Insulating a Log Cabin Floor and Roof

If you were one of my customers and you were buying a lovely new log cabin from me, especially one of our thicker wall log cabins such as 50mm upwards, I would be strongly urging you to insulate at the least the floor of your new log cabin. I would also try to nudge you to insulate the roof as well.

You may also be interested in this post on double glazing, R and U values and Log cabin thermal properties: Double Glazing in Log Cabins

The benefits are obvious for you.  You’ve decided upon your building, you’ve weighed up the benefits of the thicker logs and of course the double glazing.  But, a lot of heat is lost from the floor and it’s cold rising up and of course loads is lost through the roof.  Ideally we want these areas insulated and to the same or similar as the wall thickness.

Lots of retailers supply ‘insulation kits’ with their buildings, we don’t, but we could, we could make a bit of money out of it as well.  But seeing as the cabins are costly enough as it is do you really want to add more cost if you can help it.  So, instead of me supplying you a special insulation package and making some money from it I’ll tell you how to do it yourself and save money or better still spend the saving on better quality insulation.  All the insulation I talk about is ordered through any builders merchant, most of which will deliver to you at the fraction of the cost of a retailers special ‘Insulation pack’.

I like the Celotex brand of board, I’ve used several types over the years but get on best with this one.

celotex-log-cabin-insulation

Here’s a link where you can download more details on the product:  Product Details

I know lots of other manufacturers do a similar product, some better and some worse, that part is up to you but I prefer a solid fibre board to work with as above.

Of course there are lots of ways to accomplish an insulated roof and floor.  Some fitters favour adding it underneath the roof and boarding it out thus keeping the insulation in.  Some will put a frame on top of the roof and use rockwool and ply over the top.  I have never used these methods as I can’t see the benefit other than perhaps a saving in the insulation material cost itself.

So, my rough and simple guide on how to insulate your roof and floor of your new log cabin.

Insulate the floor

A quick one before explaining this:  Have you considered a DPM?  A damp proof membrane either within your base or on top of it.  It’s well worth it and prevents any damp coming up and into your building. (http://en.wikipedia.org/wiki/Damp_proofing)

I build my log cabin as usual on the 44mm tanalised timber foundation beams we supply with every building and I’ve now completed the build.  I’m left with the floor to put down and the roof covering to apply.  If I’m using a floor pack I will set out my floor bearers as normal.  I then cut up my insulation board which I’ve ordered from the local builders merchant at 50mm deep.  This sits perfectly between and within the bearers, the joists support my build while the insulation boards supports it. If you want to be exact to the joists then use 40mm.

Insulation board is placed within the bearers

Insulation board is placed within the bearers

Now I simply lay my floor boards as normal, happy that the floor is insulted.

Another method is to not bother with the floor pack and to fill the inner area of the cabin with the insulation boards.  On top of that you use far cheaper OSB sheets or chipboard flooring, this is especially relevant if you are later putting down carpet as OSB is certainly cheaper than our nice T&G pine.

Obviously you’ll need to work out how much board you need with a simple calculation of length x breadth to find the square meter and order the equivalent from your local builders merchant.

Insulating the Roof of your log cabin:

The roof is a little trickier to do and takes a little more work.  Before we start you need to decide what thickness of insulation board to use.  50mm, the same as the floor is very convenient and often used.  You could also go up to 70mm to gain the same R value.  I have also used 100mm when specified by planners.  Regardless the same principle applies.

Work out how much you need by calculating one side of the roof area and times it by two.  As well as the insulation boards you will also need to order longer clout nails.  These need to be long enough to go through the final roof surface, insulation and into the roof timber boards.  If you’re using 50mm insulation then order 65mm nails for the flats of the roof and 70mm clout nails for the ridges.

Lay the boards so they are flush with the leading edge.  Bare in mind this is going to be exposed so consider how you’re going to cover it.  In this example we were using 50mm board and turned the roof trim the other way up:

Roof trim along the leading edge ready for the insulation board

Roof trim along the leading edge ready for the insulation board

You can also cover this portion later with additional timber but it is worth considering it at this point.  You may need to source locally the additional trim timber.

