Insulating a skylight

[Kaymo]

Hi,
Wasn't sure where to post this as it could also have gone in roofing or windows, but there seem to be more conversations in this one about insulation so here goes!

I have recently moved into a 150 year old house, which has a 1980's L-shaped kitchen/living room extension at the back and 2 of these skylights. There are a number of things that could be done to improve the insulation including modern double glazing and wall insulation, but I suspect that this might be the most effective starting point. They are about 1.3m squared and appear to be an acrylic double bubble.

I am wondering firstly if I am right to see this as the most likely source of heat loss, and secondly what the best thing to do about it is?

My presumption is that I probably wouldn't get a huge improvement by changing the skylight itself, as any glass type material this big on the roof is bound to be a poor insulator but I have been considering the following options:

1. Cut a square piece of acrylic and fix to the bottom of the skylight shaft. (I had thought that as well as providing an extra layer of glazing it would also prevent convected heat going up into the skylight shaft area)
2. Make some type of shutter to close over at night or in winter. Presumably would need to be in wood.
3. Make some type of insulated bung to fix in place in winter and remove in summer

Just wondered if anyone had any thoughts, or experience on doing something similar?

 

[^woody^]

You will get a massive improvement in insulation values with a modern insulated surround and glass double glazing

 

[Kaymo]

Thanks Woody. I accept that there would be an insulation improvement with a modern skylight fitted. However, for a couple of skylights this size I would be looking at £1.5-2k not including installation and I am not convinced that I would ever see a return on investment at that price.

I am working on the assumption that I would make most of the insulation gains of a new skylight by blocking off the airflow to the skylight with one of the DIY options I mentioned at probably less than £100.

 

[^woody^]

So your not bothered about it looking crap?

By all means give it a go, but i cant see it making much difference. A flat piece of acrylic will be a bit wobbly at that span, and there will most likely be internal condensation to deal with.

 

[Kaymo]

I don't see why you would automatically assume that it would look crap, or that it wouldn't make any difference. The acrylic would probably be the least effective insulator amongst the options I listed but it would still make a difference because if nothing else it would prevent any convected heat entering the shaft, and reduce the chimney effect of heat being sucked up towards the coldest point. Back in January as a test I taped a polythene sheet across the opening and it made an immediate noticeable difference for this same reason.

I take your point that the acrylic might be too flexible and that's something along with condensation that I would have to consider, but I think if it was fixed to a frame and well sealed both of these issues might work out. A well sealed set of shutters or a semi-permanent insulating block would provide better insulation than any modern window unit but obviously blocks the light which is a trade-off.

To be honest if I had £3k in my back pocket I would just ring a roofer and get them to replace it but I don't, and in any case the ROI would probably be longer than the life of the window unit, which is why I am on a DIY forum hoping to discuss a more cost effective DIY solution.

 

[^woody^]

I think if you are going to do it then, it should be done properly. Either design something with a glass double glazed unit of safety and thermally coated glass. Or block it off with ply and frame and insulation layer.

There is always a potential condensation risk though with any such air void that you will create.

But as roof lights are always an asset, i do wonder if the negatives of any proposed work outweighs the benefits of just leaving them as is.

 

[unclebob1]

My dad did something similar, but he used a double glazed sealed unit.

Prepared a wooden frame of tapered, carved etc and primed timber battons, once ready, I held the glass unit up, while he secured the frame to the upstand.

Once in place, he topped the battons with draft excluder's and then filled and tidy'ed up the timber so cant be seen.

No problem with steam, but he has a vent in the gap between the bubble and the sealed unit to allow movement of air. Though this has introduced an element of how did that fly get in and die in there...

 

[London Mike 49]

If you search for insulated skylight covers there are some out there and they don't look that complicated to fit.

Mike

 

[Kaymo]

Thanks Mike, I took a look for these as suggested but the ones I found were very pricey, and one that had a price calculator, but for the size I am looking for the price was approaching the cost of a new skylight!

Thanks Unclebob, this sounds very similar to what I was thinking of doing, using shaped battens that curve to meet the existing ceiling, painted white to match the roof. The double glazed sealed unit is interesting, did a quick price check and these can be had for about £140 each. I wondered about this before but thought it might be too heavy. Do you remember roughly what size your skylight was, and where you got the sealed unit?

 

[unclebob1]

The sky light is prob about 50 by 100 (off the top of my head), the unit came from the factory where my dad works so cant help with that! Most high street glazing companies should be able to supply you with the unit - just make sure its either toughened or laminated, Explain to them where it is going, and make sure it complies with the regs.

 

[Kaymo]

Thanks Bob, will take a look at that. Mine is a good bit bigger which is why I am nervous about the weight but I should be able to find out an approximate weight before ordering.

 

[unclebob1]

Speak to glazing guys re weight - they should be able to give you that as its usually on all the delivery paper work for truck loading etc! is your surround plasterboard or plaster directly to the stand?


In my dad’s case, he had timber joist behind so screwed the frame directly to the joists along all four sides. if you have plasterboard then it will prob need cutting back to take the weight!

 

[tomfe]

4mm unit is 20kg/m2 6mm is 30kg/m2

If you just want to stop the drafts silicone the thing shut and put a bead around all the edges etc

 

[Kaymo]

Thanks Tomfe for the weights.

I considered sealing it shut, and probably will do that too but thought I would wait off on that because if I do close the shaft at the inside ceiling level the trickle draft might help with condensation, and if it doesn't I can always seal it from outside later.

I believe that closing it off at ceiling level would be the best solution because I have read that these shafts have a chimney effect, which tends to suck convected heat up into them, but mainly because the original skylight is 1980's construction with a large metal frame and thin acyclic type cover, both of which will be conducting a lot of heat to the outside. The decision I need to make is really whether to have a permanent clear solution like the double glazed unit, or an insulated bung that gets put in there in the winter - I've seen some examples of the latter in the northern USA where it gets really cold in the winter - although this might be overkill here.

Now that it is warming up a bit I have some time to make a decision

 

[Kaymo]

Thought I would just update this old thread with a pic as I went ahead and implemented what I was talking about above. Good spec of double glazing insert with low-e glass, its just sitting on the wooden battons, on which have a foam weather strip between the glass and the wood which compresses and seals. Has worked a treat and is barely noticable.