Insulate Masonry Fireplace

Our house has two large masonry fireplaces which we never use. In Winter, these are a source of cold air entering the house. The dampers are closed, but still a lot of cold air is admitted thru these fireplaces. Over the Summer, I want to insulate these openings so they don't allow cold air to come in next Winter. Thought I would get some styrofoam board and cut a plug to fit the from opening of the fireplace. Would appreciate any other ideas anyone may have.
Thanks, Des.
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Glass doors.

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I looked into glass doors and 99% have air gaps, Zero sealing, only maybe 1000 $ units looked good to stop air with only R 1 I took 3/4" L angle iron steel from Ace l and screwed them all around inside my fireplace with Tapcons Then 4" foamboard and stuck 1/2" stripmagnets on the foamboard , I painted the foamboard the color of the fireplace. It pops in or out and has 100% airtight seal with R 20 Cost apx 20 $ and all that cold is kept out.
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I have a cap that closes and pulls down , It doesnt stop cold air because the brick is cold, A plug is best good doors second best.
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Thanks for that. Sounds like a real good and inexpensive solution to the problem. I will give it a try. Des

around
the
Cost
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Ask this old house had an epsode on this, They installed a spring loaded vent cap. ~$400.00 It had a pull that you could open or close the vent cap from inside the fire box.
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That is a Lyemance Top Sealing Damper. I have them on my commercal site:
http://www.FireLogs.com/Lyemance.htm http://www.FireLogs.com/Literature-Lyemance.htm
John Galbreath Jr. http://www.FireLogs.com

fire
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