Now lay one layer of insulation boards and fix into place using one clout nail in each corner and one in the center.  You can then felt or shingle it up to that first board.  Don’t be tempted to do the whole roof with the insulation as you will eventually have to get on the roof to tile or felt it and with the whole roof done it can be very slippery.

Once a board is complete with tiles or felt move on to the next and carry on adding boards and tiles until you reach the top.

Insulating a log cabin roof

Insulating a log cabin roof

For tips on shingling your roof please see this post: https://www.tuin.co.uk/blog/fitting-felt-shingles-for-log-cabins/

insulate-log-cabin

The last consideration is the bargeboards to the front and back.  You can either move the supplied one up or double up the barge boards as below, again you may need to source this additional timber locally:

bargeboard

The same principle also applied to hipped, octagonal and hexagonal roof.  The only slight difference is that  you will finish the corners of the ridges slightly differently where by you will cut them flush with the end of the roof boards.  You will then cut a fillet to fill in the ‘v’ that naturally forms.

One last tip, if you haven’t got a timber saw or a proper board saw, your wives bread knife works a treat for cutting the insulation boards!

Insulated Log Cabin

Insulated Log Cabin

More on insulating a log cabin can be seen here, it’s a bit of a rant about the current trends in the ‘Log cabin industry’ and all the rip off’s that abound. Please see here: http://www.tuin.co.uk/blog/more-on-log-cabin-insulation/

You may also be interested in this post on double glazing, R and U values and Log cabin thermal properties: Double Glazing in Log Cabins

Please see the following article of how to make insulated walls, partition walls and how to use thicker insulation in the roof: Dealing with expansion and contraction in Log Cabins

55 thoughts on “Insulating a Log Cabin Floor and Roof

  1. This is a great guide, but I have one question. Once you’ve put the celotex on the roof, do you need to board over the top of it with OSB or similar before laying the felt or singles?

    • If you’re using 45mm- 50mm it’s not necessary to board it in my opinion as the nails will go through to the roof below and still provide a good fixing. If you’re using anything thicker then the roof would be battened and OSB or similar would go on top to enable the final covering to be fixed.

        • I’ve recently done this with 50mm celotex and it’s a perfectly good method. I ordered 70mm clout nails and the only issue I found is that as you’ve covered up the boards with insulation you can’t tell where the nails will come through, so you sometimes hit the T&G joint between boards and can see nail points in the joint underneath (if you’re unlucky enough to hit them exactly). If I were doing it again I’d go with 65mm clout nails.

  2. I will soon be insulating a flat roof with the EPDM sheet on a Yorick cabin. I have been wondering firstly is it worth using a vapour control layer over the deck before I put 75/100mm of polystyrene on and do i fix the insulation to the deck or just rely on the weight of the rubber over the top and… should i board over the top of the polystyrene? Thanks for your advice in advance!

    • For a flat roof it is better to batten around the side and then across the roof, infill this will your chosen insulation and then a board of OSB or similar is fixed on top. The EPDM is then glued to the board.

  3. Hi

    Will be attempring to insulate my Emma log cabin when it comes next week.

    Any tips for insulating a pyramid roof or are there any videos showing how?

    Do I need to install a breather membrane between the roof and 50mm celotex and batten round the edges?

    Kind regards

    Jay

    • Using insulation on a pyramid roof is the same as an apex roof. You will need a deeper bargeboard or batten to conceal the insulation at the bottom the same as an apex roof. I do not normally use a breather membrane and have not come across a reason why this would be necessary on a log cabin roof.

  4. We’re thinking of getting a cabin for use as a garden office/den. Obviously we want it to be warm in Winter and cool in Summer with as little input of power as possible. Therefore insulating is important. I’m OK with doing the roof and floor but what about the walls? Probably will go for 58mm walls but how would you fit celotex to the inside? Normally I’d batten the walls, fit the boards inside and then clad but obviously if the wall boards can’t be fixed together this won’t work.

