# HRV vs. ERV after spray foaming.



## Wildo (Jul 1, 2014)

We will be getting our house spray foamed from basement on up late this summer and it will most likely be too tight, in which case we will need an air exchange system to prevent moisture/ air quality problems.  I have been doing some research already and have found some valuable info but most of it is contradictory based upon manufacturers advertised statements.  The contradictions usually appear to be what type (hrv or erv) is dependent upon climate & location yet other sources say what type depends on home type, occupancy, heat source and other factors.  Sadly I didn't find as much info on green building advisor as I had hoped, although my search skills really suck.

  Any help that anyone who has expertise/ knowledge of these things would be most appreciated.

We heat with several woodstoves as well as moniter heaters for those ridiculous nights or when we are away.
We run a 70 pt. dehumidifier regularly for a good part of the year.
It gets really f'in cold here in the winter, very humid in the summer with temps from -40 ish to 100ish for extremes.
There are three of us in the house, daily showers, home cooking.
The house has four floors including basement and attic.

I am also an electricity Nazi so efficiency is mandatory. Any other pertinent info can be provided.

         Thanks


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## woodgeek (Jul 2, 2014)

For 50 bucks you can find someone just to do a blower door afterwards to see where you are at in terms of leakage.  If you are 3-5 ACH50, you do not need extra air in the winter, but only in the shoulder seasons and summer—the stack effect driven flow will be plenty.  I have never heard of an existing house that got tighter than that without >$50k of work.

Doesn't make sense to pay for an HRV to only operate when it is not cold outside....you can just crack a window 6 mos of the year.  Or set up an exhaust fan to run continuously at low speed, like 30-50cfm on a continuous basis during the warm months.

My 1960 house has gone from around 20 ACH50 to 5 ACH50, and in the real cold weather, I still need to run a small humidifier.  That is, excess moisture is not a problem in the winter...and we do not run the shower fans in the winter, either.


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## maple1 (Jul 2, 2014)

Have you done enough research to determine how much each option would cost?


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## Wildo (Jul 2, 2014)

I can not have any chance of a moisture/mold problem since my wife is highly allergic to certain types of mold.  That is the reason behind the spray foam, the attic will be sealed up very tightly so we won't have to add soffit and gable vents(one part of our problem was the lack of ventilation) because we have pitched ceilings.  All rafter bays will be filled.  The walls will be filled and the sills and basement will be foamed too.  Both  foam contractors that have given me quotes so far said our house will most likely need an air exchange system.  So I'm researching since I know little about them.  A blower door test will be performed after insulating, but I want to have a good idea of what we will need to do if necessary.
      Cost appears to be around $2k  but some cheaper options may solve any issues such as a small/med. unit in the attic, or individual units in different parts of the house.
A solar attic vent and a open window with a recirculating carbon rangehood in the kitchen and bathroom fans may be a cheaper alternative that was recommended.  However like I said I am not that familiar with these things, and I am concerned with providing enough makeup air for the woodstoves.


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## woodgeek (Jul 2, 2014)

Sounds like a plan.  Humidity control is improved summer and winter with air-sealing, and outdoor allergens (if an issue) are highly reduced.  HRVs are much less trouble prone than ERVs, and $2k will get you a basic system.  I just read a study that said that infiltrating air through small cracks in a house is often better filtered than air coming in from an HRV!   I would still wait for the blower door, and decide later between an exhaust only versus HRV based plan.


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## semipro (Jul 2, 2014)

woodgeek said:


> I just read a study that said that infiltrating air through small cracks in a house is often better filtered than air coming in from an HRV!


Probably because air entering the cracks then passes through fiberglass batts.  
I've seen enough dirty fiberglass insulation to know that it makes a great air filter.


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## maple1 (Jul 3, 2014)

In my limited experience & knowledge, I'm thinking that the price difference between an HRV & 'regular' ventilator should be minimal. I think in what I have seen on the building supply shelves here, it's only a couple hundred more for an HRV unit. I would think you would want each solution ducted the same - so based on that I would just get the HRV unit. We have one (a Venmar), put it in when we built 18 years ago. This past winter it didn't run much at all - but that might be because of the harsher than normal winter making for increased stack effect. Usually it runs intermittently to keep humidity below setpoint. It might also be because my timing & the weather was just right in the fall that my wood went in when it was pretty well bone dry.

Your location, and desire to tighten up your house as much as possible (that might keep going even beyond the spray foaming) also would lead me to the HRV route - but you might also be facing certain installation issues related to a retrofit situation that might complicate things. Which might turn out to be the tipping point.


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## Wildo (Jul 3, 2014)

If we go the full air exchange route the unit at the top of my list is a venmar eko 1.5 because of cost $1600 and its efficiency.  Right now my attic and 2nd floor have been gutted so I can do the install easily.


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