I use a reptile mat for seedlings so I'm not familiar with that one. I use corrugated cardboard instead of styrofoam and haven't had an issue. I'd be worried if I saw signs of melting. What temperature is it set to?

Geo, thanks a lot for the help. I wasn’t sure which foam to use and ended up getting this light blue foam that’s now melting

Would this pink foam be the type you’re talking about?

https://www.homedepot.com/p/Foamular-FOAMULAR-250-3-in-x-4-ft-x-8-ft-R-15-Scored-Squared-Edge-Rigid-Foam-Board-Insulation-Sheathing-9GB/207179251

Is it safer to maybe put the heat mats on 2x4’s? The foam definitely helped the mats keep the soil warmer when I tested cardboard, wood panels, and foam under the mats a couple of weeks ago, so hopefully I can find something similar without heat accidents.

Nik, I’m not sure how hot the actual mat is...I think around 90-95 degrees. I do know the soil(perlite) in the plant containers is 80-85 degrees. I have the soil thermometer in one of the cups.

Was also thinking of just putting aluminum foil between the heat mats and the styrofoam. Or just between the mat and my tabletop, and take the styrofoam out completey. Do you think that might keep the heat from being absorbed into the tabletop?

Foil would definitely NOT protect the tabletop. This is the foam i mean

https://www.homedepot.com/p/Thermasheath-Rmax-Thermasheath-3-2-in-x-4-ft-x-8-ft-R-13-1-Polyisocyanurate-Rigid-Foam-Insulation-Board-613010/100573703

Place foil on top of this if it isn't foil faced. Some is some isn't. The pink foam is exactly like the blue polystyrene from a different manufacturer.

Thanks geo. Unfortunately that foam you linked isn’t available at Home Depot or Lowe’s by me. Not sure why.

I will go to look for something like that. Not sure what the difference is with that and this blue foam I ended up with. Should I look for something with the highest “R” value I can find? I believe this light blue foam I’m using has an R-5 rating. It says a thermal barrier may be required, so maybe I can keep using this but put aluminum foil on top between the pots and this foam?

sorry for so many questions but I appreciate the help figuring this out. Here’s a picture of the one that burned

To protect the table surface from the heat mats I have (not sure which brand, but I can look later) I have silicone baking sheets between the mats and the wooden desk surface. They are safe to 450 degrees Fahrenheit and are relatively cheap. They are about the size of a cookie sheet and can be found in the cookware/bakeware section of stores like Walmart, Target, etc. Possibly even grocery stores. Usually rolled up to about the size of a paper towel roll prior to purchase for in-store display.

They may not provide any absorption/reflection benefits for your seedlings but at least you know they won't melt from the heat mats. I don't think any of them get that hot!

Tyler

Brendan... The type of foam you would need would be of a finer denser cell structure, generally of a tannish yellow or pale brown color. The difference chenically is that unlike the polystyrene, it doesnot have a true melting point, but eventually will char at a high temp but not at any temp a heat mat will get to.

Thanks Stewart and Geo.

Stewart that may be the easiest solution since I don't have multiple big box stores here to find the right foam unfortunately. I was surprised it started melting this foam, it gets hot but I didn't think it was that hot.

Geo, I know the type of high density foam actually, I built furniture out of it a few years ago (and coated it with liquid and spray car paint). I got that high density stuff in the city though so I'll have to see about whether I can find it upstate for my plants. Was really tough stuff, I had to use a heavy mallet and metal chisel to get into it.

Geo, last question...do you think I can just put aluminum foil underneath this foam that I have? Do you think that will help from the foam melting to the heat mat, or will the foil only cause it to get hotter/melt more?

Thanks again for the tips!

I use a reptile heat mat - thin and waterproof - and I regulate the temperature by plugging it into a lighting dimmer switch. A thermometer in the soil helps me keep it at 25 to 30 C.

Brendan... Sorry I've been late getting back to you, was away from my computer till a few hours ago. I would not use that blue foam at all in that situation. Even a couple of sheets of cardboard cut from a packing box will give you protection and more temp if nothing better is available, but foil on top of the foam would not help you much.

Ok thanks Geo, I'll probably just cut some large cardboard to lay underneath the mats.

I was worried that if the mats are hot enough to melt the foam, they might cause the cardboard to catch fire.

Brendan... Shame on you. Obviously you're not a Science Fiction fan or you would remember "Farenheit 451" book. Was about a future and book burnings and 451 F is supposedly the ignition temp of paper. 451F is about 232C so well above what that mat will do. LOL

Seriously, I'm just being silly. Heavy cardboard and perhaps a cheet of foil on top should work quite well. Have a great and happy holiday season.