  5. Hi Today I Insulated the outside of my log cabin roof. Didn’t want to use the rigid board as I am using part of inside as sauna. Obviously building in framing expansion method used insulation foil etc etc.
    Any way back to the roof. I figured after talking to the joiner across from me that there would be a risk of sweating between cold foil and hot roof, even in heated non sauna part.
    So I came up with this design. 47×75 on there edge all round outside, then every 600 I put 3 400 batons with a gap between them on there edge of course 75 up.After this I filled it with 50mm Rockwool flexi, this left me a 25mm air gap above. On top of the framing I put 18mm OSB 3. Then I fitted a 120mm x20 board in redwood from builders merchant round the bottom edge to hide the framing and match the cabin. After this I drilled through 25 mm holes aligned with the air gap above insulation and popped in manthorpe g952 round vents. It works a treat and is cheaper, more effective as it breathes and looks sturdy. Plus you have a solid base for your clout nails instead of bashing through insulation which sounds a bit too flimsy for me. I’m glad I spoke to the joiner before I started this. It’s more work but it’s definately worth it.

    Sauna insulation on log cabin roof
    log cabin insulation

    • Thank you for this information, it’s an alternative method for others to consider if there is a sauna inside. It could possibly be over complicating it but the vents are very important if you have an air gap.

  6. How would you insulate the floor if building on a timber base? Confused were the DPM would sit, and if you put the insulation in the bottom frame, or if you need to build a further frame on top to add insulation & flooring?

    • If you are using a timber frame as your base then you would insulate between the joists that are integral to the base. You would not usually make a seperate frame as it would not be needed, the floor boards would then go directly on top of the joists within the frame. This is an example where the boards go directly on top of the joists within the frame.
      Floor joists within the frame

      But, there are no hard and fast rules on this, you can also put the floor joists on top of the frame as per this example:
      Floor joists on top of the frame

      In both of these examples the insulation would go between the joists.

      Regarding DPM the purpose of this is to stop ground contact and damp coming up through the ground. With a timber frame base you are already taking the building away from the ground so the task has already been accomplished.

  7. Thanks for you website – info is soooooooo helpful.
    I have a question about your method of insulating the roof – why do you not need an air gap between the insulation and the roofing material? Will condensation not then build up around the insulation and eventually cause the roof boards to rot?

    • Condensation can become a big problem when insulating under the roof boards, when it is on top I have never had a problem as the moisture build up is inside the building not on top of the roof. It’s been nearly twenty years since I have been doing it this way and I have not had a call back so far. But, I maybe wrong and others may have a different viewpoint or thoughts and I would be pleased to hear them if anyone thinks my personal method is wrong.

  8. Thanks for a very informative site:Regarding insulating the floor i have ordered a 27mm floor pack for my Stian which will be sitting on the composite foundation beams.I was wondering as the joist are 44m and fitting 50mm insulation will this mean that the boards will sit on the insulation with a 6 mm gap to the joists below.I was considering blind screwing using Lost-Tite Screw however will this not distort boards when fixing ?

    • If you are using 50mm insulation you will have a gap between the joist and floor board due to the joist height. However, the insulation we recommend is load bearing and the floor boards will sit directly onto this and supported by the insulation board, the joists are only there to secure the boards to and carry no load.

      I have not noticed boards distorting when nailed but I have not used the fixings you are referring to.

      Alternatively a thinner insulation can also be used.

      • I had the same concern when insulating my floor so I used 40mm celotex instead of 50mm just to make sure. However we are not planning on using the cabin in the depths of Winter so the slight decrease in insulation shouldn’t make too much difference.

  9. Some of the videos I’ve seen show people fitting a wooden frame around the cabin roof and then fitting the insulation boards inside this wooden frame, would you recommend this? My concern is that if the insulation is open to wildlife on the side elevations of the ‘Royal Two Log’ cabin they may burrow in to it or peck at it and damage the insulation.

