Box in a box

[nanuuk]

Due to a fire 3 years ago our freezer roof was comprimised and being in a rush it was decided to build a freezer in a freezer if you will.What was done was a zeroloc paneled box was built within the walls of an existing room which is about 200'x200'x32'. thay used 6" thick panels and the box is not much smaller than the existing room. There is a space between the new zeroloc roof and the old roof due to wilson trusses of about 4'and this created a problem with moisture in the form of water because the air space is quite cold about 20 degrees. We put a dehumidifier on the roof which worked to dry the space but the cost was high to keep this option and a new roof was put on this section of the building because it was due to be replaced anyway and yes this option should of been done from the begining (politics) so the dehumidifier was removed. So now we are back to where we were before the fire with a good vapour barriered roof but because of this crawl space we are now facing a frost problem. We have been looking at removing the zeroloc roof but the cost is huge any suggestions on how to get rid of the horror frost problem with out going back to a dehumidifier.

[Brian_UK]

Just a clarification question due to worldwide readers....

is your 20° F or C ?

[nanuuk]

Sorry about that 20F.

[Brian_UK]

Thanks, thought so but best to check

[frank]

So..... you have a condensation problem in the space between the old freezer and the new freezer.

Did this problem occur before the fire?
What I'm trying to say is - before the fire, you had a freezer with no condensation problems? right?

But since the fire, you have constructed another freezer in the same space but you have condensation problems.

It seems like you have a vapour barrier problem or insufficient insulation since the rebuild.

[Plank!]

Sounds like you need air movement between the two ceilings. You have an insulated box surrounding a working freezer. No matter how good the insulation is, its going to get cold in that space with out large volumes of air through it.
What you have now is a panel twice as thick as before, with a gap in the middle. Where does it get any heat to keep the condensation down, I'd be looking at putting a row of fans along the middle of the roof, with vents around the sides, blow ambient air thru the void.

Would maybe be worth checking out Franks idea too, get a thermal scan of the panels, just to be on the safe side.

Regards
Steve

[nanuuk]

The fan idea or fans idea is what we are looking at doing and here's how we are thinking of doing it. The evaps 4 of them in all are 3' from the cieling evenly spaced out along the back of the freezer. We are looking at making 4 openings one infront of each evap with an intake fan and ducting do direct the air flow then doing the same at the other end with 4 fans pulling the air back into the freezer and using the evaps to act as an dehumidifier to help dry the air in the crawl space and hopefully reaching freezer temp in the crawl space what do you think is it worth a try. The other suggestion made to me was put more openings in the ceiling but I don't know if that will help move that air out of the crawl space without making the a lot of openings in the ceiling I mean how many openings are we talking here it's a large ceiling?

[nanuuk]

No takers on any suggestions sure would like to hear some ones position on the fan idea good or bad thanx.

[nh3wizard]

I believe getting some air movement across / between the two roofs is your least expensive route, intakes and exhaust. Is this happening across the whole ceiling area or in spots? 200 x 200 is a huge area to be moving the air.

[nanuuk]

Yes that is correct we have to move the air accross the whole ceiling area. The crawl space is devided into 5 sections due to I beam support for the roof so each section is apporox 40'x200'. We have added an extra opening up at each end so there is 5 on one side and 5 at the other end in order to ventilate each cavity if we go that root.

[Plank!]

Do you have trace heating between the wall panels?
I'm concerned that the moisture getting in at roof level is also going to freeze between the two walls, this will eventually push the new walls into the chamber.

I think that if the second roof can't be removed the dehumidifier is the best option.

Regards
Steve

[Gary]

I'm wondering if the cavity couldn't be pressurized with air from the freezer in order to prevent moist outdoor air from entering.

Cold air is dry air, therefore the moisture in the cavity must be coming from the outside. Any openings to the outside should be tightly sealed and any access to the cavity area should be from inside the freezer.

[Gary]

I'm concerned that the moisture getting in at roof level is also going to freeze between the two walls, this will eventually push the new walls into the chamber.

If the space between the walls is minimal, perhaps the void could be filled with sprayed-in foam.

[Gary]

We are looking at making 4 openings one infront of each evap with an intake fan and ducting do direct the air flow then doing the same at the other end with 4 fans pulling the air back into the freezer and using the evaps to act as an dehumidifier to help dry the air in the crawl space and hopefully reaching freezer temp in the crawl space what do you think is it worth a try.

Rather than using fans to pull the air back in, why not use pressure relief valves, thus pressurizing the cavity and preventing moist outside air from entering?

It would not be necessary nor desireable to reach freezer temp in the cavity. All it really needs is dry air. Even the existing frost would eventually sublimate.

[nanuuk]

We looked at filling the voids with foam because it would be a one time fix hopefully and would improve our R factor but the voids are many so the cost was a problem. On the note of pressurizing the void what do you suggest? What type of relief v/vs do you reccomend looking at to use the idea sounds interesting thanx very much for the time Gary.

[Brian_UK]

Rather than using fans to pull the air back in, why not use pressure relief valves, thus pressurizing the cavity and preventing moist outside air from entering?

I may be missing something here Gary so please bear with me...

I understand the idea of PRVs but unless there is a supply fan somewhere feeding into the cold room how is the air going to get pressurised from the cold room into the crawl space?

[Gary]

Fans would pump cold room air up into the cavity. Excess cavity air would be relieved back into the cold room at the opposite end.

To avoid causing a vacuum in the cold room, the cavity would need to be tightly sealed.

[Brian_UK]

OK, got you, thanks Gary.

[Gary]

We looked at filling the voids with foam because it would be a one time fix hopefully and would improve our R factor but the voids are many so the cost was a problem. On the note of pressurizing the void what do you suggest? What type of relief v/vs do you reccomend looking at to use the idea sounds interesting thanx very much for the time Gary.

Cold room manufacturers have pressure reliefs designed specifically for cold rooms.