I converted a bungalow a few years ago with a new roof, in each bedroom we have one Velux and one normal window. We are just getting around to sorting out the curtains and blinds.

My better half is going to try and make some blackout Roman blinds for the south facing windows and I’m looking at the rather extensive choice of Velux Blinds.. for the Velux windows which face East and West.

We would like black out material as I sometimes work shifts, my question is around heat gain. Velux do various options which they claim help reduce heat gain in the rooms, both internal blinds and external shutters or blinds. I can understand the external options are better as they stop the heat coming into the room to start with, how much better though? The come at quite an extra cost. The external shutters are around £650 per window! The external blinds are about £400. The internal blinds run in rails so will trap only about 5mm of air between it and the inside pane of glass which could get heated.. Does that make much of a difference to the whole room? Internal blinds are about £200 ish..

I also don’t want to spend a lot of time, effort and money to make perfectly heat resistant Velux windows if most of the heating comes from all the roof and external walls in a bungalow conversion anyway. Does anyone know the figures roughly? I understand it obviously depends how well insulated your roof is but a general idea would help. Mine is better insulated than building regs but by no means passive.. Last summer during the peak it certainly got very warm at night without any curtains or blinds beneath the new roof.

The main window is south facing so again am conscious of spending money on the East/West Velux if most heat is going to enter through this south facing window. We can and will close the black out Roman blinds on this window during the day to help.. is there another material my other half can sew into the blind to help reflect the heat, albeit inside the room already?

I hope you understand my questions. Anyone with any experience of Velux heat and black out blinds before and after?

Am I just better to spend the extra money on a good bedroom fan!?

Many thanks..

As a guide, external blinds are going to be better at blocking the sun than internal blinds. For internal blinds to keep heat out, they need to reflect a high proportion of the sunlight back through the window before it's absorbed - which is what external blinds do. So reflective slatted internal blinds (silver Venetians?) may be reasonably effective, as might a metallic external facing lining in a non-slatted internal blind.

Evaluating blinds against the rest of the room may not be easy. If you have very good airtightness and insulation, that will limit the heat gain through ventilation and the building fabric, and a reflective roof surface / coating would be a big plus. MVHR with a proper summer bypass will help too.

FWIW I expect to spend a similar amount on slatted timber external blinds per window for my refurb in France (plus high airtightness levels & reasonably high insulation, plus MVHR).

I am wrestling with the same issue as you. I have blackout blinds on my Velux windows which are the genuine Velux articles not generic equivalents (you get what you pay for I've discovered!). Whilst these are good at blocking the light (except around the margins to some extent) the heat gain is still huge during the summer months. Unfortunately, like you, I cannot quantify the the relative proportions of heating from the windows versus that from the roof/walls. I have added 5cm of IWI (Celotex) to the sloping ceilings and the dwarf walls so that has had some effect but not as much as I had hoped.

The larger bedroom has two large Velux windows (1.3.m x 1.3m.) with a total glass area of 3.4 sq.m. The sloping ceiling is 9.7 sq.m. so the area % of glass is approx 35%. My gut feeling is that most of the heat gain has to be from the windows. I am contemplating trying a solar film like this:

https://www.windowfilm.co.uk/buy-online/window-film-by-the-metre/solar-control/high-reflective-silver-window-film

(I have no connection with this company - I just Googled solar window film. Other suppliers are available!).

It works out at about £50 per window. However it will be tricky to install as of course the window is at about 45 degrees hence I will be fighting gravity to get the film into position. The installed cost would be prohibitive as I live in west Wales and the company charge for the time away from base. I am tempted to have a go on a smaller window (the one in the bathroom) and see how that goes first!

Posted By: Ed DaviesA quick way to estimate the heat getting through (per m²) would be to use an IR thermometer to measure the temperature of the inside surfaces.

Are your blackout blinds black? A lighter/more reflective surface on the sunny side ought to help.

My three large Velux windows are facing west so this time of year there is not much solar heating effect on that side, so may have to wait a while to get decent readings. Our blackout blinds are beige in colour with a whitish reflective layer on the glass side.

I have ordered a small amount of the reflective window film to try on the small Velux window on the landing and see what effect that has.

I agree that external blinds are much better, but I have been pleasantly surprised by how well our honeycomb blinds work inside the windows. They stop the sun striking the floor etc inside the room, and whilst they get warm themselves, the MVHR seems to be able to deal with taking most of that heat away.

We've also fitted flyscreens to the outside of some windows. Partly so we can open the windows for ventilation in summer, but they also help to reduce insolation.

It does seem pretty obvious that it would be more effective to have blinds placed externally to prevent the sun’s rays hitting the glass in the first place. I have thought about that but where we are is a very windy area and I don’t think such devices would last very long! Likewise when I talked with the technical guy at the solar film company he did not recommend applying solar film externally because that also would not last very long due to exposure to the weather.

I like the idea of the honeycomb blinds especially as contributor djh has had a positive experience with them. I presume these were used with Velux windows – do they work OK with windows at 45 degrees to the vertical?

Posted By: djh

Posted By: Jeff BI like the idea of the honeycomb blinds especially as contributor djh has had a positive experience with them. I presume these were used with Velux windows – do they work OK with windows at 45 degrees to the vertical?

We don't have any Velux windows so I have no experience of that question. All our windows are vertical so I have no experience of other angles. Having said that, our blinds are installed next to the glass using tensioned guide wires and I believe such systems are designed to operate at any angle on any type of window.

Would you mind giving the manufacturer's details please?

Posted By: djh

Posted By: Jeff BWould you mind giving the manufacturer's details please?

I don't mind, but I'm afraid I don't remember where I bought them. One of the internet blind-sellers :)

They are honeycomb blinds. They are the blackout type with reflective surfaces inside the honeycomb cells. And they have guide wires down the side that are terminated within the sash frame so the blind is close to the glass and moves with it when the window is opened.

A quick search doesn't show me anywhere I remember :(

OK, thanks for looking. I've emailed one of the Internet companies who do the honeycomb blinds for advice.

Posted By: djhJust noticed that on the operating handles, it says 'Cosiflor' and that leads to:

https://www.bloecker.de/products/for-specialist-retailers/technical-product-information/cosiflor-honeycomb-blinds/?lang=en" rel="nofollow" >https://www.bloecker.de/products/for-specialist-retailers/technical-product-information/cosiflor-honeycomb-blinds/?lang=en

Dunno where I bought them though :(

Thanks. I have found a UK supplier and have sent them an email to check if these will work with Velux windows.