    • The Royal Two is an apex log cabin. If you are using 50mm or less then the side elevation and also the apex will be covered by the boards at the side and front and no entry should be made. You may though need to source slightly wider boards for the leading edge locally.

      If you are putting in thicker insulation then it is a good idea to cell the roof with framing and then a sheet of OSB / ply on top of that. The final roof covering would then attach to that.

      If you have a flat roof log cabin then is is also necessary to insulate in this way as the final roof covering will be felt, EPDM or roof membrane which cannot be fitted like felt shingles.

      The leading edge and apexes would finish as normal.

      • My log cabin has an apex roof. I was planning on using some 40-50mm insulation on the outside as previously mentioned and then put some good quality felt over the top. I notice though that above you say that would not be possible with felt only shingles. Why is that?

        • If you are using shingles these are very hard to fold over flat due to the thickness of them. Felt is far thinner and can be folded easily and trimmed. If you need to cover exposed insulation you can use a timber trim on the leading edge which is usual or raise / wider barge boards on the apex. But, if you wish to use felt under the roof shingles this can then be folded over. I have seen some people use this method OVER the leading edge trims as they will eventually rot as they are exposed and a felt folded over will protect them.

  10. We have a log cabin that uses 6 foundation beams that run in parallel rather than around the perimeter. If we were to insulate the floor then the insulation would be exposed at the edges. Is this okay or could water be soaked up through the insulation and into the floorboards? If I were to cover the gaps between the parallel beams would I need to put ventilation holes in?

    • Other suppliers do things a different way, I do not see why they use parallel beams and would suggest our method of a ring beam every time. But you have to make sure the base is to the footprint of the building and that it is flat and level. This will create a natural seal. A layer of DPC also helps with this.

      If you have parallel beams you will need to seal the edges, I would suggest infilling with timber the gaps. Make sure though your base is sealed. In extreme infested areas such as farms I have known customers to lay chicken wire in before applying insulation.

      Ventilation holes are only necessary if you expect water within the bearers caused by not using a DPM or a bad, none sealed base.

  11. I’ve just managed to insulate and shingle one side of my Lotte roof but it’s now raining overnight and is forecast to continue tomorrow. The completed sides of the celotex (it’s not actually celotex but recticel, basically the same stuff) are exposed to the rain. Will it be ok?

    • I remember fitting a huge building that took over three days to finish the roof, it was the height of winter and during those three days we had snow, slush and rain on the roof with exposed celotex and it did not come to any harm, I have not used your make of insulation but I would imagine it will be fine if it is a similar material.

  12. Hi I have just order foil board insulation for the roof . I have gone with 60mm and was intending to fix direct to roof and the the fix the shingles … I have just read down this post and see you have started to talk about framing and osb if the insulation is greater than 50mm. Do you see any issues if I don’t frame it with the 60mm board

    Regards. Jonathan

    • When you go over 50mm you’re asking a lot of the nails holding that depth of insulation, Also I’m not sure if you can get nails long enough. I would not use anything above 45 – 50mm thick insulation without framing and boarding the roof to make sure there are no problems.

      Of course though, it may well be fine, but it is not something I have done or could recommend.

  13. Hi there, I have an Eila on order and am just finalising the insulation. I am using 50mm foil backed boards (celotex or similar). Can I stick EPDM direct to the insulation boards or will I have to board over the celotex with some ply or osb? Thanks, Tim

    • If you have a flat roof is is far better to frame it and put OSB or similar on top with the insulation inside. EPDM or the Roof Membrane will not stick well to just the insulation board.

  14. Richard I am currently waiting for the arrival of my Justine log cabin and have started to look at insulation for the roof and floor. Is it the Celotex FR5000 that you would recommend?

    • I normally like the foil back board, it is the Celotex PIR (Polyisocyanurate) 50mm x 1200 x 2400mm. I have recently been told though that kingspan offer a similar board that has higher thermal performance so that maybe worth looking at too.

