I do apologize if my questions sound horribly ill informed but...........I am that.
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Old 10-24-2009, 11:40 PM   #4
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Since you're insulating for that area to be cold, put your insulation's vapor barrier (the paper face) toward what will be the warm side. Hope that makes sense.

You do not want two vapor barriers. Since the wall is interior it probably wouldn't be detrimental but it is a no-no for the most part.
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Old 10-27-2009, 06:15 PM   #5
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One more thought-- I'm hearing of more people insulating interior walls to create a sound barrier. While this might not be your intent going in, it could provide a nice added bonus. It will help buffer sound a little, so if one room will be more high traffic it will help those using the other.
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Old 10-27-2009, 09:15 PM   #6
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No need for a vapor barrier. A vapor barrier is only for separating conditioned living space from unconditioned. Since you are conditioning both spaces there is no need.
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Old 11-11-2009, 05:19 PM   #7
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I have a similar question. I am in the process of building a wall to divide my garage/workshop into two separate sections. Both are fairly large areas and I would like to only heat each space when it is needed. Can I insulate this new wall with regular faced batts of fiberglass insulation? I know the paper usually faces the heated area but since the heated and unheated areas will change will this matter? Would this cause some sort of moisture problem? Would it be better to use unfaced batts?

Thanks in advance for any advice.
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Old 11-12-2009, 06:31 AM   #8
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If you were using the insulation in an interior wall for sound absorption, then the vapor retarder would not matter since both sides would be heated above dew point. There could be no condensation. Since you are saying that you plan to heat one OR the other, but not necessarily both sides, then you need to leave out the vapor retarder. If you used it, then when the side with the vapor retarder was heated, it would work properly. When the side with the vapor retarder was cool, then you could potentially cause a condensation problem.
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Old 11-12-2009, 07:19 AM   #9
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Sit back and think a bit about what you are asking; you ask if you need a vapour barrier between two rooms that have temperature differences. A vapour barrier, as defined elsewhere prevents almost any transfer of water vapour between two areas whereas a vapour retarder does almost the same thing but to a lesser extent i.e the may be transfer of water vapour from either direction, to some degree, but less than a vapour barrier and slower.

Where I come from we use vapour barriers; vapour retarders are not used extensively and that's because we experience cold temperatures in winter and need to heat, and hot temperatures in summer when we need to air-condition. Unde these conditions, moisture becomes a factor because we put permanent vapour barriers on the (warm) inside - when common sense says that during the summer, we should put them on the outside!

We insulate heavily to stay warm - but we manage the moisture levels in our homes by the use of 'vapour level management' - but that doesn't mean you should, wherever you live...you may not have an interior moisture level problem or have a completely different one (because you live in a warmer zone, or a dryer zone or a more humid zone than we do) so to given you an answer means little unless we know where you are, what existing measures you are taking, what heating system you may or may not have, what construction walls etc etc.

Is this two bedrooms in South Carolina? a garage and den in Illlinois? a LR and bedroom in a 11th floor condo in Arizona? is one room air-conditioned the other not? do your inside humidity levels ever get to 25% during the winter? do you ever get to below 65% RH outside or in? Do you have paint on the walls now? what heating system do you have? is one room kept at 70degF the other at 55degF or something else? do you have an air barrier on the outside?

Again, up here, we have to account for each of these factors in deciding what to use...admittedly we're more sensitive to all of this, but one thing is for certain: no one answer applies to everyone.
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Old 11-12-2009, 11:34 PM   #10
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Thanks for the replies.

So it seems that maybe a little more info would help. First of all, I am located in the Washington DC area. The building is a 30' X 60' pole building with a concrete floor and 10' ceiling. All the exterior walls have been sprayed with INCYLTHANE foam insulation and are at least R-14. I've built an interior wall with standard 2 X 4 construction on 16" centers to divide the space. On one side of the wall will be a woodshop (30 X 40). On the other side will be a small auto shop (20 X 20). On the auto shop side there is another interior wall which defines a large storage area (10 X 20).

I plan to keep the storage area heated to at least 40 or 45 degrees at all times so paint, etc. will not freeze. The other areas will only be heated when they are being used. Not sure what kind of heating system I will have yet but I am thinking of baseboard electric heat in the storage area and oil heat for the other portions of the building.

From what I'm hearing so far it seems the unfaced batts may be my best bet.

I have installed the faced insulation before but never the unfaced. With no vapor barrier to staple through I'm not sure about installing the unfaced batts. I assume there has to be some way to insure that the insulation doesn't eventually creep down in the cavity and leave unprotected airspace at the top.

Thanks again for the advice.
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Old 11-13-2009, 06:29 AM   #11
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Good! OK so what you're told us says that you don't need a vapour barrier (for example 6mil plastic sheathing) and you only have a marginal need for a vapour retarder. Plus, since you have an effective air movement barrier (in the foam), your worries of air leakage are small. You don't have a high humidity concern in the DC area, although you do need heating and air conditioning, if available. The electric heating system will "dry" the air, so there's even less moisture...

Furthermore, vapour infiltration will be mainly from inside activities and from the slab but these will be uniformly dispersed. Therefore, the choice of putting a vapour retarder in the walls between areas is completely yours. If you do use faced fibreglass batts, you then have the choice of which side the facing should face; better is facing the warmer side.

But I'd go with unfaced batts, drywall, primer and a coat of paint; the paint will act as a vapour retarder - and you save a bit. You might consider mineral wool for a better fire rating.

Good luck!
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Old 11-17-2009, 07:02 PM   #12
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We also insulated an interior wall - in a bathroom. I used Roxul (mineral wool) and it is an excellent sound barrier as well. But we should have put in a vapour barrier in this instance (I think). We will try to be diligent & keep the bath fan on to vent out any moisture. Time will tell!
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Old 11-18-2009, 06:27 AM   #13
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There really is no need to add a vapor retarder in an interior wall regardless of whether it is insulated or not. Provided that that both sides of the wall are kept at the same temperature, moisture will not condense. Good practice is to run the exhaust fan anytime moisture releasing activities are taking place.

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