Hah thanks Geo :)

i put cardboard hnder them all today. I still can’t beleive that foam was melting so much. The mats are hot but they don’t seem that hot. Hopefully this will still keep the plants happy. Have a great holiday

Hi @geoforce

I'm having a hard time finding that insulation with the foil side (and high R value) at any stores near me.

I found a blue foam that has an R value of 13 at my home depot, but I'm worried about it melting because it's the same brand as my previous blue foam (R value 5)

Do you know if simple drywall with tinfoil on top might be good for insulating the heat from the heat mat?

I've read it needs to be above 175F to burn, and I don't think the mat gets that hot.

Heading to Lowes and Home Depot today to look for something since the soil temps aren't as high on the cardboard as they were with the foam

Drywall is certainly unmeltable, but it's a lousy insulator. Can't believe you can't find the right stuff at your lowes. Here is a lowes link to the kind that is best for what you are looking for:

https://www.lowes.com/pd/Johns-Manville-Common-1-in-x-4-ft-x-8-ft-Actual-1-in-x-4-ft-x-8-ft-AP-Foil-R-6-Faced-Polyisocyanurate-Foam-Board-Insulation/3851111

Every store in my area has it and you can check on lowes site to check in stores in your ares.

Thanks geo, I will look for that Manville foam today. I think I've seen that kind there now that I think of it. Thanks again!

I am not sure if R value really matters so much for your purpose? I have used heat mat - for starting vegetable seeds in spring - and without any protection. It was on a utility-type of desk so I didn't worry much about any damage, but there was none from heat. More from the water I spilled! I also used reptile heat mat and that seems to be hotter; inserted only cardboard from a flattened box under.

If you worry about damaging tabletop, maybe you can use just a plank of wood under the mat? Perhaps you are over thinking it? Hope you don't take this as criticism! Just trying to get some simple ideas...

Some years ago I made a larger wooden frame from 2x6. Made bottom from a metal mesh/lath like this (supported in intervals, so it won't collapse):

(I had some on hand leftover from a tiling job - not the best because it is sharp, but it worked :) Then I put a layer of filter cloth over it. Next was layer of sand, in which I buried heat cable (type used for roof de-icing). Plants were all potted, and I just set them on top of the sand. It was quite good to give them bottom heat. I do not have it any more because of space - it was quite large to keep many pots on. It was kept in a room that got close to 0*C in winter, but roots of plants were just fine. This would be overkill for your purpose I think...but could be made similarly with the heat mat. I would use a piece of Styrofoam from packaging if using sand, and set the heat mat on top of it. Maybe a tray could be used, so any water will drain into it.

Hi Rina, Thanks for the tips! I like the idea of a metal grid and heat cables. I was actually looking at heat cables the other day at Home Depot.

Do you know what temp the soil in your pots was on that setup you built?

I am trying not to overthink this experiment I'm doing with semi-hydro and heat mats, but I had to come up with another way to insulate the heat from my mats when I realized it was melting the foam it was on top of. The foam really helped the heat mats transfer the heat upward and keep the perlite in the plants cups aroung 84-88 degrees F which the plants seemed to love.

Unfortunately the heat mats only get to be a little hotter than the ambient room temperature. After reading as much as I could about heat mats, it seems this is pretty common. But that meant that heat in my room has to stay pretty high, so I'm hoping to find a way to use an insulator and keep the room temp a little lower. I'm going to try geoforce's insulation foam idea once I get back to my house. We'll see how it works.

Brendan

Sorry, I do not know how warm it was - I didn't take temps then, and it was a while ago. It worked very well, so I am assuming it was all ok. I buried cable because I didn't want to 'cook' the plants, just keep roots warmer. If I had unheated GH now, I would likely do the same. I kept overwintering plants on that table. Succulents overwinter well if kept dry and cool, temps. around 41-45*F are ok for that. If it is warmer (my conditions now are approx. lowest temp 58*F or so), plants would be watered a bit, more often if it is warmer. IMO, growing them hydro/semi-hydro would be different since roots are moist and needs to be warmer - but I never tried that. Mesh on the bottom was there because as it got warmer in Spring, I started watering all of them and it didn't matter if water leaked below.

For semi-hydro, I think I would try layer of sand in a tray, with heat mat on top of it. Np with sand melting :) Maybe I should try that before I speak...:)

Heat mats works very well when I start seeds in Spring, so I am assuming they would do the same for plants on pots.