  15. This is a great article and I’m planning on insulating a cabin extension soon. I recall cutting the insulation boards in my existing cabin and found this to be a pain to do. Been watching some youtube videos on cutting boards but great to hear your preferred technique. Afraid, no specialist table saws in my home!

    For the OSB boards I will be getting it from the timber supplier but they don’t do insulation boards.

    • I’m pleased to hear this article is a help to you. The best place for insulation boards is a general builders supply yard such as Jewsons, Travis Perkins etc.

      • Indeed but they come in standard sizes and I believe none will do any cuts for you like they may for timber.

        Thinking I will use my patio chairs as cutting horses, clamp a trim on both ends cut along with a sharpened wall paper remover. Well that’s the theory at least…

        • I do not know any company that will make the cuts for you. It is easy though, a timber saw or an old bread knife with cut through it easily as will a stanley blade for precision.

  16. Hi Richard; Lots of great info as usual!

    Just took delivery of a Flow Cabin, and was wondering about the following:
    a) Using loft style insulation under the (floating) floor?

    b) Using same insulation, inside the cabin roof plus plasterboard and therefore any condensation issues?

    All much cheaper and similar R values to kingspan/Celotex etc.

    c)If I were to use 50mm Celotex on top of the roof, can I fix EDPM directly to it?

    Many thanks

    Tim Brigg

    • I’m pleased to hear the flow log cabin arrived safely for you.

      I cannot see any reason why fiberglass insulation cannot be used in the floor. I do prefer something like Celotex though as it’s easier to use and handle and does not compress.

      I would not recommend at all insulating the roof inside. Condensation can be a big issue, you will need to make sure it is well vented and I cannot give advice on what and how much. If the insulation is on top of the roof you do not ever have to worry about it.

      The flow is a flat roof log cabin. It is best to cell the roof with framing and then insulate with whatever material you choose and then put an OSB or similar board on top, then apply the final roof covering.

      EPDM or ERM will not be able to fixed directly on top of any insulation. Only felt tiles can do this with at least three clot nails used.

      EPDM or ERM is glued onto the surface and I would not recommend it being glued directly to the insulation.

      • Thanks Richard.
        One last question, hopefully;
        As we have a raised wooden base, supported on brick columns, is there any need to fix a DPM to the underside of it?
        If I do use loft insulation in the floor, I would as it would support the insulation, but celotex would be self supporting as it can fit in snug between the joists.
        Thanks again,
        Tim 🙂

        • I think you may as well fix a DPM, damp is still rising from the ground and it would not hurt to do it. If you use celotex you can fix battens either side of the joist to support the panel.

      • Hi Richard,

        Thanks for the help once again.
        Cabin up, mostly straightforward (purlins are hard work though!).
        Anyway, re your suggestion to construct roof with boards first, then kingspan and 11mm OSB covered in EPDM:
        Do you see any problem using the same order of things, except fixing the OSB directly to the top of the cabin, then working in reverse with kingspan next and have the roof boards underneath (between the purlins – a minor faff as they would need cut to size).
        This is only to reduce the height a little, and keep the barge boards fixed “downwards” which is my preferred option.

        Many thanks

        Tim 🙂

        • I would not recommend doing this as you are then putting the insulation inside the building. This can then cause problems with condensation. If this is the only way to do it make sure you create a void and vent it to allow condensation to escape. My preferred method is always to have the insulation on top of the roof.

    • Alternate to OSB I believe is marine ply. Think OSB would need to be treated. I believe a google search for warm deck roofs will provide a good selection of articles including one from rubber4roof from which I will be getting my EPDM. They also have some useful YouTube videos to guide you along…

      • You are correct regarding the Marine Ply. In my experience though I have never needed to treat the OSB. I like the Rubber4Roof site and it is cheaper than us for EPDM, they have a quicker delivery as well for it, I often recommend to customers to try them or even ebay for sourcing EPDM.

        Regardless where you get it from EPDM is the very best product to use on a flat roof.

