I have some frost and moisture behind my insulation in my new build. the wall is 2x6 with roxul and foam on the outside taped and won't breath well to the outside at all. The vapor barrier inside will be membrain by certain teed. the house was fully insulated, the geothermal contractor fired up the geo system, this last friday and also left the backup heat on at the same time bringing the house upto just under 80 degrees which didn't help the frost issue. the garage walls which were kept a lot colder around 65 degrees hardly frosted at all. should I pull all the insulation and replace or do you think the frost and moisture will dry to the inside of the home after membrane and drywall is up. also how does one go about drying the wall cavities in the winter time? won't more frost start without insulation and more heat added to the house with an external vented heater? thanks

Dana is the expert around these parts about this subject. In simple terms, moisture inside building assemblies results from some layer in the assembly getting colder than the dew point temp causing the water vapor to condense (and in your case, then freeze). So these issues are about the temp and RH both inside and outside the building, and what material is used for the assembly layers (R-value and permeance) and how they are layered. Dana can likely recommend straight away how best to resolve your issue. We also have both Psychrometrics software and Building Assembly Moisture Analysis software that can be used for figuring out what is going on too:

Borst Psychrometrics Software

Borst Building Assembly Moisture Analysis Software

what zone do you live in first of all?

I have received a lot of advice from Dana on this as my wall is similar to yours: 2x6 with batts, membrain internal vapor retarder taped and overlapped, and 1.5in hard insulation on exterior

As sailaway said, the RH inside is a key. Also you don't seem to have your Membrain up yet, so vapor is not retarded at all. So the second question is how much exterior insulation do you have? 1in? what type of insulation is it? R-value?

The rules are quite specific in you wanting to have enough insulation outside so as to keep the wall sheathing/wood warm enough to not condense. Now you also had raised the temp up to 80 which is higher than normal, so that contributed to it as well.

Another factor might be your basement slab, was it recently poured? All that humidity of the concrete drying for the first 30 days will contribute to a higher humidity rate in the home.

Do you have a vapor barrier under the concrete or outside your foundation walls mitigating humidity from the Earth seeping in through your concrete into the home?

The wall should be built with enough insulation exterior to prevent condensation in the first place. Membrain is then used as a backup plan in case the limits are exceeded temporarily, mitigating vapor so condensation will be minimized. Once summer returns and ambient RH changes, the Membrain will "open" and allow the wall to dry to the interior. But you want the walll warm enough with enough exterior insulation to begin with (depending on climate this changes).

That's a good point (really all good points)...about the variability of the MemBrain permeance. We have it listed as 1 perm, but I wonder if this is correct or if there isn't a better value to use?

Questions.
What is the zip code?
What is the exterior insulation thickness and type?
What is the cavity depth?
Plywood or OSB?

What was the outdoor temp and relative Humidity when the problem occurred
What was the indoor temp and humidity?
How long has the structure been framed and dryed in?
How long has the concrete cured?

Start with that and we will go from there.

Cheers,
Eric

A few comments:

The air tightness of the assembly to the interior is far more important than the vapor retardency. If it's leaking air into the cavity from a humid conditioned interior, you will get continued moisture accumulation.

There is a lot of moisture in the wood & concrete of new construction.  Get some cheap RH montors and use dehumidifiers to keep the interior air under 35% RH @ 70F, which willl promote drying toward the interior.  It may take more than one year for it to reach a stable level, but with the higher drying capacity of MemBrain it will eventually stabilize.

The amount & type of exterior foam matter and are local climate dependent.  The dew point of 35% RH/ 70F air(the upper bound of where it should be controlled in winter) is about 41F/5C. If the average mid-winter temperature at the sheathing is below that, you will be relying heavily on the MemBrain to keep the sheathing dry.

But for the record, where are you, and how much foam (and what type) do you have on the exterior?

Garages are notoriously air-leaky- impossible to make truly air tight, and there aren't interior sources of humidity such as cooking/bathing/breathing activity. The dew point of the garage will pretty much track that of the outdoor air, which is why you don't have moisture accumulating in those cavities.

If the MemBrain has yet to be installed, that's 100% of the problem- humid air from the nearly air-tight interior is convecting (however slowly) through the rock wool to hit the cold sheathing.  But before installing it it's a good idea to pull the batts and give the sheathing & studs a week of 70F/35% RH drying before closing it all in.

The permeance of MemBrain varies with the RH of the air adjacent to it (on either side).  When the sheathing is cold and adsorbing the moisture of the entrained air in the cavity it lowers the RH of the cavity air, lowering the RH at the MemBrain.  The permeance typically drops to 1 perm or less at about 35-40% RH, which is fairly vapor tight (tight enough to meet Canadian National Building Code requirements for vapor barriers), and is lower still at lower RH:



But in the spring when the sheathing temp rises releasing the adsorbed moisture into the cavity, the cavity air starts cyling between 60-70% RH during the days, but below 40% at night, which means it can dry MUCH faster than it re-accumulates moisture.

Under these conditions you really don't need to sweat even fairly high moisture levels in the sheathing, since it's only damp when it's cold- so cold that mold can't grow quickly.  By the time of year when the average temp at the sheathing is high enough to support mold/rot growth, the assembly has dried to the point where it's too dry to support mold growth.  So, as long as you start out without pond-soaked or heavily frosted/saturated wood in the studwall when you put up the MemBrain, it'll do just fine.


Asphalt-kraft facers are a cruder less air-tight version of a smart vapor retarder, but they also work just fine if the interior gypsum is air-tight.

the house is still under construction and the membrane wasn't put up yet before the heat was cranked on by mistake.

the building detail of the wall does not work for my climate Im above north dakota in manitoba. 2x6 wall with 1.5" of polyiso with roxul batts. there is a double layer of tyvek under the foam and tar paper overtop of it. This model has been ran through WUFI and it shows its not optimum but if moisture is kept decent in the structure it should be fine for most days.

the house has a crawl space that was kind of moist, I currently have 2 drieaz 7000xli in the house drying it out and have the heat turned down. Im willing to pull all the insulation and use it for interior sound proofing and get fresh roxul for the outside walls if need be

just wondering how someone would go about getting rid of the frost/moisture and drying the stud bays in the middle of winter with cold weather -10 to -25 (c) right now on average

thanks for all the quick replys

Thanks Dana! That's what I suspected, but I hadn't seen that MemBrain ASTM 96 permeance data before.

Posted By tjetson on 03 Dec 2014 11:51 AM
the house is still under construction and the membrane wasn't put up yet before the heat was cranked on by mistake.

the building detail of the wall does not work for my climate Im above north dakota in manitoba. 2x6 wall with 1.5" of polyiso with roxul batts. there is a double layer of tyvek under the foam and tar paper overtop of it. This model has been ran through WUFI and it shows its not optimum but if moisture is kept decent in the structure it should be fine for most days.

the house has a crawl space that was kind of moist, I currently have 2 drieaz 7000xli in the house drying it out and have the heat turned down. Im willing to pull all the insulation and use it for interior sound proofing and get fresh roxul for the outside walls if need be

just wondering how someone would go about getting rid of the frost/moisture and drying the stud bays in the middle of winter with cold weather -10 to -25 (c) right now on average

thanks for all the quick replys

MemBrain should work fine in that stackup, but you are starting out with a moisture burden much higher than would get from normal wintertime vapor diffusion loads.

The way to get rid of the frost/moisture in the stud bays is to remove the batts and heat the house, and ventilate or dehumidify to 35% RH or lower.  That raises the temperature of the sheathing to room temp, driving off the moisture. It sucks to have to do that when it's -25C, but that's the only way to make it happen in winter.   A week of drying would take the first 90% of the moisture out, but if you have a wood-moisture tester you may want to check that it's below 25% before putting new insulation in. (That's pretty high moisture content, but it will be able to dry from those levels before summer.)  But if you're doing it in winter, as soon as that insulation goes in you need to put up the MemBrain, not a day later.

If it's not too late (though it probably is),  1.5" of polyiso has terrible mid-winter performance in Manitoba, less than half it's labeled R-value.  From a dew-point control perspective (and energy use perspective) you would be better off with 1.5" of EPS, which would outperform it's labeled peformance by more than 10% in your stackup.

I learned about polyiso and cold weather after installing it stucco is on now nothing I can do now
Dana
The service you provide to this forum is amazing the number of people you help I'd like to tip you do you take bitcoin tips or send you a gift cert for a local resturant at the very least

If you simultaneously heat and ventilate some (an HRV would be even better) and make sure you don't have any other sources of interior moisture, you can drive interior RH to < 10% (exact number depends on conditions). This is far lower than a dehumidifier and will dry things out much more effectively.

I am un aware of any heat pump manufacturer that does not void their warranties if the unit is used as a construction heater.

As well they should!

You will need a dry source of heat in adequate amounts e.g. sealed combustion, direct-vent furnace...rental. We crack upper floor windows to drive off moisture and watch outdoor rH, usually "relatively' drier during the day.

How much is the warranty actually worth in the first place?

badgerboilderMN: how long do you find does it take to dry things out in the old climate of mn or mb? Im going to start trying to dry it out today. Section by section north wall first. If i take all insulation out at once i don't think my furnace will keep up.

Might have to get a direct vent natural gas heater to try and dry it quicker

And how many folks do we reckon actually tell the heat pump manufacturer that they did this?

Too many factors to predict reliably. Pray for wind, dry air and sunshine...

We are working on a 6,000 sq.ft. three story new construction and I just ordered a 200,000 buth rental to keep it warm enough to install PEX fire and water.

Standard dehumifiers will work with a drain nearby and give you some indication of the moisture you are taking out of the space, room-by-room if need be.

I suspect the backup heat is electric as Manitoba has some of the cheaper electric rates. Electric heat is dry heat so it will not contribute to the problem. The outside air should also be quite dry at this time of the year so heating and ventilating should look after the problem but some patience will be required.

Change air filters often.

I pulled some insulation today, the south facing wall has 0 frost. The moisture level in some plywood is 11% and 15%
the north wall had some frost pulled the insulation and within it melting and an hour later the levels were 23% to 28%
if i pull the roxul some is frozen to the plywood so it becomes useless for reinstalling.

Im thinking i should just complete this process room by room. I worry about wrecking the stucco, the scratch and finish is already on the house. the exterior is going to frost now in the open stud bays.

the frost is so minimal on the southern walls and garage Im almost thinking it might be less damage to cover it with membrane and hope it drys to the inside.