  17. Hi Richard, I’ve just received the composite foundation beams and they sit 10mm proud all around my 100mm concrete base with 20mm gravel outside of this. I want to put in an insulated floor with celotex as you’ve suggested. Do I just put in some floor bearers inside, on the slab floor as the log cabin will sit on the profiled composite foundation beams and then add in the celotex inbetween before using the T&G flooring on top? The tanalised floor bearers will therefore sit on the concrete and would that allow damp to rise into the floor or sides of the cabin? Should I be sealing the profiled foundation beams all around the perimeter to stop any water flow? Thanks. Mark

    • Once the cabin is built on the foundation beams if you are using them then the floor ban be put in. If you choose to use floor joists these can be laid with the insulation in between them.

      It is important though to make sure you have a damp proof membrane in the base or at least on top of it. Please see the information I offer on log cabin bases for examples.

      If your base is not 100% level and you have gaps between the base and foundation then you will need to seal this.

  18. Hi Richard,

    To avoid nail so many holes through insulation board when we put nail through shingles and insulation board to the roof board, will it be better put a OSB board on top of insulation board,fix them to the original roof board, then nail the shingles to the OSB boards? I mean for 50mm insulation board. Do you think will it be ok? Also is it ok to put one layer underlay felt under shingles to make extra protection?

    Many thanks

    Jane

  19. Hi Richard,
    In the information you give regarding roof and floor insulation, you mention that some people insulate the roof from the inside of their cabins. Although you say that your preferred method is on top of the roof you don’t actually dismiss doing it this way. Leading us to think that it can be done internally. However in the part entitled insulating the roof you then say it causes condensation doing it this way. A contradiction of paragraph one.
    Can you please advise us as unfortunately we didn’t realise that there could be a problem insulating from underneath the roof. Sadly we have already felted the roof in readiness for the Shirley to be added. Help please!
    Kind regards
    Jim & Ann Smith

    • I always prefer to insulate on top of the roof, this takes all the problems associated with condensation out of the roof. Of course I maybe wrong but it has worked well over the years for me.

      You can though insulate inside if you wish to. But, like any building including your house, allowances have to made for the moisture we expel living in it, the appliances we have and the moisture coming from the surrounding atmosphere and of course the ground / base.

      Linked above in this artice is another article where I give advice on insulating within a building: Dealing with Contraction and Expansion of Log Cabins

      The examples given should help to answer your question and avoid a contradiction, if any is found. The main point being to make sure you create a void and to ventilate that void well to allow any moisture to escape.

      Additionally insulating on top of the roof saves you unnecessary expense, a great deal of work, additional cladding or fascia and allows full height of the cabin internally. On occasions this is not always possible due to height restrictions. If you are forced to you can insulate under the roof but please make sure the void is vented otherwise, eventually the insulation wall fall down with moisture / water build up.

      We don’t really find felt necessary under shingles I do not recommend this personally, if the shingles are fitted correctly and as advised. The felt is only there to protect your building should the shingles blow off (apex roofs). If of course they are fitted as I advise they will not come off. My own, personal recommendation would be to leave the felt as is. Apply the insulation and then the shingles. You may need longer clout nails for this. Alternatively remove the felt completely. If you wish as some people do you can add a breathable membrane between the T&G roof and the insulation although I have not found this necessary either. Then you can add the final shingles. Alternatively, insulate from underneath leaving a void and a vent. What this void depth maybe I cannot advise as I have not done this.

  20. Believe insulating under the roof is referred to as cold deck construction. For the void I’ve seen a figure of 50mm but also seen a solution using rock wool (breathable insulation) over rigid foam (polystyrene) over vapour barrier and finally over roof cladding. Think on this blog I’ve also seen someone installing air vent grills into the void. Also seen a YouTube video that showed a simpler installation using multi layered foil insulation blanket tacked under roof boards and then clad. Looked really easy actually though understand the blanket may not be the cheapest material.

    Currently stuck on my project so doing more research on this topic then I really should.

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